Energy transfer in LH2 of Rhodospirillum molischianum, 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 8 70 nm at room temperature. The observed energy transfer time of similar to1 .0 ps from B800 to B850 at room temperature is longer than the correspondin g rates in Rhodopseudomonas acidophila and Rhodobacter sphaeroides. We obse rved 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 8 00 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 co nfiguration interaction exciton method (CIEM) [Linnanto et al. J. Phys. Che m. 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 t ransfer from the B800 ring to the B850 ring in the complex. The 1,2E(2) sta tes at 824 nm predicted by the calculations serve as an additional energy t ransfer channel as indicated by the observed fast transfer rate at 830 mn. excitation.