Change search
Refine search result
17181920212223 951 - 1000 of 1256
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Rows per page
  • 5
  • 10
  • 20
  • 50
  • 100
  • 250
Sort
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
Select
The maximal number of hits you can export is 250. When you want to export more records please use the Create feeds function.
  • 951.
    Selander, Nicklas
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Catalytic Functionalization of Allylic Substrates by Palladium Pincer Complexes2010Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    This thesis is based on the development of novel catalytic reactions for the synthesis and application of organometallic reagents. The main focus is directed towards organoboronate derivatives. We developed an efficient procedure for converting allylic alcohols to the corresponding allylboronates using palladium pincer complexes as catalysts. The reactions were performed under mild conditions with high selectivity, allowing further one-pot transformations. Using this approach, a variety of stereodefined homoallylic alcohols and amino acid derivatives were synthesized via trapping of the in situ generated allylboronate derivatives with an appropriate electrophile. The synthetic scope of these types of multi-component reactions is broad as many different substrate allylic alcohols may be used together with various electrophiles. Several aspects of these reactions were studied, including different reagents, catalysts and electrophiles.

    Furthermore, we studied the possibility to use oxidizing reagents as an essential component in the functionalization of olefins. Two main strategies were utilized for these catalytic methods using palladium pincer complexes. The functional group was either transferred from the oxidizing reagent, or introduced via an oxidation-transmetallation route. We propose that both methods involve palladium(IV) intermediates thus expanding both the coordination sphere of palladium and the synthetic scope of pincer complex catalysis.

    Download full text (pdf)
    FULLTEXT01
  • 952.
    Selander, Nicklas
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Development of Multi-Component Reactions using Catalytically Generated Allyl Metal Reagents2008Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    This licentiate thesis is based on the development of catalytic reactions for the synthesis and application of organometallic reagents. By use of palladium pincer-complex catalysts, we have developed an efficient procedure for the synthesis of allylboronates starting from allylic alcohols. These reactions were further extended by including various one-pot multi-component reactions, using the in situ generated allylboronates. Furthermore, novel unsymmetrical palladium pincer-complexes were synthesized and studied in auto-tandem catalysis.

    Download full text (pdf)
    FULLTEXT01
  • 953.
    Selander, Nicklas
    et al.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Kipke, Andreas
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Sebelius, Sara
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Szabó, Kálmán J.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Petasis borono-Mannich reaction and allylation of carbonyl compounds via transient allyl boronates generated by palladium-catalyzed substitution of allyl alcohols: An efficient one-pot route to stereodefined alpha-amino acids and homoallyl alcohols2007In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 129, no 44, p. 13723-13731Article in journal (Refereed)
    Abstract [en]

    An efficient one-pot procedure was designed by integration of the pincer-complex-catalyzed borylation of allyl alcohols in the Petasis borono-Mannich reaction and in allylation of aldehydes and ketones. These procedures are suitable for one-pot synthesis of α-amino acids and homoallyl alcohols from easily available allyl alcohol, amine, aldehyde, or ketone substrates. In the presented transformations, the active allylating agents are in situ generated allyl boronic acid derivatives. These transient intermediates are proved to be reasonably acid-, base-, alcohol-, water-, and air-stable species, which allows a high level of compatibility with the reaction conditions of the allylation of various aldehyde/ketone and imine electrophiles. The boronate source of the reaction is diboronic acid or in situ hydrolyzed diboronate ester ensuring that the waste product of the reaction is nontoxic boric acid. The regio- and stereoselectivity of the reaction is excellent, as almost all products form as single regio- and stereoisomers. The described procedure is suitable to create quaternary carbon centers in branched allylic products without formation of the corresponding linear allylic isomers. Furthermore, products comprising three stereocenters were formed as single products without formation of other diastereomers. Because of the highly disciplined consecutive processes, up to four-step, four-component transformations could be performed selectively as a one-pot sequence. For example, stereodefined pyroglutamic acid could be prepared from a simple allyl alcohol, a commercially available amine, and glyoxylic acid in a one-step procedure. The presented method also grants an easy access to stereodefined 1,7-dienes that are useful substrates for Grubbs ring-closing metathesis.

  • 954.
    Selander, Nicklas
    et al.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Olsson, Vilhelm J.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Szabó, Kálmán
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Allylation of carbonyl compounds and in situ formed imines via transient allyl boronates generated by palladium pincer complex catalyzed substitution of allyl alcohols2008In: Abstracts of Papers, 235th ACS National Meeting, New Orleans, LA, United States, April 6-10, 2008, 2008Conference paper (Other academic)
  • 955.
    Selander, Nicklas
    et al.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Olsson, Vilhelm J.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Szabó, Kálmán J.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Scope and mechanism of the palladium pincer complex catalyzed boronation of allyl alcohols2008In: Abstracts of Papers, 235th ACS National Meeting, New Orleans, LA, United States, April 6-10, 2008, 2008Conference paper (Other academic)
  • 956.
    Selander, Nicklas
    et al.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Paasch, Jennifer R.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Szabó, Kálmán J.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Palladium-Catalyzed Allylic C-OH Functionalization for Efficient Synthesis of Functionalized Allylsilanes2011In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 133, no 3, p. 409-411Article in journal (Refereed)
    Abstract [en]

    A new method is described for palladium-catalyzed allylic silylation using allylic alcohols and disilanes as precursors. The reactions proceed smoothly under mild and neutral conditions, and this method is suitable for synthesis of regio-and stereodefined allylsilanes. The presented silylation reaction can be easily extended to include synthesis of allylboronates by change of the dimetallic reagent. The presented synthetic procedure offers a broad platform for the selective synthesis of functionalized allyl metal reagents, which are useful precursors in advanced organic chemistry and natural product synthesis.

  • 957.
    Selander, Nicklas
    et al.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Sebelius, Sara
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Estay, Cesar
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Szabó, Kálmán J.
    Highly Selective and Robust Palladium-Catalysed Carbon-Carbon Coupling between Allyl Alcohols and Aldehydes via Transient Allylboronic Acids2006In: European Journal of Organic Chemistry, ISSN 1434-193X, E-ISSN 1099-0690, no 18, p. 4085-4087Article in journal (Refereed)
    Abstract [en]

    The highly regio- and stereoselective coupling of allyl alcohols with aldehydes could be achieved with 5 mol-% of SeCSe pincer complex catalyst and p-toluenesulfonic acid in the presence of diboronic acid. The transformations have a broad synthetic scope, and the high yields were obtained without the use of an inert atmosphere and carefully dried solvents.

