Theoretical study of the EPR spectrum of the S sub(3)TyrZ super( times ) metalloradical intermediate state of the O sub(2)-evolving complex of photosystem II
The intermediates trapped during the transitions between the consecutive S-states of the oxygen-evolving complex (OEC) of photosystem II (PSII) contain the free radical TyrZ super( times ) interacting magnetically with the Mn-cluster (Mn sub(4)Ca). In this paper, we present a theoretical study of th...
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Veröffentlicht in: | Photosynthesis research 2016-12, Vol.130 (1-3), p.417-426 |
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Sprache: | eng |
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Zusammenfassung: | The intermediates trapped during the transitions between the consecutive S-states of the oxygen-evolving complex (OEC) of photosystem II (PSII) contain the free radical TyrZ super( times ) interacting magnetically with the Mn-cluster (Mn sub(4)Ca). In this paper, we present a theoretical study of the EPR spectrum of the S sub(3)TyrZ super( times ) metalloradical intermediate state, which has been recently detected in MeOH-containing PSII preparations. For this analysis, we use two different approximations: the first, simpler one, is the point-dipole approach, where the two interacting spins are the S = 1/2 of TyrZ super( times ) and the ground spin state of S = 3 of the OEC being in the S sub(3) state. The second approximation is based on previous proposals indicating that the ground spin state (S sub(G) = 3) of the S sub(3) state arises from an antiferromagnetic exchange coupling between the S = 9/2 of the Mn(IV) sub(3)CaO sub(4) and the S = 3/2 of the external Mn(IV) of the OEC. Under the above assumption, the second approximation involves three interacting spins, denoted S sub(A)(Mn(IV) sub(3)Ca) = 9/2, S sub(B)(Mn(IV)) = 3/2 and S sub(C)(TyrZ super( times )) = 1/2. Accordingly, the tyrosine radical is exposed to dipolar interactions with both fragments of the OEC, while an antiferromagnetic exchange coupling within the "3 + 1" structural motif of the OEC is also considered. By application of the first-point-dipole approach, the inter-spin distance that simulates the experimental spectrum is not consistent with the theoretical models that were recently reported for the OEC in the S sub(3) state. Instead, the recent models are consistent with the results of the analysis that is performed by using the second, more detailed, approach. |
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ISSN: | 0166-8595 1573-5079 |
DOI: | 10.1007/s11120-016-0274-6 |