Ammonia as a substrate-water analogue in photosynthetic water oxidation: Influence on activation barrier of the O-formation step.

Information on binding and rearrangement of pivotal water molecules could support understanding of light-driven water oxidation at the catalytic MnCaO cluster of photosystem II (PSII). To address this point, the binding of ammonia (NH)-a possible substrate-water analogue-has been investigated and discussed in the context of putative reaction mechanisms. By time-resolved detection of O formation after light-flash excitation, we discriminate three NH/NH binding sites jointly characterized by a K value around 25 mM (of NH), but differing in their influence on the O-formation step. At 100 mM NHCl (pH 7.5), we observe (1) a PSII fraction with complete inhibition of O-formation, (2) fast O-formation with a time constant of 1.7 ms at 20 °C (Fast-PSII), and (3) slow O-formation with a time constant of 36 ms at 20 °C (Slow-PSII). For the Fast-PSII, we determine an activation enthalpy of 223 ± 11 meV. Activation enthalpy and entropy of the Fast-PSII are essentially identical to the corresponding figures in the absence NH/NH binding. For the Slow-PSII, the activation enthalpy is 323 ± 11 meV and thus significantly increased, whereas the activation entropy remains essentially unchanged. We conclude: (1) The fully-inhibitory binding site could relate to bound NH replacing one of the two substrate-water molecules. (2) The Fast-PSII may relate to NH/NH binding in the S-state of PSII followed by unbinding before onset of the OO bond formation step, but also more intricate mechanisms are not excluded. (3) In the Slow-PSII, NH/NH binding increases the energetic barrier of the OO bond formation step significantly.