Coupling of exciton motion in the core antenna and primary charge separation in the reaction center

The relation between exciton motion in the LH1 antenna and primary charge separation in the reaction center of purple bacteria is briefly reviewed. It is argued that in models based on hopping excitons described strictly by Forster theory, transfer-to-trap-limited kinetics is quite unlikely according to the relation between the exciton trapping kinetics and 'N', the size of the photosynthetic unit in such models. Because the results of several recent experiments have been interpreted in terms of transfer-to-trap limited kinetics, this presents a conflict between these experimental interpretations and strictly Forster-based theoretical models. Two possible resolutions are proposed. One arises from the random phase-redistribution trapping kinetics of partially coherent excitons, a kinetics uniquely independent of both N and the rate constant for primary charge separation in the reaction center. The other comes from multiple-pathways models of the multipicosecond nonexponentiality of the decay of P*, the electronically excited primary electron donor in the reaction center. In these models, because it depends only on a certain averaged electron-transfer time constant, the exciton lifetime may be relatively insensitive to variations of individual electron-transfer rate constants - thereby undercutting the argument appearing in recent literature that by default the exciton kinetics must be transfer-to-trap limited.