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A Quantum Chemical Study of Hydrogen Abstraction from Manganese-Coordinated Water by a Tyrosyl Radical: A Model for Water Oxidation in Photosystem II

Blomberg, Margareta R A; Siegbahn, Per E M; Styring, Stenbjörn LU ; Babcock, Gerald T; Åkermark, Björn and Korall, Peter (1997) In Journal of the American Chemical Society 119(35). p.8285-8292
Abstract
Recently, water oxidation in photosystem II was proposed to involve direct abstraction of hydrogen atoms from water molecules terminally ligated to manganese ions in the oxygen-evolving complex by the oxidized tyrosine radical, TyrZ. This model is tested here by performing quantum chemical calculations. An empirically parametrized hybrid density functional method is used, and both monomeric and dimeric manganese model systems are studied. It is found that, by coordination to a manganese center, the first O-H bond strength of water is lowered from 113.4 to 84.3 kcal/mol. This O-H bond strength is only 2.8 kcal/mol stronger than that in tyrosine. Using an extended basis set, we find that this difference decreases still further. The second... (More)
Recently, water oxidation in photosystem II was proposed to involve direct abstraction of hydrogen atoms from water molecules terminally ligated to manganese ions in the oxygen-evolving complex by the oxidized tyrosine radical, TyrZ. This model is tested here by performing quantum chemical calculations. An empirically parametrized hybrid density functional method is used, and both monomeric and dimeric manganese model systems are studied. It is found that, by coordination to a manganese center, the first O-H bond strength of water is lowered from 113.4 to 84.3 kcal/mol. This O-H bond strength is only 2.8 kcal/mol stronger than that in tyrosine. Using an extended basis set, we find that this difference decreases still further. The second hydrogen abstraction energy is quite similar. Since thermoneutrality in the reaction (or a weak exothermicity) is a requirement for the hydrogen abstraction model, the present calculations support this model. Possible functions of a coordinated chloride and a nearby calcium complex are suggested. Five- or six-coordination and ferro- or antiferromagnetic spin couplings of the manganese centers are discussed. (Less)
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author
organization
publishing date
type
Contribution to journal
publication status
published
subject
in
Journal of the American Chemical Society
volume
119
issue
35
pages
8285 - 8292
publisher
The American Chemical Society
external identifiers
  • scopus:0030755238
ISSN
1520-5126
DOI
10.1021/ja9642323
language
English
LU publication?
yes
id
b6f5c76b-1f10-4256-834d-4dd15b66e740 (old id 126129)
date added to LUP
2007-07-04 13:51:34
date last changed
2017-01-01 08:04:06
@article{b6f5c76b-1f10-4256-834d-4dd15b66e740,
  abstract     = {Recently, water oxidation in photosystem II was proposed to involve direct abstraction of hydrogen atoms from water molecules terminally ligated to manganese ions in the oxygen-evolving complex by the oxidized tyrosine radical, TyrZ. This model is tested here by performing quantum chemical calculations. An empirically parametrized hybrid density functional method is used, and both monomeric and dimeric manganese model systems are studied. It is found that, by coordination to a manganese center, the first O-H bond strength of water is lowered from 113.4 to 84.3 kcal/mol. This O-H bond strength is only 2.8 kcal/mol stronger than that in tyrosine. Using an extended basis set, we find that this difference decreases still further. The second hydrogen abstraction energy is quite similar. Since thermoneutrality in the reaction (or a weak exothermicity) is a requirement for the hydrogen abstraction model, the present calculations support this model. Possible functions of a coordinated chloride and a nearby calcium complex are suggested. Five- or six-coordination and ferro- or antiferromagnetic spin couplings of the manganese centers are discussed.},
  author       = {Blomberg, Margareta R A and Siegbahn, Per E M and Styring, Stenbjörn and Babcock, Gerald T and Åkermark, Björn and Korall, Peter},
  issn         = {1520-5126},
  language     = {eng},
  number       = {35},
  pages        = {8285--8292},
  publisher    = {The American Chemical Society},
  series       = {Journal of the American Chemical Society},
  title        = {A Quantum Chemical Study of Hydrogen Abstraction from Manganese-Coordinated Water by a Tyrosyl Radical: A Model for Water Oxidation in Photosystem II},
  url          = {http://dx.doi.org/10.1021/ja9642323},
  volume       = {119},
  year         = {1997},
}