TIME-RESOLVED POLARIZED ABSORPTION OF C-PHYCOCYANIN FROM THE CYANOBACTERIUM WESTIELLOPSIS-PROLIFICA

The energy transfer processes within C-phycocyanin monomers and trimer s isolated from the cyanobacterium Westiellopsis prolifica were studie d using picosecond polarized absorption techniques. In C-phycocyanin m onomers, the fast depolarization time of about 52 ps was interpreted t o be due to transfer from beta(s) to beta(f) in one beta subunit. The long-lived anisotropic relaxation component in the range 2.4-4.6 ns wa s due to Brownian rotation of the chromophore-protein molecule. In C-p hycocyanin trimers, two kinetic components of about 33 ps and 123-198 ps were observed and assigned to different isotropic relaxation proces ses. However, the alpha and beta(f) chromophores in adjacent alphabeta monomers are expected to form electronic interaction, which results i n pairwise delocalization of the excitation between the two chromophor es (K. Sauer and H. Scheer, Biochim. Biophys. Acta, 936 (1988) 157-170 ). We attributed the shortest time constant of about 33 ps to a hetero geneous, directed relaxation from the upper exciton state to the groun d state due to the site heterogeneity, which is suggested from hole-bu rning experiments (W. Kohler et al., Chem. Phys. Lett., 143 (1988) 169 -173). The lifetime in the range 123-198 ps is probably due mainly to homogeneous energy transfer in the same monomer, such as 1beta(s)-->1b eta(f), and/or excitation equilibration between beta(f) chromophores i n different monomers, such as 1beta(f)<->2beta(f). These isotropic lif etimes in C-phycocyanin trimers are in agreement with the anisotropic relaxations also studied in this paper.