Electrostatic interactions and exciton coupling in photosynthetic light-harvesting complexes and reaction centers

Protein-pigment complexes are integral to many biochemical reactions. The properties of the pigments in these complexes depend in large part on the protein which serves as both scaffold and solvent. The relative positions of the pigments determine the electronic coupling of the system while local protein charges and vibrational motions alter the energies of the molecules. Photosynthetic light-harvesting complexes and the photosynthetic reaction center serve as excellent models for understanding protein-pigment interactions. The electronic coupling between the pigments of the Light-Harvesting Complex II from Rhodopseudomonas acidophila is explored by exciton calculations that treat the excitation of the complex as a combination of monomer and charge-transfer transitions. Comparisons of the calculated absorption and circular dichroism spectra to experimental spectra provide a measure of the delocalization of the excitation and the inhomogeneity of the protein environment. The time-dependent relaxation dynamics of the initial, coherently excited superposition state is also described. In the photosynthetic reaction center from Rhodobacter sphaeroides, the effects of ionizable amino acids on the solvation energy of the oxidized primary electron donor are explored by site-directed mutagenesis. Changes in the reduction potential caused by the mutations are measured and compared to theoretical models that treat the solvation by the protein and surrounding water. Changes in the electrostatic environment also affect the electronic coupling of the two bacteriochlorophylls within the primary electron donor. The mutations alter the relative energies of the basis states in which the charge is localized on one bacteriochlorophyll or the other, and shift the spin distribution and the absorption of the oxidized state. A vibronic model that includes both symmetric and antisymmetric vibrational modes is introduced to explain consistently the changes in the spin distribution, reduction potential and absorption spectrum of the oxidized state. The results reveal the importance of the protein's vibrational modes to the electronic coupling between the pigments.