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(+.)Q(A)(-.) in Zn-substitut ed 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 d ifferent reconstituted reaction centers. The shape of the electron spin-pol arized P(+.)Q(A)(-.) EPR spectrum strongly depends on the lifetime and magn etic properties of its precursor radical pair P(+.)Phi(A)(-.) whose lifetim e was manipulated by replacing the native ubiquinone-10 (UQ-10) with duroqu inone (DQ) or 2-ethyl anthraquinone (AQ). From spectral simulations incorpo rating the transfer of spin-correlation between the two radical pairs, info rmation about the magnetic interactions and dynamics of the intermediate pr imary P(+.)Phi(A)(-.) radical pair was obtained. When the lifetime of P(+.) Phi(A)(-.) is longer than a few nanoseconds, the influence of the magnitude and sign of the exchange interaction J(P Phi) between P+. and Phi(A)(-.) o n the shape of the observed ESP spectrum is significant. This effect is eve n more pronounced at the relatively high D-band microwave frequency, facili tating accurate determination of J(P Phi) = -0.9 +/- 0.1 mT, for both DQ an d AQ-reconstituted RCs, a value not significantly different from that deter mined here for the UQ-10 reconstituted sample (J(P Phi) = -0.7 +/- 0.5 mT). The singlet and triplet rate constants (k(S) and k(T)) for the intermediat e radical pair P(+.)Phi(A)(-.) in the duroquinone- or 2-ethyl anthraquinone -reconstituted samples were (2 +/- 1) x 10(7) s(-1) and (2 +/- 1) x 10(8) s (-1), respectively. The exchange interaction between P+. and Phi(A)(-.) is J(PQ) = -0.5 +/- 0.3 mu T, -0.2 +/- 0.1 mu T, and -0.5 +/- 0.2 mu T for UQ- 10, AQ, and DQ, respectively. X-band ESEEM experiments showed that the dipo lar interaction between P+. and Phi(A)(-.) is -0.12 +/- 0.01 mT, independen t of the quinone, corresponding to a center-to-center distance of 28.5 +/- 0.8 Angstrom.