Time-resolved absorption difference spectroscopy of the LH-1 antenna of Rhodopseudomonas viridis

The LH-1 antenna of the purple bacterium Rhodopseudomonas viridis has been investigated using femtosecond pump-probe spectroscopy. We find that qualitatively the relaxation precesses are comparable to those in Rhodobacter sphaeroides and Rhodospirillum rubrum. Both the spectral relaxation and depolarization take place in approximately 150 fs, which suggests that the excitations in the LH-1 ring are delocalized over two to three pigments. Strong oscillations of the pump-probe signal are observed at all temperatures. We conclude that these must be of a vibrational origin and could be related to the observed fine structure seen in the low-temperature absorption spectrum of Rps. viridis membranes. The main frequencies of the oscillations are 65 and 105 cm(-1), and the damping time is similar to 700 fs. From the amplitude of the oscillations and the slow damping time, we conclude that excitations are transferred in a vibrationally coherent manner from site to site.