FLUORESCENCE RELAXATION KINETICS AND QUANTUM YIELD FROM THE ISOLATED PHYCOBILIPROTEINS OF THE BLUE-GREEN ALGA NOSTOC SP. MEASURED AS A FUNCTION OF SINGLE PICOSECOND PULSE INTENSITY, I

Abstract— Phycobilisomes from the blue-green alga Nostoc sp. are known to contain the phycobiliproteins: c-phycoerythrin (c-PE), c-phycocyanin (c-PC) and four forms of allophycocyanin (APC I, II, III, and B). We have made a detailed study of the effects of the intensity of a single 6 ps excitation pulse on the decay kinetics and the yield of fluorescence in the individual isolated phycobiliproteins at pH 7 and 23°C. The risetime of the fluorescence of c-PE, c-PC and APC was > 12 ps. We found that the decay of the fluorescence was exponential at intensities of 10¹⁴ photons/cm² in all the phycobiliproteins; the lifetimes being 1552 ± 31ps for c-PE, 2111 ± 83ps for c-PC, 1932 ± 165ps for APC I, 1870 ± 90ps for APC II, 1816 ± 88ps for APC III, (1869 ± 62ps for the averaged APC's I, II, and III), and 2667 ± 233 ps for APC B. We also found that the fluorescence decay became non-exponential in c-PE at excitation intensities < 10¹⁴ photons/cm², but was exponential for all the other phycobiliproteins even at a pulse intensity of 10¹⁵ photons/cm². The relaxation times of c-PE and c-PC decreased with excitation intensity above 10¹⁴ photons/cm². For c-PE and c-PC the relative fluorescence vs excitation intensity was readily described by a relationship derived for a model in which exciton–exciton annihilation occurs. In APC the fluorescence yield and relaxation time were only slightly dependent on the excitation intensity. The results are interpreted to indicate the occurrence of singlet–singlet annihilation intramolecularly among the several phycobilin chromophores within the individual phycobiliprotein molecules in solution. The s to f transfer time is less than 12ps in c-PC.