TRANSPORT OF EXCITATION-ENERGY IN THYLAKOID DISC IN CHLOROPLAST AT NORMAL TEMPERATURE

The energy of sunlight absorbed by an antenna chlorophyll inside a thy lakoid disc in chloroplast is known to migrate to the reaction center in the form of an exciton. At normal temperature both the mechanisms o f resonance transfer and exciton hopping contribute comparably. The fi nite-temperature theory of excitons in a molecular aggregate is transl ated in the language of solid state physics as the treatment of the ex citon in a thermal bath of phonons. For the sake of simplicity, the ex citon-phonon interaction can be viewed as linear in lattice displaceme nts with higher-order terms neglected. In the interaction picture, the effects of the thermal bath on the dynamics of the exciton can be inc orporated into a time-dependent effective potential that involves term s arising from the fluctuation of the medium coordinates from their eq uilibrium values. The probability of site-to-site exciton transfer is written as a correlation function whose evolution in time can be deter mined by the cumulant expansion technique. The exciton clothed by phon ons can be defined in a natural way. This procedure leads to coarse-gr aining, and the correlation function for the coarse-grained exciton is defined in terms of the dressed states and the dressed operators. The zeroth-order term in the cumulant expansion corresponds to the resona nce transfer of the dressed exciton while the second- and the higher-o rder terms lead to an expression for the probability of hopping. The t ransfer probabilities for a clothed exciton is derived under the Debye approximation for a cubic lattice. These expressions can be used to d etermine the nearest-neighbour transfer probabilities in a reasonably realistic model of the thylakoid disc which in turn can be used for a numerical simulation of excitons dynamics. The model aggregate can be spatially and orientationally disordered. So the transfer probabilitie s at different sites in different directions are all different which i s in sharp contrast with the so-called random walk model. In an earlie r computer experiment we have shown that if all the excitons are consi dered to be created simultaneously, physical processes occurring at wi dely varying time scales (like exciton creation, exciton transfer, exc iton decay by fluorescence, exciton trapping, phonon dynamics and elec tron transfers) are found to be time-wise self-consistent with one ano ther. In this work we view exciton generation as a continuous process and derive a few analytical results. An algorithm for a very realistic numerical simulation of exciton generation and its utilization in chl oroplast is also presented.