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

Abstract

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 and sign of the exchange interaction JPΦ between P+• and ΦA-• on the shape of the observed ESP spectrum is significant. This effect is even more pronounced at the relatively high D-band microwave frequency, facilitating accurate determination of JPΦ = −0.9 ± 0.1 mT, for both DQ and AQ-reconstituted RCs, a value not significantly different from that determined here for the UQ-10 reconstituted sample (JPΦ = −0.7 ± 0.5 mT). The singlet and triplet rate constants (kS and kT) for the intermediate radical pair P+• ΦA-• in the duroquinone- or 2-ethyl anthraquinone-reconstituted samples were (2 ± 1) × 107 s-1 and (2 ± 1) × 108 s-1, respectively. The exchange interaction between P+• and ΦA-• is JPQ = −0.5 ± 0.3 μT, −0.2 ± 0.1 μT, and −0.5 ± 0.2 μT for UQ-10, AQ, and DQ, respectively. X-band ESEEM experiments showed that the dipolar interaction between P+• and ΦA-• is −0.12 ± 0.01 mT, independent of the quinone, corresponding to a center-to-center distance of 28.5 ± 0.8 Å.