Picosecond dynamics of directed excitation transfer in spectrally heterogeneous light-harvesting antenna of purple bacteria

Picosecond spectrally resolved fluorescence kinetics measurements, together with model simulations of the obtained data, based on coupled rate equations, have been employed to determine the rates and the pathways of heterogeneous excitation transfer in Rhodobacter sphaeroides and Chromatium minutissimum. The presence of a directed excitation flow from the short-wavelength bacteriochlorophyll forms to the long-wavelength one and from the latter to reaction centres has been revealed. As a result, the overall excitation trapping time in the bacteria investigated has been found to be about 60 ps both at 77 K and at room temperature, i.e. the same as in Rhodospirillum rubrum, although the number of antenna bacteriochlorophyll molecules per reaction centre is several times larger. A comparison of the experimental and theoretical kinetic data shows that, besides obvious spectral heterogeneity of the bacteriochlorophyll antenna represented by well-resolved B800, B850 and B875 spectra forms, an intrinsic spectral inhomogeneiry of these forms is likely to play an essential role in the excitation transfer. The obtained picture of the mutual arrangement of different complexes in membranes is similar to the one suggested earlier, except that the presence of at least two typesof B800-850 complexes, the ones closely associated with B875 and the more remote ones,has been discovered. The excitation transfer to B875 has been shown to take about 10 and 50 ps for the first and the second type of B850 molecules, respectively. The intracomplex B800 → B850 transfer time is an order of magnitude smaller, about 1 ps. These three time constants seem to be practically independent of the reaction centre state and the temperature in the 300 K-77 K interval. At high excitation intensities (more than 1 · 1010 photons per pulse) a shortening of the long-wavelength fluorescence decay time and a short-wavelength shiftof the corresponding band maximum have been observed. Both effects are due to the annihilation of singlet and triplet excitations.