  • 958.
    Selander, Nicklas
    et al.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Sebelius, Sara
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Olsson, Vilhelm J.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Szabó, Kálmán J.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Allylation of aldehydes by allyl alcohols via palladium pincer complex catalyzed formation of transient allylboronic acids2007In: Abstracts of Papers, 233rd ACS National Meeting, Chicago, IL, United States, March 25-29, 2007, 2007Conference paper (Other academic)
  • 959.
    Selander, Nicklas
    et al.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Szabó, Kálmán
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    [2,6-Bis[(phenylseleno-κSe)methyl]phenyl-κC]chloropalladium2009In: Encyclopedia of Reagents for Organic Synthesis, John Wiley & Sons, Ltd. , 2009Chapter in book (Other academic)
  • 960.
    Selander, Nicklas
    et al.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Szabó, Kálmán
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Performance of SCS palladium pincer complexes in borylation of allylic alcohols. Control of the regioselectivity in the one-pot borylation-allylation process2009In: Journal of Organic Chemistry, ISSN 0022-3263, E-ISSN 1520-6904, Vol. 74, no 15, p. 5695-5698Article in journal (Refereed)
    Abstract [en]

    One-pot borylation−allylation reactions of aldehydes and allylic alcohols were performed under various reaction conditions. The borylation of allylic alcohols was performed using a very efficient SCS palladium pincer-complex catalyst. The regioselectivity of the allylation depends on the applied solvent. The reaction in CHCl3 gave the linear allylic product; however, when MeOH was added to the reaction mixture, the branched allylic product was formed.

  • 961.
    Selander, Nicklas
    et al.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Szabó, Kálmán J.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Catalysis by Palladium Pincer Complexes2011In: Chemical Reviews, ISSN 0009-2665, E-ISSN 1520-6890, Vol. 111, no 3, p. 2048-2076Article, review/survey (Refereed)
  • 962.
    Selander, Nicklas
    et al.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Szabó, Kálmán J
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Catalytic borylation of allylic alcohols using SCS-palladium pincer complexes: Control of the regioselectivity in one-pot borylation-allylation reactions2010In: Abstracts of Papers, 239th ACS National Meeting, San Francisco, CA, United States, March 21-25, 2010, American Chemical Society , 2010Conference paper (Other academic)
  • 963.
    Selander, Nicklas
    et al.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Szabó, Kálmán J.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Efficient synthesis of α-amino acids via organoboronate reagents2009In: Asymmetric Synthesis and Application of α-Amino Acids / [ed] Vadim A. Soloshonok, Kunisuke Izawa, Washington: American Chemical Society , 2009, p. 190-202Chapter in book (Other academic)
  • 964.
    Selander, Nicklas
    et al.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Szabó, Kálmán J
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Palladium pincer complexes: Selective and versatile catalysts for the synthesis of organoboronates2010In: Abstracts of Papers, 239th ACS National Meeting, San Francisco, CA, United States , March 21-25, 2010, American Chemical Society , 2010Conference paper (Other academic)
  • 965.
    Selander, Nicklas
    et al.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Szabó, Kálmán J
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Palladium-catalyzed functionalization of olefins under oxidative conditions2010In: Abstracts of Papers, 240th ACS National Meeting, Boston, MA, United States, August 22-26, 2010, American Chemical Society , 2010Conference paper (Other academic)
  • 966.
    Selander, Nicklas
    et al.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Szabó, Kálmán J.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Single-pot triple catalytic transformations based on coupling of in situ generated allyl boronates with in situ hydrolyzed acetals2008In: Chemical Communications, ISSN 1359-7345, E-ISSN 1364-548X, no 29, p. 3420-3422Article in journal (Refereed)
  • 967.
    Selander, Nicklas
    et al.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Szabó, Kálmán J.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Synthesis and transformation of organoboronates and stannanes by pincer-complex catalysts2009In: Dalton Transactions, ISSN 1477-9226, E-ISSN 1477-9234, no 32, p. 6267-6279Article in journal (Refereed)
  • 968.
    Selander, Nicklas
    et al.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Szabó, Kálmán J.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Synthesis of stereodefined substituted cycloalkenes by a one-pot catalytic boronation-allylation-metathesis sequence2008In: Advanced Synthesis and Catalysis, ISSN 1615-4150, E-ISSN 1615-4169, Vol. 350, no 13, p. 2045-2051Article in journal (Refereed)
    Abstract [en]

    Stereodefined cyclohexene and cyclopentene derivatives were prepared by the coupling of allylic alcohols and other allylic precursors with unsaturated aldehydes. These reactions are based on a multicatalytic one-pot approach involving palladium pincer complex-catalyzed boronation, allylation and ring-closing metathesis reactions. This reaction sequence can be performed in an operationally simple procedure affording the cycloalkene products in high overall yields and excellent regio- and stereoselectivities. The presented procedure has a broad synthetic scope and high functional group tolerance, which allows the synthesis of bicyclic lactone and spirane skeletons and various substitution patterns including hydroxy, silyl, vinyl, allyl, and sulfonyl groups. The studied catalytic one-pot reactions involve up to three individual processes performed by up to four acid- and transition metal-catalyzed events.

  • 969.
    Selander, Nicklas
    et al.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Willy, Benjamin
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Szabó, Kálmán J.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Selective C-H Borylation of Alkenes by Palladium Pincer Complex Catalyzed Oxidative Functionalization2010In: Angewandte Chemie International Edition, ISSN 1433-7851, E-ISSN 1521-3773, Vol. 49, no 24, p. 4051-4053Article in journal (Refereed)
  • 970.
    Shakeri, Mozaffar
    et al.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Engström, Karin
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Sandström, Anders G.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Bäckvall, Jan-Erling
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Highly enantioselective resolution of β-amino esters by Candida antarctica lipase A immobilized in mesocellular foam: application to dynamic kinetic resolution2010In: ChemCatChem, ISSN 1867-3899, Vol. 2, no 5, p. 534-538Article in journal (Refereed)
    Abstract [en]

    Candida antarctica lipase A (CALA) immobilized in functionalized mesocellular foam in the presence of sucrose, followed by lyophilization, led to a dramatic increase in the enantioselectivity as well as an improved thermostability of the enzyme. The immobilized lipase was used for kinetic resolution (KR) and dynamic kinetic resolution (DKR) of the β-amino ester, ethyl 3-amino-3-phenylpropanoate. The temperature of optimum activity of CALA shifted from 20–30 °C to 80–90 °C on immobilization in the MCF. An “enantiomeric ratio” E (E=νA/νB; νA and νB are the rate constants for entantiomers A and B) of 69 and a conversion of 43 % in 1 h were obtained at 80 °C, whereas non-immobilized CALA lost its activity at T≥50 °C. The obtained immobilized CALA showed an E value of greater than 500 at 22 °C. Combination of the immobilized CALA with a ruthenium complex, acting as a racemization catalyst, allowed for a successful DKR of ethyl 3-amino-3-phenylpropanoate resulting in 85 % conversion and 89 % ee.

