Excitations and excitons in photosystem I

The elementary requirements for energy transfer in a photosynthetic antenna are discussed. Energy migration between chlorophyll molecules in the PSI antenna reaction centre complex was modelled using a geometry based on the recent X-ray structure. The migration is trap-limited and an intrinsic electron transfer rate constant of 1.3 ps(-1) was obtained by modifying the model to account for experimental results obtained from antennae of different sizes. Exciton coupling between pairs of Chlorophyll molecules was calculated and it is found that to prevent self-quenching their interplanar separation must be greater than or equal to 5 Angstrom. In the PSI reaction centre, exciton coupling between adjacent chlorophylls was found to be around 300 cm(-1) with no particularly large value between the special pair being necessary to describe the experimentally observed transient spectra of P-700(+)-P-700 and A(0)(-)-A(0). Electrochromic interaction is particularly important in reproducing the A(0)(-)-A(0) spectrum.