Interpretation of multifrequency transient EPR spectra of the P ⁺₇₀₀ A₀Q ⁻_K state in photosystem I complexes with a sequential correlated radical pair model: Wild type versus A₀ mutants

Abstract

Transient electron paramagnetic resonance (TR-EPR) spectra of the electron-hole pair state P ⁺₇₀₀ A₀Q ⁻_K in photosystem I are numerically calculated. Parameter variation concerns mainly the exchange integralJ of the precursor spin pair state P ⁺₇₀₀ A ⁻₀ Q_K and its lifetime τ. A prominent emissive feature in the high-field region (P ⁺₇₀₀ part) of the EPR spectrum turns out to be diminished with increasing lifetime τ of the precursor pair state in the case of positive exchange couplingJ>0 (ferromagnetic type). Correspondingly, the emissive feature becomes more pronounced with increasing lifetime τ in the case of negative exchange couplingJ<0 (antiferromagnetic type). These results can be used to interpret the changes in the pattern observed in TR-EPR spectra comparing wild-type and specific A₀ mutants. The central ligating amino acid residue to the A₀ chlorophyll cofactor is mutated from native methionine (M) to leucine (L) in either the PsaA or the PsaB branch. Changes are observed only for the A-side mutant: M688L(PsaA). They are consistent with the following parameters in the precursor pair P ⁺₇₀₀ A ⁻₀ :J≈0.5÷1.0 mT and τ=1.5÷2 ns (as compared to τ∼0.05 ns in the wild type).