SUBPICOSECOND EXCITED-STATE RELAXATION OF THE ACCESSORY BACTERIOCHLOROPHYLLS IN NATIVE AND MODIFIED REACTION CENTERS OF RB SPHAEROIDES R26

Energy transfer within native and borohydride treated (chemically modi fied) reaction centers (RCs) isolated from Rhodobacter sphaeroides R26 was investigated, in particular from the excited accessory bacterioch lorophylls (B) to the primary donor (P). The decay kinetics of chemic ally modified and native RCs show some similarities as well as distinc t differences. In reduced RCs, B decays with a time constant of 100 f s, 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 sepa ration. In native RCs, however, an induced absorption is observed with a maximum at a delay of 500 fs, which is absent in chemically modifie d RCs. The initial bleaching in modified and in native RCs is characte rized by an anisotropy of 0.4. After the excitation is transferred fro m B to P, the anisotropy in modified RCs is decreased to a value of a bout 0.2. In native RCs the time-resolved anisotropy varies and depend s strongly on the wavelength of detection. These observations are anal yzed and discussed in terms of the individual contributions of B-A and B-B 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 similar to 400 fs.