Subpicosecond Excited-State Relaxation of the Accessory Bacteriochlorophylls in Native and Modified Reaction Centers of Rb. sphaeroides R26

Abstract

Energy transfer within native and borohydride treated (chemically modified) reaction centers (RCs) isolated from Rhodobacter sphaeroides R26 was investigated, in particular from the excited accessory bacteriochlorophylls (B*) to the primary donor (P). The decay kinetics of chemically modified and native RCs show some similarities as well as distinct differences. In reduced RCs, B* decays with a time constant of 100 fs, both in native and in modified RCs, followed by a partial recovery of the bleaching with a time constant of about 3 ps due to charge separation. In native RCs, however, an induced absorption is observed with a maximum at a delay of 500 fs, which is absent in chemically modified RCs. The initial bleaching in modified and in native RCs is characterized by an anisotropy of 0.4. After the excitation is transferred from B* to P, the anisotropy in modified RCs is decreased to a value of about 0.2. In native RCs the time-resolved anisotropy varies and depends strongly on the wavelength of detection. These observations are analyzed and discussed in terms of the individual contributions of BA and BB to the absorbance kinetics at different wavelengths. In oxidized RCs, in addition to a fast relaxation of about 100 fs in the decay of B*, we observed, a decay component with a time constant of ∼400 fs.