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

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journal contribution
posted on 19.05.1999 by Marion E. van Brederode, Ivo H. M. van Stokkum, Evaldas Katilius, Frank van Mourik, Michael R. Jones, Rienk van Grondelle
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+QA- 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+QA- via the pathway BA* → BA+HA- → D+HA- → D+QA-. 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 BB is involved in the charge separation process in this complex.