Kinetic model of primary energy transfer and trapping in photosynthetic membranes.

The picosecond time-domain incoherent singlet excitation transfer and trapping kinetics in core antenna of photosynthetic bacteria are studied in case of low excitation intensities by numerical integration of the appropriate master equation in a wide temperature range of 4-300 K. The essential features of our two-dimensional-lattice model are as follows: Förster excitation transfer theory, spectral heterogeneity of both the light-harvesting antenna and the reaction center, treatment of temperature effects through temperature dependence of spectral bands, inclusion of inner structure of the trap, and transition dipole moment orientation. The fluorescence kinetics is analyzed in terms of distributions of various kinetic components, and the influence of different inhomogeneities (orientational, spectral) is studied.A reasonably good agreement between theoretical and experimental fluorescence decay kinetics for purple photosynthetic bacterium Rhodospirillum rubrum is achieved at high temperatures by assuming relatively large antenna spectral inhomogeneity: 20 nm at the whole bandwidth of 40 nm. The mean residence time in the antenna lattice site (it is assumed to be the aggregate of four bacteriochlorophyll a molecules bound to proteins) is estimated to be approximately 12 ps. At 4 K only qualitative agreement between model and experiment is gained. The failure of quantitative fitting is perhaps due to the lack of knowledge about the real structure of antenna or local heating and cooling effects not taken into account.