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 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.