Primary Charge Separation Routes in the BChl:BPhe Heterodimer Reaction Centers of Rhodobacter sphaeroides^†

Energy transfer and the primary charge separation process are studied as a function of excitation wavelength in membrane-bound reaction centers of Rhodobacter sphaeroides in which the excitonically coupled bacteriochlorophyll homodimer is converted to a bacteriochlorophyll-bacteriopheophytin heterodimer, denoted D [Bylina, E. J., and Youvan, D. C. (1988) Proc. Natl. Acad. Sci. U.S.A. 85, 7226]. In the HM202L heterodimer reaction center, excitation of D using 880 nm excitation light results in a 43 ps decay of the excited heterodimer, D*. The decay of D* results for about 30% in the formation of the charge separated state D⁺Q_A^- and for about 70% in a decay directly to the ground state. Upon excitation of the monomeric bacteriochlorophylls using 798 nm excitation light, approximately 60% of the excitation energy is transferred downhill to D, forming D*. Clear evidence is obtained that the other 40% of the excitations results in the formation of D⁺Q_A^- via the pathway B_A* → B_A⁺H_A^- → D⁺H_A^- → D⁺Q_A^-. In the membrane-bound “reversed” heterodimer reaction center HL173L, the simplest interpretation of the transient absorption spectra following B excitation is that charge separation occurs solely via the slow D*-driven route. However, since a bleach at 812 nm is associated with the spectrum of D* in the HL173L reaction center, it cannot be excluded that a state including B_B is involved in the charge separation process in this complex.