Electron spin polarization model applied to sequential electron transfer in iron-containing photosynthetic bacterial reaction centers with different quinones as O_A

Electron spin polarization develops on P⁺ [QFe²⁺]^- in iron-containing photosynthetic bacterial reaction centers (RC) of Rhodobacter sphaeroides. The spin-polarized electron paramagnetic resonance (EPR) spectra of the oxidized primary donor (P⁺) depend on τ_H, the lifetime of the radical pair P⁺H^- formed prior to P⁺[QFe²⁺]^-. The polarized EPR signal can be described by the sequential electron transfer polarization (SETP) model in which the chemically induced dynamic electron polarization (CIDEP) developed in P⁺H^- is projected onto the correlated radical pair polarization (CRPP) developed in P⁺[QFe²⁺]^-. Replacing the native ubiquinone-10 with various anthraquinones and naphthoquinones alters both the free energy and rate of electron transfer from H^- to QFe²⁺, which in turn modifies τ_H-. At long τ_H-, the polarized P⁺ EPR signal is dominated by the CIDEP component of SETP. At short τ_H-, the signal is dominated by the CRPP component, while at intermediate τ_H-'s the signal can only be described using the full SETP model. The ranges of τ_H, where polarization is dominated by interactions on the prior or observed radical pair are influenced by the EPR microwave frequency and RC isotopic composition. Experimental spectra of spin-polarized P⁺ from a series of Rb, sphaeroides RCs having τ_H-'s ranging from 0.33 to 25 ns are modeled with SETP. The model accounts for differences in the polarization line shape with deuteration of the RCs or increase in the EPR microwave frequency.