Significance of hydrogen bonding networks in the proton-coupled electron transfer reactions of photosystem II from a quantum-mechanics perspective

The photosynthetic protein complex, photosystem II (PSII), conducts the light-driven water-splitting reaction with unrivaled efficiency. Proton-coupled electron transfer (PCET) reactions at the redox-active tyrosine residues are thought to play a critical role in the water-splitting chemistry. Addressing the fundamental question as to why the tyrosine residue, Y-Z, is kinetically competent in comparison to a symmetrically placed tyrosine residue, Y-D, is important for the elucidation of the mechanism of PCET in the water-splitting reaction of PSII. Here, using all-quantum-mechanical calculations we study PCET at the Y-Z and Y-D residues of PSII. We find that when Y-Z is in its protein matrix under physiological conditions, the HOMO of Y-Z constitutes the HOMO of the whole system. In contrast, the HOMO of Y-D is buried under the electronic states localized elsewhere in the protein matrix and PCET at Y-D requires the transfer of the phenolic proton, which elevates the HOMO of Y-D to become the HOMO of the whole system. This leads to the oxidation of Y-D, albeit on a slower timescale. Our study reveals that the key differences between the electronic structure of Y-Z and Y-D are primarily determined by the protonation state of the respective hydrogen-bonding partners, D1-His190 and D2-His189, or more generally by the H-bonding network of the protein matrix.