Alteration of the axial met ligand to electron acceptor A₀ in photosystem I: Effect on the generation of P₇₀₀^·+ A₁^·- radical pairs as studied by W-band transient EPR

Photosystem I (PS I) mutants from the cyanobacterium Synechocystis sp. PCC 6803 bearing point mutations to the axial ligands of A_0A (M688N_PsaA) and A_0B (M668N_PsaB) were studied by high-field W-band electron paramagnetic resonance (EPR) spectroscopy. It was found that the EPR observables of PS I from the M668N_PsaB mutant were virtual identical to that of the wild type (WT), and are clearly distinct from the M688N_PsaA mutant. In particular, the P₇₀₀^·+ decay kinetics in the M688N_PsaA mutant is significantly slower than in the WT or the M668N_PsaB mutant. The analysis of the out-of-phase electron-electron dipolar electron spin echo envelope modulation shows that in the M668N_PsaB mutant, the estimated distance of 26.0 ± 0.3 Å agrees well with the 25.8 Å distance for the P₇₀₀^·+A_1A^·- radical pair measured in the X-ray crystal structure. In the M688N_PsaA mutant, two populations are found with estimated distances of 26.0 ± 0.3 and 25.0 ± 0.3 Å in a ratio of 0.7-0.3, which agree well with the 25.8 Å distance for the P₇₀₀^·+A_1A^·- radical pair and the 24.6 Å distance for the P₇₀₀^·+A_1B^·- radical pair measured in the X-ray crystal structure. The data confirm that under the experimental conditions employed in this work, which involve dark-adapted samples without the pre-reduction of the iron-sulfur clusters, electron transport in cyanobacterial PS I is asymmetrical at 100 K, with the majority of electron transfer taking place through the A-branch of cofactors.