Temperature broadening of LH2 absorption in glycerol solution.

In order to determine the relationship between the pigment-protein and the pigment-pigment interactions, the measurements of absorption spectra of the peripheral light-harvesting complex LH2 from the purple bacteria Rhodobacter sphaeroides solvated in glycerol/buffer solution were carried out in a wide temperature range, from 4 to 250 K. The SDFs used for simulating the temperature dependence of B800 and B850 bands were determined in a parametric form. To fit experimental spectra the overall exciton-phonon coupling had to be assumed to be weak for B850 (lambda/2V approximately 0.3, where lambda is the reorganization energy and V is the nearest-neighbor dipole-dipole coupling for bacteriochlorophylls). At physiological temperatures the intermediate nuclear bath dynamics compares with the magnitude of energy gap fluctuations. Slower dynamics with kappa approximately 0.39, where kappa is the ratio of the nuclear relaxation rate and the line width parameter, determines the spectral shape of B850 whilst faster modulations characterize B800 (kappa approximately 2.39). The static disorder for the B800 band is relatively high with the characteristic value of the inhomogeneous bandwidth Gamma(inh) approximately 120 cm-1, while for the B850 band this value is almost equal to the dipole-dipole coupling strength (Gamma(inh) approximately 360 cm-1). It has been found that the LH2 absorption spectrum is likely to be influenced by the temperature dependence of the dielectric constant of the solution in the high temperature range, when the glycerol/buffer solution is in the liquid state.