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Theoretical and experimental sulfur K-edge X-ray absorption spectroscopic (XANES) study of cysteine, cystine, methionine and methionine sulfoxide
Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry.
Stockholm University, Faculty of Science, Department of Physics.
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2009 (English)In: Dalton Transactions, ISSN 1477-9226, Vol. 18, 3542-3558 p.Article in journal (Refereed) Published
Abstract [en]

The experimental sulfur K-edge x-ray absorption near edge structure (XANES) spectra of the amino acids cysteine, homocysteine, penicillamine, methionine, the oxidation products methionine sulfoxide and the disulfide cystine, have been analyzed by transition potential DFT calculations. The absolute energies and intensities of the main pre-edge sulfur 1s electron transitions have been computed to determine the character of the receiving low energy unoccupied molecular orbitals (MO), and to investigate the influence of external interactions, especially by introducing water molecules hydrogen bonded to the ionic species present in different pH ranges. When the thiol group deprotonates for the three amino acids cysteine, homocysteine and penicillamine the energy of the main transition, to an MO with antibonding σ*(S-H) character, reduces by ~1.1 eV and the receiving MO obtains σ*(S-C) character. The energy shifts due to hydrogen bonding were in most cases found to be relatively small, although the transition intensities could vary significantly due to the changes induced in the molecular charge distribution, thereby affecting the shapes of the spectral features. For the cysteine and penicillamine zwitterions deconvolution of the experimental spectra allowed the microscopic acid dissociation constants to be extracted separately for the thiol and the protonated amine groups, pKaT = 8.5 ± 0.1 and 8.2 ± 0.1, and pKaN = 8.9 ± 0.1 and 8.8 ± 0.1, respectively, in both cases with the thiol group the more acidic. Coordination of cysteine to nickel(II) or mercury(II) introduced a new low energy transition involving metal ion orbitals in the receiving LUMO. The small experimentally observed energy differences between the similar main absorption features of the cysteine and methionine zwitterions, 0.2-0.3 eV in comparable surrounding, as well as a minor difference in their intensity ratio, are reflected in the calculated transitions. The S K-edge XANES spectrum of the disulfide cystine displays a characteristic double peak with the lower energy transition (2469.9 eV) into the antibonding σ*(S-S) MO. The second peak, at 2471.5 eV in aqueous solution, contains several transitions into MO:s with σ*(S-C) character involving also charge transfer to the water molecules hydrating the protonated amine groups (NH3+) of cystine. The experimental energy difference between the two peaks increases by 0.2 eV for solid cystine, while no such increase occurs for the –S-S- bond between the cysteine residues in the oxidized disulfide of glutathione, where the amine groups are engaged in peptide bonds. This study shows that externally induced changes in the intramolecular bonding, e.g. by coordination, conformation geometry or hydrogen bonding, can significantly influence the S K-edge spectra, and emphasizes the importance of a similar chemical surrounding of the model compounds for standard spectra of sulfur functional groups, used to deconvolute composite experimental spectra.

Place, publisher, year, edition, pages
2009. Vol. 18, 3542-3558 p.
URN: urn:nbn:se:su:diva-24304OAI: diva2:197186
Part of urn:nbn:se:su:diva-6843Available from: 2007-05-15 Created: 2007-05-04 Last updated: 2010-01-26Bibliographically approved
In thesis
1. Structure and bonding of sulfur-containing molecules and complexes
Open this publication in new window or tab >>Structure and bonding of sulfur-containing molecules and complexes
2007 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Synchrotron-based spectroscopic techniques enable investigations of the many important biological and environmental functions of the ubiquitous element sulfur. In this thesis the methods for interpreting sulfur K-edge X-ray absorption near edge structure (XANES) spectra are developed and applied for analyses of functional sulfur groups. The influence of coordination, pH, hydrogen bonding, etc., on the sulfur 1s electronic excitations is evaluated by transition potential density functional theory. Analyses have been performed of reduced sulfur compounds in marine-archaeological wood from historical shipwrecks, including the Vasa, Stockholm, Sweden and the Mary Rose, Portsmouth, U.K.. The accumulation of sulfur as thiols in lignin-rich parts of the wood on the seabed is also a probable pathway in the natural sulfur cycle for how reduced sulfur enters fossil fuels via humic matter in anaerobic marine sediments. Sulfur K-edge XANES spectra for several biochemical model compounds and for coexisting isomeric sulfur species in cysteine and sulfite(IV) aqueous solutions have been analyzed with the aid of theoretical calculations. Cysteine derivatives are important for biochemical detoxification, and mercury(II) cysteine complexes in solution have been structurally characterized by means of Extended X-ray Absorption Fine Structure (EXAFS), Raman and 199Hg NMR spectroscopy. Lanthanoid(III) ions were found to coordinate eight dimethyl sulfoxide oxygen atoms in a distorted square antiprism in the solid state and in solution, by combining crystallography, EXAFS, XANES and vibrational spectroscopy. The mean M-O bond distances for the disordered crystal structures are in good agreement with those from the lattice-independent EXAFS studies. The different sulfur K-edge XANES spectra for the dimethyl sulfoxide ligands in the hexasolvated complexes of the trivalent group 13 metal ions, Tl(III), In(III), Ga(III) and Al(III), were interpreted by theoretical calculations.

Place, publisher, year, edition, pages
Stockholm: Institutionen för fysikalisk kemi, oorganisk kemi och strukturkemi, 2007. 105 p.
Sulfur species, x-ray absorption and vibrational spectroscopy, TP-DFT calculations
National Category
Chemical Sciences
Research subject
Structural Chemistry
urn:nbn:se:su:diva-6843 (URN)978-91-7155-423-9 (ISBN)
Public defence
2007-06-05, Magnélisalen, Kemiska övningslaboratoriet, Svante Arrhenius väg 12 A, Stockholm, 10:00 (English)
Available from: 2007-05-15 Created: 2007-05-04 Last updated: 2010-01-15Bibliographically approved

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