Studies of the energy transfer among allophycocyanin from phycobilisomes of Polysiphonia urceolata by time-resolved fluorescence isotropic and anisotropic spectroscopy

The excitation energy transfer processes in the allophycocyanin (APC) monomer and trimer from phycobilisomes of Polysiphonia urceolata were studied using picosecond time-resolved fluorescence isotropic and anisotropic spectroscopy, Based on our experimental results, conclusions could be drawn as follows: (1) After the processes of exciton localization are finished, the localized excitation energy on any chromophore can be transferred to the other chromophores due to the weak couplings between them, and the processes among three beta(84)-phycocyaninbilin (PCB) chromophores in the center of the ring shape of the APC trimer are more important than those of between alpha(84)- and beta(84)-PCB chromophores in the same monomer, (2) The decay time constants of 95 +/- 5 ps and 40 +/- 5 ps components, observed by us in this work, were assigned to the excitation energy transfer or redistribution between alpha(84)- and beta(84)-PCB chromophores in the same monomer of the APC trimer and among three beta(84)-PCB chromophores in the center of the ring shape of the APC trimer, respectively. Specifically, the assignment of the decay constants for the 40 +/- 5 ps component was different from those of previous results, (3) Based on the model of Debreczeny, and using the fluorescence residual anisotropy r(infinity) with a probing wavelength of 650 nm, the angles between the C-3 symmetry axis and transition dipoles of alpha(84)- and beta(84)-C-3 chromophores were found to be theta(alpha 84) = 67 degrees and theta(beta 84) = 148 degrees, respectively, which are in agreement with the prediction of the X-ray crystal structure of APC. (4) The results show that anisotropy decays, observed with the APC trimer, did exhibit a strongly probing wavelength dependence that did not show up in the monomer.