Studies of the Energy Transfer among Allophycocyanin from Phycobilisomes of Polysiphonia urceolata by Time-Resolved Fluorescence Isotropic and Anisotropic Spectroscopy

Abstract

Abstract— 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 p84-phycocyaninbilin (PCB) chromophores in the center of the ring shape of the APC trimer are more important than those of between a84- and p84-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 α84- and β84-PCB chromophores in the same monomer of the APC trimer and among three β84-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(∞) with a probing wavelength of 650 nm, the angles between the C3 symmetry axis and transition dipoles of α84- and -PCB chromophores were found to be φa84= 67° and φβ84= 148°, 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.