Exciton model of the primary processes of bacterial photosynthesis: Spectra and kinetics of energy transfer

Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 62, No. 2, pp. 77–82, March–April, 1995.     

Exciton Derealization in the Light-Harvesting LH2 Complex of Photosynthetic Purple Bacteria

Abstract— Absorption and circular dichroism spectra for both long-wavelength bands of the antenna LH2 complex from photosynthetic purple bacteria have been calculated. Calculations were performed taking into account resonant interactions between all 27 bacteriochlorophyll (BChl) molecules for different values of site inhomogeneity. The exciton derealization length within BChl800 and BChl850 rings was estimated on the basis of calculated exciton wavefunctions for different temperatures and site inhomogeneity values. The relationship between anomalously high bleaching values in pump-probe spectra and the exciton derealization length is discussed.     

Spectral differences between monomers and trimers of photosystem I depend on the interaction between peripheral chlorophylls of neighboring monomers in trimer

The spatial position of the long-wavelength chlorophylls in trimer of pigment-protein complex of photosystem I (PSI) have been determined bymodeling the optical fluorescence absorption and emission spectra for two hypothetical models of PSI trimer. The calculation has been performed using X-ray diffraction data on the spatial position of chlorophylls in PSI monomer; the pigment site energies were taken from the studies of other researchers, while interactions between monomers in trimer are considered as fitting parameters. The interaction energy between the chlorophylls spaced by a distance smaller than 10 Å was estimated based on the concept of extended dipole?dipole interaction. The model under study allowed us to evaluate the influence of the exciton interaction between peripheral pigments on the optical response of PSI trimer. The intensity and shape of stationary fluorescence line turned out to be sensitive to the PSI monomer packing in trimer. A visualization of the density matrix for low-energy exciton states has made it possible to estimate the localization of long-wavelength chlorophylls in PSI trimer.