Low-intensity pump-probe spectroscopy on the B800 to B850 transfer in the light harvesting 2 complex of Rhodobacter sphaeroides

The isolated LH2 (B800–850) complex of Rb. sphaeroides has been studied at 77 K using low-intensity one-colour pump-probe spectroscopy. Delta absorbance transients were measured at several wavelengths within the 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. More to the blue much faster rates are found which are ascribed to downhill energy-transfer among the B800 pigments. To our surprise, increasing the average intensity from 4 W/cm2 to higher excitation densities significantly slows the decay of B800∗. Since this effect is permanent we conclude that it is due to some form of photodamage. We propose that this observation explains the slower decay reported by others.