Energy Transfer in LH2 of Rhodospirillum M olischianum, Studied by Subpicosecond Spectroscopy and Configuration Interaction Exciton Calculations

Two color transient absorption measurements were performed on a LH2 complex from Rhodospirillum molischianum by using several excitation wavelengths (790, 800, 810, and 830 nm) and probing in the spectral region from 790 to 870 nm at room temperature. The observed energy transfer time of ∼1.0 ps from B800 to B850 at room temperature is longer than the corresponding rates in Rhodopseudomonas acidophila and Rhodobacter sphaeroides. We observed variations (0.9−1.2 ps) of B800−850 energy transfer times at different B800 excitation wavelengths, the fastest time (0.9 ps) was obtained with 800 nm excitation. At 830 nm excitation the energy transfer to the B850 ring takes place within 0.5 ps. The measured kinetics, as well as steady-state absorption and CD spectra, are consistent with those calculated with the configuration interaction exciton method (CIEM) [Linnanto et al. J. Phys. Chem. B 1999, 103, 8739]. Fully excitonic simulation of the CD spectrum of the LH2 of Rs. molischianum is presented for the first time. The calculations put the E 3 exciton states of B850 near the narrow excitonic B800 manifold and according to our model, these states provide the main route of energy transfer from the B800 ring to the B850 ring in the complex. The 1,2E2 states at 824 nm predicted by the calculations serve as an additional energy transfer channel as indicated by the observed fast transfer rate at 830 nm excitation.