B3LYP studies of the formation of neutral tyrosyl radical YZ• and regeneration of neutral tyrosine YZ in PSII

Tyrosine Y161 (Y-Z) plays an important role in the charge separation of photosystem II (PSII) by reducing photooxidized P680(+) and thus forming a neutral tyrosyl radical. The neutral tyrosyl radical then abstracts an electron and a proton from the manganese cluster (or other proton source) to regenerate neutral tyrosine. The production of the neutral tyrosyl radical is closely related to the oxidation of tyrosine by P680(+) and deprotonation by a neighbouring histidine. The regeneration of neutral tyrosine is more complicated, involving the manganese cluster and other protein bases. Hybrid density functional calculations (B3LYP) show that the electron transfer between tyrosine and P680(+) is coupled to the proton transfer between tyrosine and histidine. The proton is transferred from tyrosine to histidine spontaneously as tyrosine is oxidized, forming Y-z(.)--H+-His state. The theoretical results of the formation of the neutral tyrosyl radical agrees with recent experimental and previous computational results. Neutral tyrosine is difficult to regenerate directly via abstraction of a hydrogen atom from water. The possible proton transfer from histidine cation directly to water or with glutamic acid assitance are both endothermic. In both cases, the proton lies between tyrosine and histidine and shifts back to tyrosine spontaneously when the tyrosyl radical is reduced. This result is consistent with Junge's electron transfer model. However, when glutamate anion is present, rather than glutamic acid, the proton transfers spontaneously via histidine to glutamate as tyrosine is oxidized, forming Y-z(.)--His--Glu stale. (C) 2001 John Wiley & Sons, Inc.