Hydration of the oxygen-evolving complex of photosystem II probed in the dark-stable S₁ state using proton NMR dispersion profiles

The hydration of the oxygen-evolving complex (OEC) was characterized in the dark stable S₁ state of photosystem II using water R ₁(ω) NMR dispersion (NMRD) profiles. The R₁(ω) NMRD profiles were recorded over a frequency range from 0.01 MHz to 40 MHz for both intact and Mn-depleted photosystem II core complexes from Thermosynechococcus vulcanus (T. vulcanus). The intact-minus-(Mn)-depleted difference NMRD profiles show a characteristic dispersion from approximately 0.03 MHz to 1 MHz, which is interpreted on the basis of the Solomon-Bloembergen- Morgan (SBM) and the slow motion theories as being due to a paramagnetic enhanced relaxation (PRE) of water protons. Both theories are qualitatively consistent with the S_T = 1, g = 4.9 paramagnetic state previously described for the S₁ state of the OEC; however, an alternative explanation involving the loss of a separate class of long-lived internal waters due to the Mn-depletion procedure can presently not be ruled out. Using a point-dipole approximation the PRE-NMRD effect can be described as being caused by 1-2 water molecules that are located about 10 Å away from the spin center of the Mn₄CaO₅ cluster in the OEC. The application of the SBM theory to the dispersion observed for PSII in the S₁ state is questionable, because the parameters extracted do not fulfil the presupposed perturbation criterion. In contrast, the slow motion theory gives a consistent picture indicating that the water molecules are in fast chemical exchange with the bulk (τ_w < 1 μs). The modulation of the zero-field splitting (ZFS) interaction suggests a (restricted) reorientation/structural equilibrium of the Mn₄CaO₅ cluster with a characteristic time constant of τ_ZFS = 0.6-0.9 μs.