Proton Isomers Rationalize the High- and Low-Spin Forms of the S2 State Intermediate in the Water-Oxidizing Reaction of Photosystem II

• Published in: The journal of physical chemistry letters 2019-09, Vol.10 (17), p.5226-5230
• Main Authors: Corry, Thomas A, O’Malley, Patrick J
• Format: Article
• Language: eng ; jpn
• ISSN: 1948-7185; 1948-7185
• DOI: 10.1021/acs.jpclett.9b01372

A new paradigm for the high- and low-spin forms of the S2 state of nature’s water-oxidizing complex in Photosystem II is found. Broken symmetry density functional theory calculations combined with Heisenberg–Dirac–van Vleck spin ladder calculations show that an open cubane form of the water-oxidizing complex changes from a low-spin, S = 1/2, to a high-spin, S = 5/2, form on protonation of the bridging O4 oxo. We show that such models are fully compatible with structural determinations of the S2 state by X-ray free-electron laser crystallography and extended X-ray absorption fine structure and provide a clear rationale for the effect of various treatments on the relative populations of each form observed experimentally in electron paramagnetic resonance studies.