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 Rho dobacter sphaeroides in which the excitonically coupled bacteriochlorophyll homodimer is converted to a bacteriochlorophyll-bacteriopheophytin heterod imer, 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, excitatio n of D using 880 nm excitation light results in a 43 PS decay of the excite d heterodimer, D*. The decay of D* results for about 30% in the formation o f the charge separated state D(+)Q(A)(-) and for about 70% in a decay direc tly to the ground state. Upon excitation of the monomeric bacteriochlorophy lls using 798 nm excitation light, approximately 60% of the excitation ener gy is transferred downhill to D, forming D*. Clear evidence is obtained tha t the other 40% of the excitations results in the formation of D(+)Q(A)(-) via the pathway B-A* --> BA+HA- --> D+HA- 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 ble ach 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.