A quantum mechanics/molecular mechanics study of the tyrosine residue, TyrD, of Photosystem II

QM/MM calculations have been used to monitor the oxidation of the D2-Tyr160, Tyr_D, residue involved in redox reactions in Photosystem II. The results indicate that in the reduced form the residue is involved in hydrogen bond donation via its phenolic head group to the τ-nitrogen of the neighboring D2-His189 residue. Oxidation to form the radical is accompanied by spontaneous transfer of the phenolic hydrogen to the τ-nitrogen of D2-His189 leading to the formation of a tyrosyl-imidazolium ion complex. Deprotonation of the imidazolium ion leads to the formation of a tyrosyl-imidazole neutral hydrogen-bonded complex. Comparison of calculated and experimental hyperfine coupling tensors and g-tensors suggests that the neutral imidazole complex is formed at physiological temperatures while the imidazolium complex may be stabilized at cryogenic temperatures.