Radical Pair Dynamics and Interactions in Quinone-Reconstituted Photosynthetic Reaction Centers of Rb. Sphaeroides R26: A Multifrequency Magnetic Resonance Study

The secondary photoinduced radical pair state P+• QA-• in Zn-substituted and quinone-reconstituted photosynthetic reaction centers of Rhodobacter sphaeroides R26 is investigated with transient EPR spectroscopy at X- (9.3 GHz), Q- (34.7 GHz), and D-band (130.0 GHz) microwave frequencies. Novel D-band P+• QA-• electron spin-polarized spectra are presented for three different reconstituted reaction centers. The shape of the electron spin-polarized P+• QA-• EPR spectrum strongly depends on the lifetime and magnetic properties of its precursor radical pair P+• ΦA-•, whose lifetime was manipulated by replacing the native ubiquinone-10 (UQ-10) with duroquinone (DQ) or 2-ethyl anthraquinone (AQ). From spectral simulations incorporating the transfer of spin-correlation between the two radical pairs, information about the magnetic interactions and dynamics of the intermediate primary P+• ΦA-• radical pair was obtained. When the lifetime of P+• ΦA-• is longer than a few nanoseconds, the influence of the magnitude an...