Alteration of the Axial Met Ligand to Electron Acceptor A0 in Photosystem I: Effect on the Generation of P 700 ·+ A 1 ·− Radical Pairs as Studied by W-band Transient EPR

Abstract

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 P 700 ? 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 A agrees well with the 25.8 A distance for the P 700 ? 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 A in a ratio of 0.7-0.3, which agree well with the 25.8 A distance for the P 700 ? A 1A -radical