COMPARISON OF CALCULATED AND EXPERIMENTALLY RESOLVED RATE CONSTANTS FOR EXCITATION-ENERGY TRANSFER IN C-PHYCOCYANIN .1. MONOMERS

Rate constants for excitation energy transfer in the light-harvesting protein, C-phycocyanin (PC), in the monomeric aggregation state, isolated from the cyanobacterium Synechococcus sp. PCC 7002, are calculated using Forster theory and compared with the results of time-resolved fluorescence measurements. In addition to the relative distances and orientations between chromophores, obtained from the crystal structure of PC, the Forster calculations require spectroscopic resolution of several properties of the chromophore types (beta(155), alpha(84), beta(84)) found in PC monomers, including absorption and fluorescence spectra, molar absorptivities, fluorescence quantum yields, and fluorescence lifetimes. The resolution of the first two properties, the chromophore absorption and fluorescence spectra, was described in a previous paper [Debreczeny et al. J. Phys. Chem. 1993, 97, 9852-9862]. The assignments of the energy-transfer rate constants in PC monomers are confirmed here by time-resolved fluorescence anisotropy measurements of the PC monomers isolated from both the wild-type and a mutant strain (cpcB/C155S) whose PC is missing the beta(155) chromophore. These fluorescence anisotropy measurements also allow one to extract the angles between the transition dipoles of the chromophores within the beta(155)-beta(84) (34 degrees) and alpha(84)-beta(84) (27 degrees) chromophore pairs. The values of the inverse of the sum of the forward and back calculated Forster rate constants for energy transfer within the beta(155)-beta(84) alpha(84)-beta(84), and beta(155)-alpha(84) chromophore pairs are 49, 158, and 890 ps, respectively, and are in excellent agreement with the experimentally measured values. It is concluded that the Forster model of resonant energy transfer in the weak coupling limit successfully describes the dominant energy-transfer processes in this protein in the monomeric state.