A PERTURBED 2-LEVEL MODEL FOR EXCITON TRAPPING IN SMALL PHOTOSYNTHETIC SYSTEMS

The study of exciton trapping in photosynthetic systems provides signi ficant information about migration kinetics within the light harvestin g antenna (LHA) and the reaction center (RC). We discuss two random wa lk models for systems with weakly coupled pigments, with a focus on th e application to small systems (10-40 pigments/RC). Details of the exc iton transfer to and from the RC are taken into consideration, as well as migration within the LHA and quenching in the RC. The first model is obtained by adapting earlier local trap models for application to s mall systems. The exciton lifetime is approximated by the sum of three contributions related to migration in the LHA, trapping by the RC, an d quenching within the RC. The second model is more suitable for small systems and regards the finite rate of migration within the LHA as a perturbation of the simplified model, where the LHA and the RC are eac h represented by a single pigment level. In this approximation, the ex citon lifetime is the sum of a migration component and a single nonlin ear expression for the trapping and quenching of the excitons. Numeric al simulations demonstrate that both models provide accurate estimates of the exciton lifetime in the intermediate range of 20-50 sites/RC. In combination, they cover the entire range of very small to very larg e photosynthetic systems. Although initially intended for regular LHA lattices, the models can also be applied to less regular systems. This becomes essential as more details of the structure of these systems b ecome available. Analysis with these models indicates that the excited state decay in LH1 is limited by the average rate at which excitons t ransfer to the RC from neighboring sites in the LHA. By comparing this to the average rate of transfer within the LHA, various structural mo dels that have been proposed for the LH1 core antenna are discussed.