Photosystem I (PS I) mutants from the cyanobacterium Synechocystis sp. PCC 6803 bearing point mutations to the axial ligands of A0A (M688NPsaA) and A0B (M668NPsaB) were studied by high-field W-band electron paramagnetic resonance (EPR) spectroscopy. It was found that the EPR observables of PS I from the M668NPsaB mutant were virtual identical to that of the wild type (WT), and are clearly distinct from the M688NPsaA mutant. In particular, the P700·+ decay kinetics in the M688NPsaA mutant is significantly slower than in the WT or the M668NPsaB mutant. The analysis of the out-of-phase electron-electron dipolar electron spin echo envelope modulation shows that in the M668NPsaB mutant, the estimated distance of 26.0 ± 0.3 Å agrees well with the 25.8 Å distance for the P700·+A1A·- radical pair measured in the X-ray crystal structure. In the M688NPsaA 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·+A1A·- radical pair and the 24.6 Å distance for the P700·+A1B·- 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.
ASJC Scopus subject areas
- Atomic and Molecular Physics, and Optics