  • 971.
    Shakeri, Mozaffar
    et al.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Tai, Cheuk-Wai
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Göthelid, Emmanuelle
    Oscarsson, Sven
    Bäckvall, Jan-E.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Small Pd Nanoparticles Supported in Large Pores of Mesocellular Foam: An Excellent Catalyst for Racemization of Amines2011In: Chemistry - A European Journal, ISSN 0947-6539, E-ISSN 1521-3765, Vol. 17, no 47, p. 13269-13273Article in journal (Refereed)
    Abstract [en]

    Highly dispersed palladium nanoparticles (1–2 nm) supported in large-pore mesocellular foam (MCF; 29 nm) were synthesized. The Pd-nanocatalyst/MCF system was characterized by transmission electron microscopy (TEM), powder X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The performance of the Pd nanocatalyst obtained was examined for amine racemization. The Pd nanocatalyst showed higher activity and selectivity toward racemization of (S)-1-phenylethyl amine than any other amine racemization catalyst reported so far and it could be reused several times. Our data from TEM and XRD suggest a restructuring of the Pd nanocatalyst from amorphous to crystalline and an increase in Pd nanocatalyst size during the racemization reaction. This led to an unexpected increase of activity after the first use. The Pd nanocatalyst obtained can be integrated with other resolving processes of racemic organic compounds to increase the yield of chiral organic products.

  • 972. Shashkov, Alexander S.
    et al.
    Zhang, Wenwen
    Perepelov, Andrei V.
    Weintraub, Andrej
    Liu, Bin
    Widmalm, Göran
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Knirel, Yuriy A.
    Structure of the O-polysaccharide of Escherichia coli O1322016In: Carbohydrate Research, ISSN 0008-6215, E-ISSN 1873-426X, Vol. 427, p. 44-47Article in journal (Refereed)
    Abstract [en]

    Mild acid degradation of the lipopolysaccharide of Escherichia coli O132 released its O-polysaccharide. Analysis by 1D and 2D H-1 and C-13 NMR spectroscopy prior and subsequent to O-deacetylation, in conjunction with sugar analysis, revealed a linear pentasaccharide repeating unit of the O-polysaccharide having the following structure: -> 2)-alpha-D-Galf-(1 -> 3)-alpha-L-Rhap2Ac-(1 -> 4)-alpha-D-Glcp-(1 -> 2)-alpha-L-Rhap-(1 -> 3)-beta-D-GlcpNAc-(1 -> Putative functions of genes in the O-antigen gene cluster of E. coli O132 are consistent with the O-polysaccharide structure.

    Download full text (pdf)
    fulltext
  • 973.
    Shatskiy, Andrey
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Highly Active Ruthenium-Based Water Oxidation Catalyst with an Easy Access to the Catalytically Active SpeciesManuscript (preprint) (Other academic)
    Abstract [en]

    Herein is described a highly active ruthenium-based water oxidation catalyst [RuIV(mcbp)(O)(py)2] (5, mcbp2− = 2,6-bis(1-methyl-4-(carboxylate)benzimidazol-2-yl)pyridine), which can be generated from either [RuII(mcbp)(py)2] (4II) or [RuIII(Hmcbp)(py)2]2+ (4III). Complexes 4II and 4III were isolated and characterized by single crystal X-ray analysis, NMR, UV-vis, FT-IR, ESI-HRMS, EPR, and elemental analysis, and their redox properties were studied in detail by electrochemical and spectroscopic methods. Unlike for the parent catalyst [Ru(tda)(py)2] (1, tda2− = [2,2′:6′,2″-terpyridine]-6,6″-dicarboxylate), for which full transformation to the catalytically active species [RuIV(tda)(O)(py)2] (2) could not be carried out — stoichiometric generation of the catalytically active Ru-aqua complex 5 from 4II was achieved under mild conditions (pH 7.0) and short reaction times. The redox properties of the catalyst were studied and its activity for electrocatalytic water oxidation was evaluated, reaching TOFmax ≈ 40 000 s−1 at pH 9.0 (from the foot-of-the-wave analysis, FOWA), which is comparable to the activity of the state-of-the-art catalyst 2.

  • 974.
    Shatskiy, Andrey
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Ru-Based Water Oxidation Catalysts: Development and Mechanistic Studies2018Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Oxidation of water constitutes one of the most challenging processes in artificial photosynthesis, which aims at storing solar energy in the form of chemical bonds of high-energy fuels. To facilitate this process, efficient and durable water oxidation catalysts have to be developed and integrated into the complete photosynthetic cells. Importantly, the intricate complexity of such devices requires the catalyst not only to be highly efficient and robust, but also operate through a well-defined mechanism.

    This thesis describes the development and mechanistic studies of new water oxidation catalysts based on ruthenium. The first part of the thesis describes the synthesis of a dinuclear ruthenium-based catalyst active for both chemical and light-driven water oxidation. This catalyst displayed a pronounced influence of the acetonitrile co-solvent on the redox properties, which was studied in detail by electrochemical methods. In the second part, a new benzimidazole-based mononuclear catalyst was evaluated. The activity of the catalyst was studied for chemical and light-driven water oxidation, and insight into the operating mechanism was provided with the help of density functional theory calculations. In the third part of the thesis, a new mononuclear ruthenium-based catalyst was prepared and evaluated for electrochemically-driven water oxidation. This catalyst displayed activity similar to that of the current state-of-the-art water oxidation catalyst, while eliminating its main drawback, that is incomplete activation. The redox properties of the new catalyst were studied in detail by electrochemical and spectroscopic techniques, providing insight into the origins of its improved performance. Finally, in the fourth part of the thesis, a heterogeneous nanoparticulate catalyst immobilized on a solid support is described. The catalyst displayed high activity and stability during chemical and light-driven water oxidation, which was attributed to the small average particle size and efficient anchoring of the catalyst to the heterogeneous support via an oxidatively-stable linker.

    Download full text (pdf)
    Ru-Based Water Oxidation Catalysts
    Download (jpg)
    Omslagsframsida
  • 975.
    Shatskiy, Andrey
    et al.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Kärkäs, Markus D.
    Åkermark, Björn
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    The Art of Splitting Water: Storing Energy in a Readily Available and Convenient Form2019In: European Journal of Inorganic Chemistry, ISSN 1434-1948, E-ISSN 1099-1948, no 15, p. 2020-2024Article in journal (Refereed)
    Abstract [en]

    This essay for EurJIC's special issue on Redox Catalysis for Artificial Photosynthesis introduces the reader to the field of water oxidation using molecular catalysts. The most essential challenge our society must address during the 21st century is perhaps the realization of a system for producing sustainable energy on the global scale. Currently, there exists an urgent need to develop effective and economical carbon-neutral or carbon-free energy technologies. The production of solar fuels through water splitting constitutes a key enabling element. The construction of robust and efficient catalysts for oxidation of water is therefore essential. In this essay the progress and mechanistic considerations pertaining to molecular water oxidation catalysts are described and discussed from a personal perspective.

