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Optimized Spin Crossings and Transition States for Short-range Electron Transfer in Transition Metal Dimers
Stockholm University, Faculty of Science, Department of Physics.
2005 (English)In: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 109, no 20, 10513-10520 p.Article in journal (Refereed) Published
Abstract [en]

Electron-transfer reactions in eight mixed-valence manganese dimers are studied using B3LYP. One of the dimers is a model of the active site of manganese catalase, while another represents a basic building block of the oxygen-evolving complex in photosystem II. The adiabatic reactions are characterized by fully optimized transition states where the single imaginary frequency represents the electron-transfer coordinate. When there is antiferromagnetic coupling between different high-spin centers, electron transfer must be accompanied by a spin transition. Spin transitions are characterized by minimum-energy crossing points between spin surfaces. Three reaction mechanisms have been investigated. First, a single-step reaction where spin flip is concerted with electron transfer. Second, an initial transition to a center with intermediate spin that can be followed by electron transfer. Third, an initial transition to a ferromagnetic state from which the electron can be transferred adiabatically. The complexes prefer the third route with rate-determining barriers ranging from 5.7 kcal/mol to 17.2 kcal/mol for different complexes. The origins of these differences are discussed in terms of oxidation states and ligand environments. Many DFT functionals overestimate charge-transfer interactions, but for the present complexes, the error should be limited because of short Mn−Mn distances

Place, publisher, year, edition, pages
2005. Vol. 109, no 20, 10513-10520 p.
National Category
Physical Chemistry
Identifiers
URN: urn:nbn:se:su:diva-23786DOI: 10.1021/jp051116qOAI: oai:DiVA.org:su-23786DiVA: diva2:194513
Note

Part of urn:nbn:se:su:diva-486

Available from: 2005-04-27 Created: 2005-04-27 Last updated: 2014-09-30Bibliographically approved
In thesis
1. Challenges in Enzyme Catalysis - Photosystem II and Orotidine Decarboxylase: A Density Functional Theory Treatment
Open this publication in new window or tab >>Challenges in Enzyme Catalysis - Photosystem II and Orotidine Decarboxylase: A Density Functional Theory Treatment
2005 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Possibly the most fascinating biochemical mechanism remaining to be solved is the formation of oxygen from water in photosystem II. This is a critical part of the photosynthetic reaction that makes solar energy accessible to living organisms.

The present thesis uses quantum chemistry, more specifically the density functional B3LYP, to investigate a mechanism where an oxyl radical bound to manganese is the active species in O-O bond formation. Benchmark calculations on manganese systems confirm that B3LYP can be expected to give accurate results. The effect of the self-interaction error is shown to be limited. Studies of synthetic manganese complexes support the idea of a radical mechanism. A manganese complex with an oxyl radical is active in oxygen formation while manganese-oxo complexes remain inactive. Formation of the O-O bond requires a spin transition but there should be no effect on the rate. Spin transitions are also required in many short-range electron-transfer reactions.

Investigations of the superproficient enzyme orotidine decarboxylase support a mechanism that involves an invariant network of charged amino acids, acting together with at least two mobile water molecules.

Place, publisher, year, edition, pages
Stockholm: Fysikum, 2005. 77 p.
Keyword
photosystem II, oxyl radical, manganese systems, orotidine decarboxylase, reaction mechanism, density functional theory
National Category
Theoretical Chemistry
Identifiers
urn:nbn:se:su:diva-486 (URN)91-7155-057-7 (ISBN)
Public defence
2005-05-27, sal FA32, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 10:00
Opponent
Supervisors
Available from: 2005-04-27 Created: 2005-04-27Bibliographically approved

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