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The secondary photoinduced radical pair state P +• Q A -• 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 +• Q A -• electron spin-polarized spectra are presented for three different reconstituted reaction centers. The shape of the electron spin-polarized P +• Q A -• EPR spectrum strongly depends on the lifetime and magnetic properties of its precursor radical pair P +• Φ A -• , 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 +• Φ A -• radical pair was obtained. When the lifetime of P +• Φ A -• is longer than a few nanoseconds, the influence of the magnitude and sign of the exchange interaction J PΦ between P +• and Φ A -• 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 J PΦ ) -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 (J PΦ ) -0.7 ( 0.5 mT). The singlet and triplet rate constants (k S and k T ) for the intermediate radical pair P +• Φ A -• in the duroquinone-or 2-ethyl anthraquinone-reconstituted samples were (2 ( 1) × 10 7 s -1 and (2 ( 1) × 10 8 s -1 , respectively. The exchange interaction between P +• and Φ A -• is J PQ ) -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 Φ A -• is -0.12

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