  • 976.
    Shatskiy, Andrey
    et al.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Lomoth, Reiner
    Abdel-Magied, Ahmed F.
    Stockholm University, Faculty of Science, Department of Organic Chemistry. Nuclear Materials Authority, Egypt.
    Rabten, Wangchuk
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Laine, Tanja M.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Chen, Hong
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK). KTH Royal Institute of Technology, Sweden.
    Sun, Junliang
    Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
    Andersson, Pher G.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Kärkäs, Markus D.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Johnston, Eric V.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Åkermark, Björn
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Catalyst-solvent interactions in a dinuclear Ru-based water oxidation catalyst2016In: Dalton Transactions, ISSN 1477-9226, E-ISSN 1477-9234, Vol. 45, no 47, p. 19024-19033Article in journal (Refereed)
    Abstract [en]

    Photocatalytic water oxidation represents a key process in conversion of solar energy into fuels and can be facilitated by the use of molecular transition metal-based catalysts. A novel straightforward approach for covalent linking of the catalytic units to other moieties is demonstrated by preparation of a dinuclear complex containing two [Ru(pdc)(pic)(3)]-derived units (pdc = 2,6-pyridinedicarboxylate, pic = 4-picoline). The activity of this complex towards chemical and photochemical oxidation of water was evaluated and a detailed insight is given into the interactions between the catalyst and acetonitrile, a common co-solvent employed to increase solubility of water oxidation catalysts. The solvent-induced transformations were studied by electrochemical and spectroscopic techniques and the relevant quantitative parameters were extracted.

  • 977. Shen, Zhengnan
    et al.
    Mobarak, Hani
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Li, Wei
    Widmalm, Göran
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Yu, Biao
    Synthesis of β-(1→2)-Linked 6-Deoxy-L-altropyranose Oligosaccharides via Gold(I)-Catalyzed Glycosylation of an ortho-Hexynylbenzoate Donor2017In: Journal of Organic Chemistry, ISSN 0022-3263, E-ISSN 1520-6904, Vol. 82, no 6, p. 3062-3071Article in journal (Refereed)
    Abstract [en]

    The ss-(1 -> 2)-linked 6-deoxy-L-altropyranose di- to pentasaccharides 2-5, relevant to the O-antigen of the infectious Yersinia enterocolitica 0:3, were synthesized for the first time. The challenging 1,2-cis-altropyranosyl linkage was assembled effectively via glycosylation with 2-O-benzyl-3,4-di-O-benzoyl-6-deoxy-L-altropyranosyl ortho-hexynylbenzoate (7) under the catalysis of PPh3AuNTf2. NMR and molecular modeling studies showed that the pentasaccharide (5) adopted a left-handed helical conformation. [GRAPHICS]

    Download full text (pdf)
    fulltext
  • 978. Sheng, Xiang
    et al.
    Kroutil, Wolfgang
    Himo, Fahmi
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Computational Study of the Fries Rearrangement Catalyzed by Acyltransferase from Pseudomonas protegens2024In: ChemistryOpen, ISSN 2191-1363Article in journal (Refereed)
    Abstract [en]

    The acyltransferase from Pseudomonas protegens (PpATase) catalyzes in nature the reversible transformation of monoacetylphloroglucinol to diacetylphloroglucinol and phloroglucinol. Interestingly, this enzyme has been shown to catalyze the promiscuous transformation of 3-hydroxyphenyl acetate to 2′,4′-dihydroxyacetophenone, representing a biological version of the Fries rearrangement. In the present study, we report a mechanistic investigation of this activity of PpATase using quantum chemical calculations. A detailed mechanism is proposed, and the energy profile for the reaction is presented. The calculations show that the acylation of the enzyme is highly exothermic, while the acetyl transfer back to the substrate is only slightly exothermic. The deprotonation of the C6−H of the substrate is rate-limiting, and a remote aspartate residue (Asp137) is proposed to be the general base group in this step. Analysis of the binding energies of various acetyl acceptors shows that PpATase can promote both intramolecular and intermolecular Fries rearrangement towards diverse compounds. 

  • 979.
    Sheng, Xiang
    et al.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Lind, Maria E. S.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Himo, Fahmi
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Theoretical study of the reaction mechanism of phenolic acid decarboxylase2015In: The FEBS Journal, ISSN 1742-464X, E-ISSN 1742-4658, Vol. 282, no 24, p. 4703-4713Article in journal (Refereed)
    Abstract [en]

    The cofactor-free phenolic acid decarboxylases (PADs) catalyze the non-oxidative decarboxylation of phenolic acids to their corresponding p-vinyl derivatives. Phenolic acids are toxic to some organisms, and a number of them have evolved the ability to transform these compounds, including PAD-catalyzed reactions. Since the vinyl derivative products can be used as polymer precursors and are also of interest in the food-processing industry, PADs might have potential applications as biocatalysts. We have investigated the detailed reaction mechanism of PAD from Bacillus subtilis using quantum chemical methodology. A number of different mechanistic scenarios have been considered and evaluated on the basis of their energy profiles. The calculations support a mechanism in which a quinone methide intermediate is formed by protonation of the substrate double bond, followed by C-C bond cleavage. A different substrate orientation in the active site is suggested compared to the literature proposal. This suggestion is analogous to other enzymes with p-hydroxylated aromatic compounds as substrates, such as hydroxycinnamoyl-CoA hydratase-lyase and vanillyl alcohol oxidase. Furthermore, on the basis of the calculations, a different active site residue compared to previous proposals is suggested to act as the general acid in the reaction. The mechanism put forward here is consistent with the available mutagenesis experiments and the calculated energy barrier is in agreement with measured rate constants. The detailed mechanistic understanding developed here might be extended to other members of the family of PAD-type enzymes. It could also be useful to rationalize the recently developed alternative promiscuous reactivities of these enzymes.

