biostudies-literaturede Lichtenberg CU.S. Department of EnergyVetenskapsrådetÅForskOklahoma Humanities Council12763-12775https://www.ebi.ac.uk/biostudies/studies/S-EPMC8494045biostudies-literatureUnknown12(38)The molecular oxygen we breathe is produced from water-derived oxygen species bound to the Mn₄CaO₅ cluster in photosystem II (PSII). Present research points to the central oxo-bridge O5 as the 'slow exchanging substrate water (W_s)', while, in the S₂ state, the terminal water ligands W2 and W3 are both discussed as the 'fast exchanging substrate water (W_f)'. A critical point for the assignment of W_f is whether or not its exchange with bulk water is limited by barriers in the channels leading to the Mn₄CaO₅ cluster. In this study, we measured the rates of H₂ ¹⁶O/H₂ ¹⁸O substrate water exchange in the S₂ and S₃ states of PSII core complexes Chemical scienceThe exchange of the fast substrate water in the S₂ state of photosystem II is limited by diffusion of bulk water through channels - implications for the water oxidation mechanism.PMC84940452016-051832020-03809DE-SC0005291de Lichtenberg CChernev PMessinger JDebus RJKim CJfalseThe exchange of the fast substrate water in the S₂ state of photosystem II is limited by diffusion of bulk water through channels - implications for the water oxidation mechanism.The molecular oxygen we breathe is produced from water-derived oxygen species bound to the Mn₄CaO₅ cluster in photosystem II (PSII). Present research points to the central oxo-bridge O5 as the 'slow exchanging substrate water (W_s)', while, in the S₂ state, the terminal water ligands W2 and W3 are both discussed as the 'fast exchanging substrate water (W_f)'. A critical point for the assignment of W_f is whether or not its exchange with bulk water is limited by barriers in the channels leading to the Mn₄CaO₅ cluster. In this study, we measured the rates of H₂ ¹⁶O/H₂ ¹⁸O substrate water exchange in the S₂ and S₃ states of PSII core complexes 2021-01-01T00:00:00Z2021 Oct2026-05-08T20:22:14.874Z2025-02-19T00:27:53.599ZS-EPMC84940453470356310.1039/d1sc02265b