On the action of hydroxylamine, hydrazine and their derivatives on the water-oxidizing complex

Photosynthetic water oxidation proceeds by a four-step sequence of one-electron oxidations which is formally described by the transitions S0 → S1, S1 → S2, S2 → S3, S3 → (S4) → S0. State S1 is most stable in the dark. Oxygen is released during S3 → (S4) → S0. Hydroxylamine and hydrazine interact with S1. They cause a two-digit shift in the oxidation sequence as observed from the dark equilibrium, i.e. from S1 → S2 : S2 → S3 : S3 → (S4) → S0 : S0 → S1 :... in the absence of the agents, to S1 (*) → S0 : S0 → S1 : S1 → S2 : S2 → S3 :... in the presence of hydroxylamine or hydrazine.We measured the concentration dependence of this two-digit shift via the pattern of proton release which is associated with water oxidation. At saturating concentrations hydroxylamine and hydrazine shift the proton-release pattern from OH(+)(S1 → S2) : 1H(+)(S2 → S3) : 2H(S3 → S0) : 1H(+)(S0 → S1) :... to 2H(+)(S1 (*) → S0) : 1H(+)(S0 → S1) : OH(+)(S1 → S2) : 1H(+)(S2 → S3) : 2H(+)(S3 → S0) :... The 2H(+) were released upon the first excitation with a half-rise time of 3.1 ms, both with hydroxylamine and withydrazine. The concentration dependence of the shift was rather steep with an apparent Hill coefficient at half saturation of 2.43 with hydroxylamien (Förster and Junge (1985) FEBS Lett. 186, 53-57) and 1.48 with hydrazine. The concentration dependence could be explained by cooperative binding of n≥3 molecules of hydroxylamine and of n≥2 molecules of hydrazine, respectively. Tentatively, we explain the interaction of hydroxylamine and hydrazine with the water-oxidizing complex (WOC) as follows: Two bridging ligands, possible Cl(-) or OH(-), which normally connect two Mn nuclei, can be substituted by either 4 molecules of hydroxylamine or 2 molecules of hydrazine when the WOC resides in state S1.