  • 980.
    Sheng, Xiang
    et al.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Lind, Maria E. S.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Himo, Fahmi
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Theoretical Study of the Reaction Mechanism of Phenolic Acid DecarboxylaseManuscript (preprint) (Other academic)
  • 981.
    Sheng, Xiang
    et al.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Pasch, Katharina
    Payer, Stefan E.
    Ertl, Claudia
    Hofer, Gerhard
    Keller, Walter
    Braeuer, Simone
    Goessler, Walter
    Glueck, Silvia M.
    Himo, Fahmi
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Faber, Kurt
    Reaction Mechanism and Substrate Specificity of Iso-orotate Decarboxylase: A Combined Theoretical and Experimental Study2018In: Frontiers in Chemistry, E-ISSN 2296-2646, Vol. 6, article id 608Article in journal (Refereed)
    Abstract [en]

    The C-C bond cleavage catalyzed by metal-dependent iso-orotate decarboxylase (IDCase) from the thymidine salvage pathway is of interest for the elucidation of a (hypothetical) DNA demethylation pathway. IDCase appears also as a promising candidate for the synthetic regioselective carboxylation of N-heteroaromatics. Herein, we report a joint experimental-theoretical study to gain insights into the metal identity, reaction mechanism, and substrate specificity of IDCase. In contrast to previous assumptions, the enzyme is demonstrated by ICPMS/MS measurements to contain a catalytically relevant Mn(2+)rather than Zn2+. Quantum chemical calculations revealed that decarboxylation of the natural substrate (5-carboxyuracil) proceeds via a (reverse) electrophilic aromatic substitution with formation of CO2. The occurrence of previously proposed tetrahedral carboxylate intermediates with concomitant formation of HCO3- could be ruled out on the basis of prohibitively high energy barriers. In contrast to related o-benzoic acid decarboxylases, such as y-resorcylate decarboxylase and 5-carboxyvanillate decarboxylase, which exhibit a relaxed substrate tolerance for phenolic acids, IDCase shows high substrate fidelity. Structural and energy comparisons suggest that this is caused by a unique hydrogen bonding of the heterocyclic natural substrate (5-carboxyuracil) to the surrounding residues. Analysis of calculated energies also shows that the reverse carboxylation of uracil is impeded by a strongly disfavored uphill reaction.

  • 982.
    Sheng, Xiang
    et al.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Patskovsky, Yury
    Vladimirova, Anna
    Bonanno, Jeffrey B.
    Almo, Steven C.
    Himo, Fahmi
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Raushel, Frank M.
    Mechanism and Structure of gamma-Resorcylate Decarboxylase2018In: Biochemistry, ISSN 0006-2960, E-ISSN 1520-4995, Vol. 57, no 22, p. 3167-3175Article in journal (Refereed)
    Abstract [en]

    gamma-Resorcylate decarboxylase (gamma-RSD) has evolved to catalyze the reversible decarboxylation of 2,6-dihydroxybenzoate to resorcinol in a nonoxidative fashion. This enzyme is of significant interest because of its potential for the production of gamma-resorcylate and other benzoic acid derivatives under environmentally sustainable conditions. Kinetic constants for the decarboxylation of 2,6-dihydroxybenzoate catalyzed by gamma-RSD from Polaromonas sp. JS666 are reported, and the enzyme is shown to be active with 2,3-dihydroxybenzoate, 2,4,6-trihydroxybenzoate, and 2,6-dihydroxy-4-methylbenzoate. The three-dimensional structure of gamma-RSD with the inhibitor 2-nitroresorcinol (2-NR) bound in the active site is reported. 2-NR is directly ligated to a Mn2+ bound in the active site, and the nitro substituent of the inhibitor is tilted significantly from the plane of the phenyl ring. The inhibitor exhibits a binding mode different from that of the substrate bound in the previously determined structure of gamma-RSD from Rhizobtum sp. MTP-10005. On the basis of the crystal structure of the enzyme from Polaromonas sp. JS666, complementary density functional calculations were performed to investigate the reaction mechanism. In the proposed reaction mechanism, gamma-RSD binds 2,6-dihydroxybenzoate by direct coordination of the active site manganese ion to the carboxylate anion of the substrate and one of the adjacent phenolic oxygens. The enzyme subsequently catalyzes the transfer of a proton to Cl of y-resorcylate prior to the actual decarboxylation step. The reaction mechanism proposed previously, based on the structure of gamma-RSD from Rhizobtum sp. MTP-10005, is shown to be associated with high energies and thus less likely to be correct.

  • 983.
    Sheng, Xiang
    et al.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Zhu, Wen
    Huddleston, Jamison
    Xiang, Dao Fen
    Raushel, Frank M.
    Richards, Nigel G. J.
    Himo, Fahmi
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    A Combined Experimental-Theoretical Study of the LigW-Catalyzed Decarboxylation of 5-Carboxyvanillate in the Metabolic Pathway for Lignin Degradation2017In: ACS Catalysis, ISSN 2155-5435, E-ISSN 2155-5435, Vol. 7, no 8, p. 4968-4974Article in journal (Refereed)
    Abstract [en]

    Although it is a member of the amidohydrolase superfamily, LigW catalyzes the nonoxidative decarboxylation of 5-carboxyvanillate to form vanillate in the metabolic pathway for bacterial lignin degradation. We now show that membrane inlet mass spectrometry (MIMS) can be used to measure transient CO2 concentrations in real time, thereby permitting us to establish that C-C bond cleavage proceeds to give CO2 rather than HCO3- as the initial product in the LigW-catalyzed reaction. Thus, incubation of LigW at pH 7.0 with the substrate 5-carboxyvanillate results in an initial burst of CO2 formation that gradually decreases to an equilibrium value as CO2 is nonenzymatically hydrated to HCO3-. The burst of CO2 is completely eliminated with the simultaneous addition of substrate and excess carbonic anhydrase to the enzyme, demonstrating that CO2 is the initial reaction product. This finding is fully consistent with the results of density functional theory calculations, which also provide support for a mechanism in which protonation of the C5 carbon takes place prior to C-C bond cleavage. The calculated barrier of 16.8 kcal/mol for the rate-limiting step, the formation of the C5-protonated intermediate, compares well with the observed kcat value of 27 for Sphingomonas paucimobilis LigW, which corresponds to an energy barrier of 16 kcal/mol. The MIMS-based strategy is superior to alternate methods of establishing whether CO2 or HCO3- is the initial reaction product, such as the use of pH-dependent dyes to monitor very small changes in solution pH. Moreover, the MIMS-based assay is generally applicable to studies of all enzymes that produce and/or consume small-molecule, neutral gases.

  • 984. Siegbahn, Anna
    et al.
    Aili, Ulrika
    Ochocinska, Agata
    Olofsson, Martin
    Rönnols, Jerk
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Mani, Katrin
    Widmalm, Göran
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Ellervik, Ulf
    Synthesis, conformation and biology of naphthoxylosides2011In: Bioorganic & Medicinal Chemistry, ISSN 0968-0896, E-ISSN 1464-3391, Vol. 19, no 13, p. 4114-4126Article in journal (Refereed)
    Abstract [en]

    Proteoglycans (PG) are polyanionic proteins consisting of a core protein substituted with carbohydrate chains, that is, glycosaminoglycans (GAG). The biosynthesis of GAG can be manipulated by simple xylosides carrying hydrophobic aglycons, which can enter the cell and initiate the biosynthesis. While the importance of the aglycon is well investigated, there is far less information on the effect of modifications in the xylose residue.

    We have developed a new synthetic protocol, based on acetal protection and selective benzylation, for modification of the three hydroxyl groups in xylose. Thus we have synthesized twelve analogs of 2-naphthyl β-d-xylopyranoside (XylNap), where each hydroxyl group has been epimerized or replaced by methoxy, fluoro, or hydrogen.

