Computational investigation of light harvesting in purple photosynthetic bacteria

Purple photosynthetic bacteria achieve remarkably high light harvesting efficiency, thus reconciling multiple competing processes in the chromatophore. The first step in photosynthesis is the capture and transport of light energy in the form of short-lived electronic excitation called excitons. Rapid long-range exciton transport is key to the high light harvesting efficiency associated with purple bacteria. The light harvesting system of purple bacteria consists of light harvesting complex 2 (LH2), light harvesting complex 1 (LH1) and reaction center (RC) assembeled into a structure known as the chromatophore. The pigments embedded into the complexes in the chromatophore are placed close together and are tightly held by their surrounding proteins. Pigment excited states thus interact very strongly and are also strongly coupled to surrounding environmental fluctuation. Exciton dynamics in purple bacteria is thus described using the hierarchy equations of motion (HEOM) for open quantum systems, which does not rely on assumptions of relative interaction strengths and includes quantum coherence effects. An efficient implementation of the HEOM is developed and utilized to describe exciton dynamics in the light harvesting complexes of purple bacteria, and calculate excitation transfer between LH2-LH2, LH2-LH1 and LH1-RC pairs. It is shown that strong environmental coupling is reponsible for rapid exciton relaxation into equilibrium prior to inter- complex exciton transfer, thus allowing inter-complex transfer rates to be calculated with the much simpler generalized F ̈orster theory. The effect of intra-complex correlated environmental fluctuations is also examined and found to substantially affect inter-complex exciton transfer. Strong coupling between pigments within a complex results in inter-pigment quantum coherence that significantly improves the rate of inter-complex exciton transfer, vital to efficient light harvesting in purple bacteria