Primary Charge Separation Routes in the BChl:BPhe Heterodimer Reaction Centers of <i>Rhodobacter sphaeroides</i><sup>†</sup>

Energy transfer and the primary charge separation process are studied as a function of excitation wavelength in membrane-bound reaction centers of <i>Rhodobacter</i> <i>sphaeroides</i> in which the excitonically coupled bacteriochlorophyll homodimer is converted to a bacteriochlorophyll-bacteriopheophytin heterodimer, denoted D [Bylina, E. J., and Youvan, D. C. (1988) <i>Proc. Natl. Acad. Sci. U.S.A.</i> <i>85</i>, 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<sup>+</sup>Q<sub>A</sub><sup>-</sup> 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<sup>+</sup>Q<sub>A</sub><sup>-</sup> via the pathway B<sub>A</sub>* → B<sub>A</sub><sup>+</sup>H<sub>A</sub><sup>-</sup> → D<sup>+</sup>H<sub>A</sub><sup>-</sup> → D<sup>+</sup>Q<sub>A</sub><sup>-</sup>. 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<sub>B</sub> is involved in the charge separation process in this complex.