THERMAL DENATURATION OF MONOMERIC AND TRIMERIC PHYCOCYANINS STUDIED BY STATIC AND SPECTROSCOPY POLARIZED TIME-RESOLVED FLUORESCENCE
b'C-Phycocyanin (PC) and allophycocyanin (APC). as well as the a-subunit of PC. have been\\nisolated from the blue-green alga (cyanobacterium). Spirulina platensis. The effects of partial thermal\\ndenaturation of PC and of its state of aggregation have been studied by ps time-resolved, polarized\\nfluorescence spectroscopy. All measurements have been performed under low photon fluxes (< 10\\u2019 \\u2019\\nphotonsipulse x cm\\u2019) to minimize singlet-singlet annihilation processes. A complex decay is obtained\\nunder most conditions, which can be fitted satisfactorily with a bi-exponential (7\\u2019 = 70400 ps. T? =\\n1000-3000 ps) for both the isotropic and the polarized part, but with different intensities and time\\nconstants for the two decay curves. The data are interpreted in the frameworkof the model first developed\\nby Teak and Dale (Biochern. J. 116, 161 (1970)], which divides the spectroscopically different\\nchromophores in (predominantly) sensitizing (s) and fluorescing U, ones. If one assumes temperature\\ndependent losses in the energy transfer from the s to the f and between f chromophores. both the\\nbiexponential nature of the isotropic fluorescence decay and the polarization data can be rationalized. In\\nthe isotropic emission (corresponding to the population of excited states) the short lifetime is related to the\\ns-,f transfer. the longer one to the \\u201cfree\\u201c decay of the final acceptor(s) (= f). The polarized part is\\ndominated by an extremely short decay time. which is related to s+f transfer, as well as to resonance\\ntransfer between the f-chromophores.'