Intra- and interband transfers in the B800-B850 antenna of Rhodospirillum molischianum:: Redfield theory modeling of polarized pump-probe kinetics

We use an exciton model for the B800-B850 LH2 light-harvesting antenna of Rhodospirillum molischianum to explain the absorption, excitation-wavelength-dependent pump-probe kinetics, and induced absorption anisotropy at 77 K reported previously (Wendling, M.; van Mourik, F.; van Stokkum, I. H. M.; Salverda, J. M.; Michel, H.; van Grondelle, R. Low-intensity pump-probe measurements on the B800 band of Rhodospirillum molischianum. Biophys. J. 2003, 84, 440). The nonlinear response was calculated using the density matrix equation, expanded up to the third-order with respect to the external field, with the Redfield relaxation operator in the exciton basis. The model allowed us to obtain a simultaneous and quantitative fit of the data while taking into account the excitonic interactions, the static disorder, and phonon-induced relaxation of populations and coherences in the one-exciton manifold (including a nonsecular coherence transfer and population-coherence coupling). The spectral density of the exciton-phonon coupling adjusted from the fit allowed us to determine the time scales and pathways of energy transfer. The 800 nm band consists of exciton states of the outer ring (B800 states) together with the upper Davydov component of the inner ring (B850* states). The 850 nm band contains the lower Davydov states of the inner ring (13850 states). The excitation dynamics includes migration around the outer ring with 1-3 ps time constant (the B800 --> B800 transfer), 1 ps transfer to the inner ring (B800 --> B850 transfer), and 600-800 fs transfer to higher states of the inner ring (B800 --> B850* transfer). The dynamics of B850* states is characterized by very fast 100-200 fs intraband (B850* --> B850*) equilibration, 60-200 A interband (B850* --> B850) relaxation, and a slower (>250 fs) back (B850* --> B800) transfer. The relative contribution of the B800 --> B850* --> B850 pathway is comparable (approximately equal) to the contribution of the direct B800 --> B850 transfer. Both B800 --> B800 and B800 --> B850* transfers are faster for the blue side of the 800 nm band, giving rise to slower kinetics when the excitation wavelength is tuned from 788 to 800 nm. From 800 to 809 nm kinetics become faster due to the increasing contribution of the directly excited B850* states and due to better coupling (faster relaxation) to higher states of the B850 band. The anisotropy decay in the 800 nm region exhibits a fast component (300-500 fs), reflecting the decay of the one-exciton coherences that is followed by slow picosecond depolarization due to B800 --> B800 migration. Both these processes can be directly viewed by means of the density matrix in the exciton and site representation.