LOW-INTENSITY PUMP-PROBE SPECTROSCOPY ON THE B800 TO B850 TRANSFER INTHE LIGHT-HARVESTING 2 COMPLEX OF RHODOBACTER-SPHAEROIDES

The isolated LH2 (B800-850) complex of Rb. sphaeroides has been studie d at 77 K using low-intensity one-colour pump-probe spectroscopy. Delt a absorbance transients were measured at several wavelengths within th e B800 band. In the red part of the band, the excited B800 population decays mono-exponentially with a lifetime of about 1.2 +/- 0.1 ps. Mor e to the blue much faster rates are found which are ascribed to downhi ll energy-transfer among the B800 pigments. To our surprise, increasin g the average intensity from 4 W/cm(2) to higher excitation densities significantly slows the decay of Bg800. Since this effect is permanen t we conclude that it is due to some form of photodamage. We propose t hat this observation explains the slower decay reported by others.