    To gain more information about the properties of xylose, conformational studies were made on some of the analogs. It was found that the 4C1 conformation is highly predominant, accompanied by a nonnegligible population of the 2S0 conformation. However, deoxygenation at C3 results in a large portion of the 1C4 conformation.

    The GAG priming ability and proliferation activity of the twelve analogs, were investigated using a matched pair of human breast fibroblasts and human breast carcinoma cells. None of the analogs initiated the biosynthesis of GAG, but an inhibitory effect on endogenous PG production was observed for analogs fluorinated or deoxygenated at C4. From our data it seems reasonable that all three hydroxyl groups in XylNap are essential for the priming of GAG chains and for selective toxicity for tumor cells.

  • 985. Siegbahn, Anna
    et al.
    Manner, Sophie
    Persson, Andrea
    Tykesson, Emil
    Holmqvist, Karin
    Ochocinska, Agata
    Rönnols, Jerk
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Sundin, Anders
    Mani, Katrin
    Westergren-Thorsson, Gunilla
    Widmalm, Göran
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Ellervik, Ulf
    Rules for priming and inhibition of glycosaminoglycan biosynthesis; probing the beta 4GalT7 active site2014In: Chemical Science, ISSN 2041-6520, E-ISSN 2041-6539, Vol. 5, no 9, p. 3501-3508Article in journal (Refereed)
    Abstract [en]

    beta-1,4-Gatactosyltransferase 7 (beta 4GalT7) is an essential enzyme in the biosynthesis of glycosaminoglycan (GAG) chains of proteoglycans (PGs). Mammalian cells produce PGs, which are involved in biological processes such as cell growth and differentiation. The PGs consist of a core protein, with one or several GAG chains attached. Both the structure of the PGs and the GAG chains, and the expression of the enzymes involved in their biosynthesis and degradation, vary between normal cells and tumor cells. The biosynthesis of GAG chains is initiated by xylosylation of a serine residue of the core protein, followed by galactosylation by beta 4GalT7. The biosynthesis can also be initiated by exogenously added beta-D-xylopyranosides with hydrophobic aglycons, which thus can act as acceptor substrates for beta 4GalT7. To determine the structural requirements for beta 4GalT7 activity, we have cloned and expressed the enzyme and designed a focused library of 2-naphthyl beta-D-xylopyranosides with modifications of the xylose moiety. Based on enzymatic studies, that is galactosylation and its inhibition, conformational analysis and molecular modeling using the crystal structure, we propose that the binding pocket of beta 4GalT7 is very narrow, with a precise set of important hydrogen bonds. Xylose appears to be the optimal acceptor substrate for galactosylation by beta 4GalT7. However, we show that modifications of the xylose moiety of the beta-D-xylopyranosides can render inhibitors of galactosylation. Such compounds will be valuable tools for the exploration of GAG and PG biosynthesis and a starting point for development of anti-tumor agents.

    Download full text (pdf)
    fulltext
  • 986. Siegbahn, Anna
    et al.
    Thorsheim, Karin
    Ståhle, Jonas
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Manner, Sophie
    Hamark, Christoffer
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Persson, Andrea
    Tykesson, Emil
    Mani, Katrin
    Westergren-Thorsson, Gunilla
    Widmalm, Göran
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Ellervik, Ulf
    Exploration of the active site of beta 4GalT7: modifications of the aglycon of aromatic xylosides2015In: Organic and biomolecular chemistry, ISSN 1477-0520, E-ISSN 1477-0539, Vol. 13, no 11, p. 3351-3362Article in journal (Refereed)
    Abstract [en]

    Proteoglycans (PGs) are macromolecules that consist of long linear polysaccharides, glycosaminoglycan (GAG) chains, covalently attached to a core protein by the carbohydrate xylose. The biosynthesis of GAG chains is initiated by xylosylation of the core protein followed by galactosylation by the galactosyltransferase beta 4GalT7. Some beta-D-xylosides, such as 2-naphthyl beta-D-xylopyranoside, can induce GAG synthesis by serving as acceptor substrates for beta 4GalT7 and by that also compete with the GAG synthesis on core proteins. Here we present structure-activity relationships for beta 4GalT7 and xylosides with modifications of the aromatic aglycon, using enzymatic assays, cell studies, and molecular docking simulations. The results show that the aglycons reside on the outside of the active site of the enzyme and that quite bulky aglycons are accepted. By separating the aromatic aglycon from the xylose moiety by linkers, a trend towards increased galactosylation with increased linker length is observed. The galactosylation is influenced by the identity and position of substituents in the aromatic framework, and generally, only xylosides with beta-glycosidic linkages function as good substrates for beta 4GalT7. We also show that the galactosylation ability of a xyloside is increased by replacing the anomeric oxygen with sulfur, but decreased by replacing it with carbon. Finally, we propose that reaction kinetics of galactosylation by beta 4GalT7 is dependent on subtle differences in orientation of the xylose moiety.

    Download full text (pdf)
    fulltext
  • 987.
    Siegbahn, Per E. M.
    Stockholm University, Faculty of Science, Department of Organic Chemistry. Stockholm University, Faculty of Science, Department of Biochemistry and Biophysics. Stockholm University, Faculty of Science, Department of Physics.
    Computational Model Study of the Experimentally Suggested Mechanism for Nitrogenase2024In: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 128, no 4, p. 985-989Article in journal (Refereed)
    Abstract [en]

    The mechanism for N-2 activation in the E-4 state of nitrogenase was investigated by model calculations. In the experimentally suggested mechanism, the E-4 state is obtained after four reductions to the ground state. In a recent theoretical study, results for a different mechanism have been found in excellent agreement with available Electron Paramagnetic Resonance (EPR) experiments for E-4. The two hydrides in E-4 leave as H-2 concertedly with the binding of N-2. The mechanism suggested differs from the experimentally suggested one by a requirement for four activation steps prior to catalysis. In the present study, the experimentally suggested mechanism is studied using the same methods as those used in the previous study on the theoretical mechanism. The computed results make it very unlikely that a structure obtained after four reductions from the ground state has two hydrides, and the experimentally suggested mechanism does therefore not agree with the EPR experiments for E-4. Another structure with only one hydride is here suggested to be the one that has been observed to bind N-2 after only four reductions of the ground state.

