THERMAL DENATURATION OF MONOMERIC AND TRIMERIC PHYCOCYANINS STUDIED BY STATIC AND POLARIZED TIME-RESOLVED FLUORESCENCE SPECTROSCOPY

C. Phycocyanin (PC) and allophycocyanin (APC). as well as the α-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(10¹³ photons/pulse cm²) to minimize singlet-singlet annihilation processes. A complex decay is obtained under most conditions, which can be fitted satisfactorily with a bi-exponential (T₁=70–400 ps, T₂ -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 framework of the model first developed by Teale and Dale Biochem. J. 116, 161 (1970)], which divides the spectroscopically different chromophores in (predominantly) sensitizing ( s ) and fluorescing ( f ), 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 sf 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 sf transfer, as well as to resonance transfer between the f -chromophores.