[Modeling of energy migration and trapping in purple bacteria. Analysis of extreme formulae]

Homogeneous pigment ensembles similar to those of purple bacteria Rhodospirillum rubrum were studied. Two formulae were advanced for the limiting values of excitation lifetime and quantum yield of excitation trapping in these ensembles, provided all reaction centers are in an active state. It was demonstrated by mathematical modeling that these limiting values strictly depend on three parameters of molecular ensembles: the numbers of core-bacteriochlorophyll molecules per reaction center, the values of rate constants for excitation trapping in reaction centers, and excitation wasteful deactivation in all molecules. The excitation lifetime and quantum yield were proved to approach their limiting values as the rate constants of excitation intermolecular migration increase. The closeness of experimental values for two above mentioned functions to their calculated limiting values proves the migration-limited type of the photosynthetic unit investigated and a high efficiency of excitation trapping in its reaction centers.