The influence of aggregation and temperature on the excited state kinetics of C-phycocyanin from Mastiqocladus laminosus has been studied. Polarized fluorescence decay curves have been recorded using a synchronously pumped dye laser in conjunction with a synchroscan streak camera. The experimental data for all samples can be fit satisfactorily assuming a biexponential decay law. Fluorescence depolarization times have been interpreted in terms of energy transfer among the different chromophores. The influence of temperature is only moderate on the intra­molecular relaxation, but pronounced on the rates of energy transfer. Both are dependent on the size of the aggregate. The biexponential decay of the α-subunit containing only one chromophore, indicates the presence of different subsets of chromophores in these samples. The results are discussed in terms of variations of the chromophore arrangements upon temperature induced changes in the protein conformation.

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Phycobiliproteins are photosynthetic light-harvesting pigments in blue-green and red algae. They consist of 2-3 polypeptide subunits, each bearing up to 4 covalently bound linear tetrapyrrolic chromophores. In vivo the biliproteins are organized into complex structures, the phycobilisomes, which are attached to the outer thylakoid surface. Within the phycobilisome the excitation energy is transfered from the "outer" biliproteins with higher excitation energy to "inner" lying ones with lower excitation energy. In the intact alga the last step in the ener gy transfer chain leads to the reaction center. It is generally assumed that the energy transfer is based upon dipole-dipoleinteraction (Forster mechanism), but details are yet insufficiently understood. In an approach complementary to the study of energy transfer in functionally intact phycobilisomes or large fractions thereof /1/ we are currently investigating by time-resolved fluorescence spectroscopy C-Phycocyanin (PC) isolated from Mastigocladus laminosus and its subunits, which are prepared according to procedures described earlier /2,3/. All samples are dissolved in potassium phosphate buffer (80 mM, pH 6.0).

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