Modeling of Emission Spectra for Molecular Rings - LH2 And LH4 Complexes

Abstract

Computer simulation of steady state fluorescence spectra of the ring molecular systems (resembling, e.g. the light harvesting rings from LH2 and LH4 photosynthetic complexes of purple bacteria) is presented in this paper. The general organization of the LH2 and LH4 complexes is the same: identical subunits are repeated cyclically in such a way that a ring-shaped structure is formed. However, the symmetries of these rings are different: LH2 is usually nonameric but LH4 is octameric. The other difference is the presence of four bacteriochlorophyll molecules per repeating unit in LH4 rather than three ones found in LH2. Transi- tion dipole moments of bacteriochlorophylls in B850 ring of LH2 have nearly tangential orientation whereas in LH4 they are organized in a more radial fashion. The dynamical aspects in ensemble of rings are reflected in optical line shapes of electronic transitions. The observed linewidths reflect the combined influence of different types of static and dynamic disorder. To avoid the broadening of lines due to ensemble averaging one uses the single-molecule spectroscopy technique to obtain a fluorescence-excitation spectrum. For our simulations we have used the ring of tightly bound two-level systems. Static disorder is taken into account simultaneously with dynamic disorder in Markovian approximation. The cumulant-expansion method of Mukamel et al. is used for the calculation of spectral responses of the system with exciton-phonon coupling. Comparison of steady state fluorescence spectra for B850 ring from LH2 and LH4 ring is done.