Monomeric C-phycocyanin at room temperature and 77 K: Resolution of the absorption and fluorescence spectra of the individual chromophores and the energy-transfer rate constants

At both room temperature (RT) and 77 K, the absorption and fluorescence spectra of the three individual chromophore types (α₈₄, β₈₄, and β₁₅₅) found in monomeric C-phycocyanin (α^PCβ^PC), isolated from the cyanobacterium Synechococcus sp. PCC 7002, were resolved along with the rates of energy transfer between the chromophores. The cpcB/C155S mutant, whose PC is missing the β₁₅₅ chromophore, was useful in effecting this resolution. At RT, the single broad peak in the visible region of the absorption spectrum of (α^PCβ^PC) was resolved into its three-component spectra by comparing the steady-state absorption spectra of the isolated wild-type α subunit of PC (α^PC) (containing only the α₈₄ chromophore) with those of the monomeric PCs isolated from the mutant strain (α^PCβ*) and the wild-type strain (α^PCβ^PC). At 77 K, the visible region of the absorption spectrum of (α^PCβ^PC) splits into two peaks. This partial resolution at 77 K of the chromophore spectra of (α^PCβ^PC) when compared with the 77 K absorption spectra of α^PC, β^PC, and (α^PCβ*) provided a confirmation of our RT assignments of the chromophore absorption spectra. The individual fluorescence spectra of the chromophores and the rate constants for energy transfer between them in (α^PCβ^PC) were resolved by modeling the time-resolved fluorescence spectra of β^PC (containing the β₁₅₅ and β₈₄ chromophores) and (α^PCβ*). As with absorption, lowering the temperature to 77 K helped resolve the fluorescence spectra of the individual chromophores because the spectra were narrowed.