Kinetics of excitation migration and trapping in the photosynthetic unit of purple bacteria

Purple bacteria have developed an efficient apparatus to harvest sunlight. The apparatus consists of up to four types of pigment-protein complexes: (i ) the photosynthetic reaction center surrounded by (ii) the light-harvestin g complex LH1, (iii) antenna complexes LH2, which are replaced under low-li ght conditions by (iv) antenna complexes LH3 with a higher absorption maxim um. Following absorption of light anywhere in the apparatus, electronic exc itation is transferred between the pigment-protein complexes until it is us ed for the primary photoreaction in the reaction center. We calculate, usin g Forster theory, all rates for the inter-complex excitation transfer proce sses on the basis of the atomic level structures of the pigment-protein com plexes and of an effective Hamiltonian, established previously, for intraco mplex excitations. The kinetics of excitation migration in the photosynthet ic apparatus is described through a master equation which connects the calc ulated transfer rates to the overall architecture of the apparatus. For two exemplary architectures the efficiency, distribution of dissipation, and t ime evolution of excitation migration are determined. Pigment-protein compl exes are found to form an excitation reservoir, in which excitation is spre ad over many chromophores rather than forming an excitation funnel in which excitation is transferred without detours from the periphery to the RC. Th is feature permits a high quantum yield of 83% to 89%, but also protects th e apparatus from overheating by spreading dissipation over all complexes. S ubstitution of LH2 complexes by LH3 complexes or changing an architecture i n which few LH2 (LH3) complexes are in contact with LH1 to an architecture in which all LH2 (LH3) complexes are in contact with LH1 increases the quan tum yield up to 94% and decreases the degree to which dissipation is evenly distributed.