Aggregation of chlorophyll b in model systems

Chlorophyll b (Chi b) in model systems occurs in several forms. The relationship between these forms and the forms occurring in organisms is not yet clear. Chi b aggregation has been investigated less than that of Chi a. In this paper, several spectral features, e.g. fluorescence lifetimes in the picosecond time range, time-resolved delayed luminescence spectra, photoacoustic spectra and photopotential generation of Chi b embedded in model systems (poly(vinylalcohol) film or nematic liquid crystal), were measured. The decay of the fluorescence of Chi b in polymer films can be analyzed, to a good approximation, on the basis of the following three exponential components: 3300-4300 ps, 400-900 ps and 40-71 ps. These decays are tentatively related to the emission of ''dry'' monomers and dry and wet dimers and oligomers respectively. The delayed luminescence spectra of the same samples in the microsecond range (at 8-295 K) are located in a similar spectral range to the fluorescence spectra. The intensity ratio between the delayed luminescence and prompt fluorescence is higher in the long-wavelength region, in which the oligomer emission is observed, than in the short-wavelength region, in which the emission of monomers and small aggregates predominates. The yield of thermal deactivation of the aggregated forms is higher than that of the monomers. The differences between the lifetimes of the various forms can be explained by the competition between the emission of prompt fluorescence, thermal deactivation and energy trapping (which is, in part, later deactivated as delayed luminescence). The excitation energy transfer from ''dry'' monomers to aggregated forms is not very effective. The most effective process of excitation trapping followed by delayed luminescence emission occurs in oligomers of Chi b. On the basis of photopotential generation, the delayed luminescence is due, at least in part, to pigment ionization followed by slow charge recombination. The kinetics of photopotential generation and decay depend on the aggregation of the pigment.