C-Phycocyanin (PC) and allophycocyanin (APC). as well as the a-subunit of PC. have been isolated from the blue-green alga (cyanobacterium). Spirulina platensis. The effects of partial thermal denaturation of PC and of its state of aggregation have been studied by ps time-resolved, polarized fluorescence spectroscopy. All measurements have been performed under low photon fluxes (< 10' ' photonsipulse x cm') to minimize singlet-singlet annihilation processes. A complex decay is obtained under most conditions, which can be fitted satisfactorily with a bi-exponential (7' = 70400 ps. T? = 1000-3000 ps) for both the isotropic and the polarized part, but with different intensities and time constants for the two decay curves. The data are interpreted in the frameworkof the model first developed by Teak and Dale (Biochern. J . 116, 161 (1970)], which divides the spectroscopically different chromophores in (predominantly) sensitizing (s) and fluorescing U, ones. If one assumes temperature dependent losses in the energy transfer from the s to the f and between f chromophores. both the biexponential nature of the isotropic fluorescence decay and the polarization data can be rationalized. In the isotropic emission (corresponding to the population of excited states) the short lifetime is related to the s-,f transfer. the longer one to the "free" decay of the final acceptor(s) (= f). The polarized part is dominated by an extremely short decay time. which is related to s+f transfer, as well as to resonance transfer between the f-chromophores.-<, . <, 22 1

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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