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 proposed on the basis of the results from femtosecond transient measurements and other spectroscopic data. The model uses the structural data (Kuhlbrandt; et al. Nature 1994, 367, 614) and is obtained using a genetic algorithm search of the large parameter space. Forster resonance transfer 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 molecules that are proximal to the central luteins are Chl b. In contrast to prior assumptions, the basic feature of the model consists of an intermediately strong coupling (V < 100 cm(-1)) between the Chl a and b molecules in close pairs and the absence of substantial excitonic coupling between Chls a, thus indicating an overall limited influence of excitonic effects on spectra and kinetics. A theoretical estimation of exciton effects supports these model assumptions. Over most of the difference absorption spectrum good agreement between experimental and theoretical kinetics has been obtained. Energy transfer times in the symmetric LHC II trimer range from 90 fs to 5.1 ps. For the monomeric complexes only the longest lifetime is significantly affected and predicted to be just slightly longer (6.6 ps). The predicted transition dipole moment orientations result in weak coupling between the LHC II monomers. Several possible routes to improve both the data fitting and the reliability of the predictions in the future are discussed.