A PICOSECOND TIME-RESOLVED FLUORESCENCE STUDY ON THE BILIPROTEIN, PHYCOCYANIN-645

Phycocyanin 645, a photosynthetic chromoprotein isolated from a cryptomonad, has been studied by picosecond time-resolved fluorescence by comparing the results of excitation at three wavelengths, 565, 585 and 615 nm. These excitation wavelengths were selected to correspond as closely as possible using our instrumentation to the maxima of three of the four bands found by deconvolution of the absorption spectrum. These three deconvolution bands - 558, 585 and 624 nm - corresponded to the absorption maxima of chromophores that transfer their excitation energies at very high efficiencies to the lowest-energy chromophore. The fourth, lowest-energy, deconvolution band was at 650 nm, and this chromophore provided the main fluorescence emission of the protein. The decay kinetics for 565 and 615 nm excitation were at 8.9 and 14.7 ps, respectively, but excitation at 585 nm gave rise to a faster component at about 2 ps. Careful statistical analysis was performed to verify that the decays obtained by the various excitations were significantly different. These results were discussed in terms of a model in which the 585 nm and 650 nm bands were strongly coupled and shared delocalization energy. The remaining two bands - 558 and 624 nm - probably transferred excitons to the 650 nm band by very weak dipole coupling. If we assume the delocalization hypothesis to be valid, the 2 ps decay would be a measure of the internal conversion between the high- and low-energy exciton states of the delocalized pair.