Structure-spectroscopy relationship in the LHC II: a model study on the chlorophyll transition dipole moment orientations and flow of excitation energy among the chlorophylls

Abstract

In the Qy spectral region (~640-680 nm) the chlorophylls (Chls) a and b are the harvesters of light in the LHCII, the main antenna of green plants. The interactions between the Chls and between the Chls and the protein determine both spectral characteristics and energy transfer dynamics. Despite the existence of a large amount of spectroscopic data in the Qy region, the low structural resolution and the overlap of many absorption bands in a narrow spectral window preclude a comprehensive understanding of the functioning and spectroscopy. In the current structural model (Kühlbrandt et al., Nature, 1994) the Chls could only be modeled as naked tetrapyrroles. Therefore the Chls a and b could not be distinguished (the identity problem) and no distinction could be made between the molecular x- and y-axis of the Chls (the orientation problem). Both are essential to model the Chl interactions. In this contribution a large part of a recent proposal on the pigment identities (Remelli et al., J. Biol. Chem., 1999) has been integrated into the structural model. The Chl-Chl (Coulombic) interactions have been treated in the point-dipole approximation and the pigment-protein interactions are taken as empirical fit parameters. The orientations and the individual spectral characteristics of all twelve Chls have been assigned by relating the key features of the absorption and linear dichroism spectra to the structure through exciton calculations. It has been discussed that the proposed assignment can satisfactorily explain many features of the energy transfer processes between the Chls observed in the ultrafast experiments.