Energetics of the S 2 State Spin Isomers of the Oxygen - Evolving 2 Complex of Photosystem II

6 ABSTRACT: The S2 redox intermediate of the oxygen-evolving 7 complex in photosystem II is present as two spin isomers. The S = 8 1/2 isomer gives rise to a multiline electron paramagnetic resonance 9 (EPR) signal at g = 2.0, whereas the S = 5/2 isomer exhibits a broad 10 EPR signal at g = 4.1. The electronic structures of these isomers are 11 known, but their role in the catalytic cycle of water oxidation remains 12 unclear. We show that formation of the S = 1/2 state from the S = 5/2 13 state is exergonic at temperatures above 160 K. However, the S = 1/2 14 isomer decays to S1 more slowly than the S = 5/2 isomer. These 15 differences support the hypotheses that the S3 state is formed via the S2 16 state S = 5/2 isomer and that the stabilized S2 state S = 1/2 isomer plays 17 a role in minimizing S2QA − decay under light-limiting conditions.