Two-dimensional HYSCORE characterization of a histidine axial ligand ligation to Chl_3A in M668H_Psab genetic variant of Photosystem I

Photosystem I (PSI) has been shown to contain a novel six-core chlorophyll-a moiety that is highly coupled and allows for an efficient generation and stabilization of a charge-separated state. The primary acceptor, A₀, has become particularly intriguing to assign to specific chlorophylls as the ultrafast processes and redox properties are not fully understood. This work builds on the recent studies on wild-type and a M688H_PsaA A-side mutation that probes the reduced A₀^•− state. While the M688H_PsaA A-side mutation has significant electron transfer and physiological effects, the M668H_Psab mutant lacks much of these deviations from the wild type. In this study, we have applied 2D HYSCORE spectroscopy in conjunction with molecular dynamics simulations and density functional theory calculations to study of the M668H_Psab variant. Curiously, the M668H_Psab mutant shows some significant hyperfine parameters differences from both the A-side wild type and M688H_PsaA. Analysis suggests that the His imidazole is the axial ligand to the central Mg²⁺ ion in Chl_3A in the M668H_Psab mutant. The electron density over the Chl₂/Chl₃ dimer results in a small but notable shift in the delocalization. This, coupled with the electron-withdrawing properties of the ligand, however, does not significantly affect the inhibition of forward electron transfer in the His-ligated conformation, which contrasts with the changes noted previously in the M688H_PsaA variant.