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