A High-Field EPR Study of P₇₀₀^+. in Wild-Type and Mutant Photosystem I from Chlamydomonas reinhardtii

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′ (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 13¹-keto group of P_A 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 P₇₀₀^+. was shifted down by up to 2 × 10^-4, while mutations near P_A had no significant effect. We discuss the shift of the g_zz component in terms of a model with a highly asymmetric distribution of unpaired electron spin in the P₇₀₀^+. radical cation, mostly localized on P_B, and a deviation of the P_B chlorophyll structure from planarity due to the axial ligand.