MONOMERIC C-PHYCOCYANIN AT ROOM-TEMPERATURE AND 77-K - RESOLUTION OF THE ABSORPTION AND FLUORESCENCE-SPECTRA OF THE INDIVIDUAL CHROMOPHORESAND THE ENERGY-TRANSFER RATE CONSTANTS

At both room temperature (RT) and 77 K, the absorption and fluorescenc e spectra of the three individual chromophore types (alpha84, beta84, and beta155) found in monomeric C-phycocyanin (alpha(PC)beta(PC)), iso lated from the cyanobacterium Synechococcus sp. PCC 7002, were resolve d along with the rates of energy transfer between the chromophores. Th e cpcB/C155S mutant, whose PC is missing the beta155 chromophore, was useful in effecting this resolution. At RT, the single broad peak in t he visible region of the absorption spectrum of (alpha(PC)beta(PC)) wa s resolved into its three-component spectra by comparing the steady-st ate absorption spectra of the isolated wild-type alpha subunit of PC ( alpha(PC)) (containing only the alpha84 chromophore) with those of the monomeric PCs isolated from the mutant strain (alpha(PC)beta) and th e wild-type strain (alpha(PC)beta). At 77 K, the visible region of th e absorption spectrum of (alpha(PC)beta(PC)) splits into two peaks. Th is partial resolution at 77 K of the chromophore spectra of (alpha(PC) beta(PC)) when compared with the 77 K absorption spectra of alpha(PC), beta(PC), and (alpha(PC)beta) provided a confirmation of our RT assi gnments of the chromophore absorption spectra. The individual fluoresc ence spectra of the chromophores and the rate constants for energy tra nsfer between them in (alpha(PC)beta(PC)) were resolved by modeling th e time-resolved fluorescence spectra of beta(PC)(containing the beta15 5 and beta84 chromophores) and (alpha(PC)beta). As with absorption, l owering the temperature to 77 K helped resolve the fluorescence spectr a of the individual chromophores because the spectra were narrowed.