  • 988.
    Siegbahn, Per E. M.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    How Protons Move in Enzymes - The Case of Nitrogenase2023In: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 127, no 10, p. 2156-2159Article in journal (Refereed)
    Abstract [en]

    When moving protons in enzymes, water molecules are often used as intermediates. The water molecules used are not necessarily seen in the crystal structures if they move around at high rates. In a different situation, for metal containing cofactors in enzymes, it is sometimes necessary to move protons on the cofactor from the position they enter the cofactor to another position where the energy is lower. That is, for example, the situation in nitrogenase. In recent studies on that enzyme, prohibitively high barriers were sometimes found for transferring protons, and that was used as a strong argument against mechanisms where a sulfide is lost in the mechanism. A high barrier could be due to nonoptimal distances and angles at the transition state. In the present study, possibilities are investigated to use water molecules to reduce these barriers. The study is very general and could have been done for many other enzymes. The effect of water was found to be very large in the case of nitrogenase with a lowering of one barrier from 15.6 kcal/mol down to essentially zero. It is concluded that the effect of water molecules must be taken into account for meaningful results.

  • 989.
    Siegbahn, Per E. M.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Is there computational support for an unprotonated carbon in the E-4 state of nitrogenase?2018In: Journal of Computational Chemistry, ISSN 0192-8651, E-ISSN 1096-987X, Vol. 39, no 12, p. 743-747Article in journal (Refereed)
    Abstract [en]

    In the key enzyme for nitrogen fixation in nature, nitrogenase, the active site has a metal cluster with seven irons and one molybdenum bound by bridging sulfurs. Surprisingly, there is also a carbon in the center of the cluster, with a role that is not known. A mechanism has been suggested experimentally, where two hydrides leave as a hydrogen molecule in the critical E-4 state. A structure with two hydrides, two protonated sulfurs and an unprotonated carbon has been suggested for this state. Rather recently, DFT calculations supported the experimental mechanism but found an active state where the central carbon is protonated all the way to CH3. Even more recently, another DFT study was made that instead supported the experimentally suggested structure. To sort out the origin of these quite different computational results, additional calculations have here been performed using different DFT functionals. The conclusion from these calculations is very clear and shows no computational support for an unprotonated carbon in E-4.

  • 990.
    Siegbahn, Per E. M.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Model Calculations Suggest that the Central Carbon in the FeMo-Cofactor of Nitrogenase Becomes Protonated in the Process of Nitrogen Fixation2016In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 138, no 33, p. 10485-10495Article in journal (Refereed)
    Abstract [en]

    Nitrogen activation by nitrogenase is one of the most important enzymatic processes on earth. In spite of the determination of X-ray structures of increasingly higher resolution, the nitrogenase mechanism is still not understood. In the most recent X-ray structures it has been shown that a carbon resides in the center of the MoFe-cofactor. Its role is not known. Recent spectroscopic studies, mainly EPR, have come closest to obtaining a molecular mechanism for activating nitrogen. Two hydrides have been shown to play a key role in this context. In the present study, the mechanism for nitrogenase has been investigated by hybrid DFT using a cluster model. This approach has been shown to be very successful for predicting mechanisms for other redox-active enzymes, such as the one for photosystem II, but has so far not been used in its most recent form for nitrogenase. The mechanism obtained has large similarities to the one suggested by spectroscopy, with a reductive elimination of two hydrides just before nitrogen binding. However, a very surprising finding is that the central carbon becomes protonated and has to move out of the cavity as a methyl group before the hydrides can be formed. This has not been suggested before.

  • 991.
    Siegbahn, Per E. M.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Substrate Water Exchange for the Oxygen Evolving Complex in PSII in the S-1, S-2, and S-3 States2013In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 135, no 25, p. 9442-9449Article in journal (Refereed)
    Abstract [en]

    Detailed mechanisms for substrate water exchange in the oxygen evolving complex in photosystem II have been determined with DFT methods for large models. Existing interpretations of the experimental water exchange results have been quite different. By many groups, these results have been the main argument against the water oxidation mechanism suggested by DFT, in which the oxygen molecule is formed between a bridging oxo and an oxyl radical ligand in the center of the OEC. That mechanism is otherwise in line with most experiments The problem has been that the mechanism requires a rather fast exchange of a bridging oxo ligand, which is not a common finding for smaller Mn-containing model systems. However, other groups have actually favored a substrate derived oxo ligand partly based on the same experiments. In the present study, three S-states have been studied, and the rates have been well reproduced by the calculations. The surprising experimental finding that water exchange in Si is slower than the one in S-2 is reproduced and explained. The key to this rate difference is the ease by which one of the manganese centers (Mn3) is reduced. This reduction has to occur to release the substrate water from Mn3. The similar rate of the slow exchange in S-2 and S-3 has been rationalized on the basis of earlier experiments combined with the present calculations. The results strongly support the previous DFT-suggested water oxidation mechanism.

  • 992.
    Siegbahn, Per E. M.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    The S-2 to S-3 transition for water oxidation in PSII (photosystem II), revisited2018In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 20, no 35, p. 22926-22931Article in journal (Refereed)
    Abstract [en]

    The formation of O-2 from water requires four transitions, each one after the absorption of one light flash. The mechanism for one of these transitions, the one from S-2 to S-3, has been under considerable debate for the past five years. There has since long been a consensus that one water molecule becomes inserted into the OEC (oxygen evolving complex) in this transition, but the mechanism for this insertion is not clear. A major conclusion obtained in the recent studies has been that a, so called, closed cubane structure of the OEC is a key factor in this transition. Leading suggestions for water insertion have been the so called pivot and carousel mechanisms. In the present study, inspired by recent experiments, it is found that the closed cubane structure is never involved in the mechanism for this transition, nor are the different suggestions mentioned above for water insertion. In the experiments, a high spin S-2 (HS-S-2) intermediate has been detected. Here, it is suggested that this state has an open structure of the OEC.

  • 993.
    Siegbahn, Per E. M.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Water oxidation energy diagrams for photosystem II for different protonation states, and the effect of removing calcium2014In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 16, no 24, p. 11893-11900Article in journal (Refereed)
    Abstract [en]

    The main parts of the water oxidation mechanism in photosystem II have now been established both from theory and experiments. Still, there are minor questions remaining. One of them concerns the charge and the protonation state of the oxygen evolving complex (OEC). Previously, theory and experiments have agreed that the two water derived ligands on the outer manganese should be one hydroxide and one water. In the present study it is investigated whether both of them could be water. This question is addressed by a detailed study of energy diagrams, but in this context it is more conclusive to compare the redox potential of the OEC to the one of Tyr(Z). Both procedures lead to the conclusion that one of the ligands is a hydroxide. Another question concerns the protonation of the second shell His337, where the results are more ambiguous. The final part of the present study describes results when calcium is removed from the OEC. Even though protons enter to compensate the charge of the missing Ca2+, the redox potential and the pK(a) value of the OEC change dramatically and prevent the progress after S-2.

