MODEL FOR THE EXCITATION DYNAMICS IN THE LIGHT-HARVESTING COMPLEX-II FROM HIGHER-PLANTS

A model for the spectral characteristics, the transition dipole moment orientations, and the energy transfer properties of chlorophyll (Chl) a and b molecules in the light-harvesting complex (LHC) II is propose d on the basis of the results from femtosecond transient measurements and other spectroscopic data. The model uses the structural data (Kuhl brandt; et al. Nature 1994, 367, 614) and is obtained using a genetic algorithm search of the large parameter space. Forster resonance trans fer has been assumed as the mechanism of energy transfer. The spectral and orientational assignments of all twelve Chl molecules of a LHC II monomer are proposed. In the best fit model two of the seven Chl mole cules that are proximal to the central luteins are Chl b. In contrast to prior assumptions, the basic feature of the model consists of an in termediately strong coupling (V < 100 cm(-1)) between the Chl a and b molecules in close pairs and the absence of substantial excitonic coup ling between Chls a, thus indicating an overall limited influence of e xcitonic effects on spectra and kinetics. A theoretical estimation of exciton effects supports these model assumptions. Over most of the dif ference absorption spectrum good agreement between experimental and th eoretical kinetics has been obtained. Energy transfer times in the sym metric LHC II trimer range from 90 fs to 5.1 ps. For the monomeric com plexes only the longest lifetime is significantly affected and predict ed to be just slightly longer (6.6 ps). The predicted transition dipol e moment orientations result in weak coupling between the LHC II monom ers. Several possible routes to improve both the data fitting and the reliability of the predictions in the future are discussed.