Alteration of the Axial Met Ligand to Electron Acceptor A(0) in Photosystem I: Effect on the Generation of P (700) (center dot+) A (1) (center dot-) Radical Pairs as Studied by W-band Transient EPR

摘要

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) (center dot+) 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 +/- A 0.3 agrees well with the 25.8 distance for the P (700) (center dot+) A (1A) (center dot-) radical pair measured in the X-ray crystal structure. In the M688N(PsaA) mutant, two populations are found with estimated distances of 26.0 +/- A 0.3 and 25.0 +/- A 0.3 in a ratio of 0.7-0.3, which agree well with the 25.8 distance for the P (700) (center dot+) A (1A) (center dot-) radical pair and the 24.6 distance for the P (700) (center dot+) A (1B) (center dot-) 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.