Modelling photon capture, excitation energy transfer and electron transfer in photosynthesis

Photosynthetic organisms capture photons of visible light using chlorophyll-containing antennae and convert it to chemical energy in the form of a charge separated radical pair between the reaction center chlorophyll and an electron acceptor. This process occurs in two protein-bound chlorophyll systems in green plants, called photosystem I and photosystem II.; In this work, a mathematical model of photon capture is formulated which relates the measured rate constant for photo-oxidation of the photosystem I reaction center to its effective absorption cross-section, allowing for measurement of the photosystem I absorption cross-section in thylakoids or detergent-isolated photosystem I preparations.; Using the techniques of matrix algebra, a mathematical framework is outlined which provides for both numerical and analytical solutions to the kinetic equations describing the fate of the captured photon's energy as it is transferred between antenna chlorophylls, trapped by the reaction center and passed through an arbitrary number of electron transfer steps. No requirements are imposed on the spatial and spectral structure of the modelled system. Computer simulations are presented which demonstrate how variations in the spatial and spectral parameters of a photosynthetic system affect the chlorophyll fluorescence decay.; Time-correlated single photon counting and least-squares global analysis was used to obtain the fluorescence emission surface from the Chlamydomonas reinhartii mutant LM3-A4d, which lacks the photosystem II reaction center complex. An accurate description of the fluorescence decay required four exponential terms. The experimental data was further analyzed by comparison to calculated fluorescence parameters from simulated systems, and a composite model system was created which can account for the observed results. Processes required to accurately describe the fluorescence decay of photosystem I include excitation energy transfer, electron transfer and peripheral antenna-core interactions. The spatial and spectral parameters of the simulated system provide insight into the structure of photosystem I.; Analytical expressions are derived which can be used to characterize the dynamics of energy transfer in certain types of photosynthetic core complexes. It is shown, by simulation, that the derived parameters should be obtainable from experimental data from real systems.