B3LYP studies of the formation of neutral tyrosyl radical Y-Z(center dot) and regeneration of neutral tyrosine Y-Z in PSII

Tyrosine Y161 (Y-Z) plays an important role in the charge separation of pho tosystem II (PSII) by reducing photooxidized P680(+) and thus forming a neu tral tyrosyl radical. The neutral tyrosyl radical then abstracts an electro n and a proton from the manganese cluster (or other proton source) to regen erate neutral tyrosine. The production of the neutral tyrosyl radical is cl osely related to the oxidation of tyrosine by P680(+) and deprotonation by a neighbouring histidine. The regeneration of neutral tyrosine is more comp licated, involving the manganese cluster and other protein bases. Hybrid de nsity functional calculations (B3LYP) show that the electron transfer betwe en tyrosine and P680(+) is coupled to the proton transfer between tyrosine and histidine. The proton is transferred from tyrosine to histidine spontan eously as tyrosine is oxidized, forming Y-z(.)--H+-His state. The theoretic al results of the formation of the neutral tyrosyl radical agrees with rece nt experimental and previous computational results. Neutral tyrosine is dif ficult to regenerate directly via abstraction of a hydrogen atom from water . The possible proton transfer from histidine cation directly to water or w ith glutamic acid assitance are both endothermic. In both cases, the proton lies between tyrosine and histidine and shifts back to tyrosine spontaneou sly when the tyrosyl radical is reduced. This result is consistent with Jun ge's electron transfer model. However, when glutamate anion is present, rat her than glutamic acid, the proton transfers spontaneously via histidine to glutamate as tyrosine is oxidized, forming Y-z(.)--His--Glu stale. (C) 200 1 John Wiley & Sons, Inc.