Evidence that histidine forms a coordination bond to the A_0A and A_0B chlorophylls and a second H-bond to the A_1A and A_1B phylloquinones in M688H_PsaA and M668H_PsaB variants of Synechocystis sp. PCC 6803

The axial ligands of the acceptor chlorophylls, A_0A and A _0B, in Photosystem I are the Met sulfur atoms of M688_PsaA and M668_PsaB. To determine the role of the Met, His variants were generated in Synechocystis sp. PCC 6803. Molecular dynamics simulations on M688H_PsaA show that there exist low energy conformations with the His coordinated to A_0A and possibly H-bonded to A_1A. Transient EPR studies on M688H_PsaA indicate a more symmetrical electron spin distribution in the A_1A phyllosemiquinone ring consistent with the presence of an H-bond to the C1 carbonyl. Ultrafast optical studies on the variants show that the 150 fs charge separation between P ₇₀₀ and A₀ remains unaffected. Studies on the ns timescale show that 57% of the electrons are transferred from A_0A^- to A_1A in M688H_PsaA and 48% from A_0B ^- to A_1B in M668H_PsaB; the remainder recombine with P₇₀₀⁺ with 1/e times of 25 ns and 37 ns, respectively. Those electrons that reach A_1A and A_1B in the branch carrying the mutation are not transferred to F_X, but recombine with P₇₀₀⁺ with 1/e times of ~ 15 μs and ~ 5 μs, respectively. Hence, the His is coordinated to A₀ in all populations, but in a second population, the His may be additionally H-bonded to A₁. Electron transfer from A₀ to A₁ occurs only in the latter, but the higher redox potentials of A₀ and A ₁ as a result of the stronger coordination bond to A₀ and the proposed second H-bond to A₁ preclude electron transfer to the Fe/S clusters.