CHLOROPHYLL A AGGREGATES IN HYDROCARBON SOLUTION, A PICOSECOND SPECTROSCOPY AND MOLECULAR MODELING STUDY

Chlorophyll a aggregates in 3-methylpentane solution have been studied by using picosecond absorption and fluorescence spectroscopy and molecular modeling. Chlorophyll a aggregates give rise to reversible temperature changes in the absorption and fluorescence spectra. Time-dependent anisotropies were used to estimate rotational correlation times of the aggregates. These were indicative of the sizes of the aggregates. The rotational diffusion of the monomer and the two identified aggregates was hydrodynamic over the viscosity range studied (0.29-1.8 cP). Molecular mechanics calculations were used to predict the minimum energy structures of several chlorophyll a dimers suggested earlier in the literature. These structures were used to estimate exciton splitting ofthe Q(y0-0) transition and compared to the experimentally observed spectral shifts.