Comparison of calculated and experimentally resolved rate constants for excitation energy transfer in C-phycocyanin. 2. Trimers

The light-harvesting protein C-phycocyanin (PC) in the trimeric aggregation state, isolated from the cyanobacterium synechococcus sp. PCC 7002, is studied by absorption spectroscopy and by time-resolved anisotropic fluorescence spectroscopy with 1 ps time resolution. PC trimers isolated from the wild-type strain and a mutant strain, cpcB/C155S, in which the β₁₅₅ chromophore is missing are compared. The absorption spectra of the trimeric PCs, when compared with previously published spectra of the monomeric PCs, lead us to conclude that the absorption spectrum of the β₁₅₅ chromophore is similar when PC is in the monomeric and trimeric states. This means that the red shift of the absorption spectrum that occurs when PC aggregates from monomers to trimers is due to changes in the spectra of α₈₄ and/or β₈₄ chromophores. Results of present study are in excellent agreement with Foerster calculations in the weak coupling limit for prediction of decay times in a pigment protein.