High-frequency, high-field EPR at 330 GHz was used to study the photo-oxidized primary donor of photosystem I (P700+.) in wild-type and mutant forms of photosystem I in the green alga Chlamydomonas reinhardtii. The main focus was the substitution of the axial ligand of the chlorophyll a and chlorophyll a′ molecules that form the P700 heterodimer. Specifically, we examined PsaA-H676Q, in which the histidine axial ligand of the A-side chlorophyll a′ (PA) is replaced with glutamine, and PsaB-H656Q, with a similar replacement of the axial ligand of the B-side chlorophyll a (PB), as well as the double mutant (PsaA-H676Q/PsaB-H656Q), in which both axial ligands were replaced. We also examined the PsaA-T739A mutant, which replaces a threonine residue hydrogen-bonded to the 131-keto group of PA with an alanine residue. The principal g-tensor components of the P700 +. radical determined in these mutants and in wild-type photosystem I were compared with each other, with the monomeric chlorophyll cation radical (Chlz+.) in photosystem II, and with recent theoretical calculations for different model structures of the chlorophyll a+ cation radical. In mutants with a modified PB axial ligand, the gzz component of P700+. was shifted down by up to 2 × 10-4, while mutations near PA had no significant effect. We discuss the shift of the gzz component in terms of a model with a highly asymmetric distribution of unpaired electron spin in the P700+. radical cation, mostly localized on PB, and a deviation of the PB chlorophyll structure from planarity due to the axial ligand.
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