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 P-700 heterodimer. Specifically, we examined PsaA-H676Q, in which the histidine axial ligand of the A-side chlorophyll a' (P-A) is replaced with glutamine, and PsaB-H656Q, with a similar replacement of the axial ligand of the B-side chlorophyll a (P-B), 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 13(1)-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 (Chl(z)(+.)) in photosystem II, and with recent theoretical calculations for different model structures of the chlorophyll a(+) cation radical. In mutants with a modified P-B axial ligand, the g(zz) component of P-700(+.) was shifted down by up to 2 x 10(-4), while mutations near P-A 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 P-700(+.) radical cation, mostly localized on PB, and a deviation of the PB chlorophyll structure from planarity due to the axial ligand.