Time-resolved polarized absorption of C-phycocyanin from the cyanobacterium Westiellopsis prolifica

Abstract

The energy transfer processes within C-phycocyanin monomers and trimers isolated from the cyanobacterium Westiellopsis prolifica were studied using picosecond polarized absorption techniques. In C-phycocyanin monomers, the fast depolarization time of about 52 ps was interpreted to be due to transfer from β s to β f in one β subunit. The long-lived anisotropic relaxation component in the range 2.4–4.6 ns was due to Brownian rotation of the chromophore—protein molecule. In C-phycocyanin trimers, two kinetic components of about 33 ps and 123–198 ps were observed and assigned to different isotropic relaxation processes. However, the α and β f chromophores in adjacent αβ monomers are expected to form electronic interaction, which results in pairwise delocalization of the excitation between the two chromophores (K. Sauer and H. Scheer, Biochim. Biophys. Acta, 936 (1988) 157–170). We attributed the shortest time constant of about 33 ps to a heterogeneous, directed relaxation from the upper exciton state to the ground state due to the site heterogeneity, which is suggested from hole-burning experiments (W. Köhler 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 1β s → 1β f, and/or excitation equilibration between β f chromophores in different monomers, such as 1β f⌊2β f. These isotropic lifetimes in C-phycocyanin trimers are in agreement with the anisotropic relaxations also studied i this paper.