  • 994.
    Siegbahn, Per E. M.
    et al.
    Stockholm University, Faculty of Science, Department of Biochemistry and Biophysics. Stockholm University, Faculty of Science, Department of Physics.
    Himo, Fahmi
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    The quantum chemical clusterapproach for modeling enzymereactions2011In: Wiley Interdisciplinary Reviews. Computational Molecular Science, ISSN 1759-0876, E-ISSN 1759-0884, Vol. 1, no 3, p. 323-356Article in journal (Refereed)
    Abstract [en]

    This Overview describes the general concepts behind the quantum chemical clusterapproach formodeling enzyme active sites and reaction mechanisms. First, theunderlying density functional electronic structure method is briefly recapitulated.The cluster methodology is then discussed, including the important observationon the convergence of the solvation effects. The concepts are illustrated usingexamples from recent applications, such as the discrimination between differentreaction mechanisms in phosphotriesterase, the elucidation of origins of regioselectivityin the epoxide-opening reaction of haloalcohol dehalogenase, and finallythe use of the cluster methodology to establish the detailed structure of theoxygen-evolving complex in photosystem II.

  • 995.
    Siegbahn, Per E. M.
    et al.
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Liao, Rong-Zhen
    The Energetics of Hydrogen Molecule Oxidation in NiFe-hydrogenase2020In: ACS Catalysis, ISSN 2155-5435, E-ISSN 2155-5435, Vol. 10, no 10, p. 5603-5613Article in journal (Refereed)
    Abstract [en]

    The mechanism for oxidation of the hydrogen molecule by NiFe-hydrogenase is reinvestigated. In contrast to most earlier studies, the emphasis is on the entire mechanism, including the oxidation steps. An estimate of the driving force is made, and the main effects of entropy are included. Two different mechanisms are investigated, not only the standard heterolytic cleavage but also homolytic cleavage. Heterolytic cleavage occurs for a NiFe(II,II) oxidation state, while homolytic cleavage occurs for a NiFe(I,II) state. The finding of a previously unreported transition state leads to a lower barrier for the latter mechanism. To reach the homolytic mechanism, one cycle of the heterolytic mechanism is needed. It is argued that the use of the very unusual active site, including CO and CN ligands, is not due to the efficiency of the H-H cleavage but rather to a minimization of the energy loss in the oxidation steps. This means that the H-H cleavage is not preceded by a good binding of molecular H-2. Instead, the transition state is reached directly from the reactant state with a free H-2.

  • 996.
    Sigurdsson, Susannah
    Stockholm University.
    Studies aimed at improving the H-phosphonate approach for solid phase oligonucleotide synthesis2002Doctoral thesis, comprehensive summary (Other academic)
  • 997. Silva, Luiz F., Jr.
    et al.
    Olofsson, Berit
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Hypervalent iodine reagents in the total synthesis of natural products2011In: Natural product reports (Print), ISSN 0265-0568, E-ISSN 1460-4752, Vol. 28, no 10, p. 1722-1754Article, review/survey (Refereed)
    Abstract [en]

    This report describes the recent applications of hypervalent iodine reagents in the total synthesis of natural products. The large diversity of high-yielding and chemoselective reactions that can be achieved, even for highly functionalized molecules, is summarized, demonstrating that hypervalent iodine reagents have become an essential tool in synthetic organic chemistry.

    Download full text (pdf)
    NPR review
  • 998. Siqueira, Fernanda A.
    et al.
    Ishikawa, Eloisa E.
    Fogaça, André
    Faccio, Andréa T.
    Carneiro, Vânia M. T.
    Soares, Rafael R. S.
    Utaka, Aline
    Tébéka, Iris R. M.
    Bielawski, Marcin
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Olofsson, Berit
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Silva Jr., Luiz F.
    Metal-Free Synthesis of Indanes by Iodine(III)-Mediated Ring Contraction of 1,2-Dihydronaphthalenes2011In: Journal of the Brazilian Chemical Society, ISSN 0103-5053, E-ISSN 1678-4790, Vol. 22, no 9, p. 1795-1807Article in journal (Refereed)
    Abstract [en]

    A metal-free protocol was developed to synthesize indanes by ring contraction of 1,2-dihydronaphthalenes promoted by PhI(OH)OTs (HTIB or Koser’s reagent). This oxidative rearrangement can be performed in several solvents (MeOH, CH3CN, 2,2,2-trifluoroethanol (TFE), 1,1,1,3,3,3-hexafluoroisopropanol (HFIP), and a 1:4 mixture of TFE:CH2Cl2) under mild conditions. The ring contraction diastereoselectively gives functionalized trans-1,3-disubstituted indanes, which are difficult to obtain in synthetic organic chemistry.

    Download full text (pdf)
    J Braz Chem Soc
  • 999.
    Sjöberg, Kjell
    Stockholm University.
    Organic syntheses applicable to preparation of penicillins: synthetic uses of methylsulfinyl carbanion1971Doctoral thesis, comprehensive summary (Other academic)
  • 1000. Sjödin, Martin
    et al.
    Irebo, Tania
    Utas, Josefin
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Lind, Johan
    Merényi, Gabor
    Åkermark, Björn
    Stockholm University, Faculty of Science, Department of Organic Chemistry.
    Hammarström, Leif
    Kinetic Effects of Hydrogen-bonds on Proton-Coupled Electron Transfer from Phenols2006In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 128, no 40, p. 13076-13083Article in journal (Refereed)
    Abstract [en]

    The kinetics and mechanism of proton-coupled electron transfer (PCET) from a series of phenols to a laser flash generated [Ru(bpy)3]3+ oxidant in aqueous solution was investigated. The reaction followed a concerted electron−proton transfer mechanism (CEP), both for the substituted phenols with an intramolecular hydrogen bond to a carboxylate group and for those where the proton was directly transferred to water. Without internal hydrogen bonds the concerted mechanism gave a characteristic pH-dependent rate for the phenol form that followed a Marcus free energy dependence, first reported for an intramolecular PCET in Sjödin, M. et al. J. Am. Chem. Soc. 2000, 122, 3932−3962 and now demonstrated also for a bimolecular oxidation of unsubstituted phenol. With internal hydrogen bonds instead, the rate was no longer pH-dependent, because the proton was transferred to the carboxylate base. The results suggest that while a concerted reaction has a relatively high reorganization energy (λ), this may be significantly reduced by the hydrogen bonds, allowing for a lower barrier reaction path. It is further suggested that this is a general mechanism by which proton-coupled electron transfer in radical enzymes and model complexes may be promoted by hydrogen bonding. This is different from, and possibly in addition to, the generally suggested effect of hydrogen bonds on PCET in enhancing the proton vibrational wave function overlap between the reactant and donor states. In addition we demonstrate how the mechanism for phenol oxidation changes from a stepwise electron transfer−proton transfer with a stronger oxidant to a CEP with a weaker oxidant, for the same series of phenols. The hydrogen bonded CEP reaction may thus allow for a low energy barrier path that can operate efficiently at low driving forces, which is ideal for PCET reactions in biological systems.

17181920212223 951 - 1000 of 1256
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf