Excitation Energy Transfer Between Sensitizing Chromophores of Phycocyanin 612

The visible absorption and circular dichroism spectra of phycocyanin 612 were each resolved into four components. The deconvolution required the blue side of each spectrum to be fitted by a half‐Lorentzian band and the remainder of the spectrum by Gaussian components. The energy levels of the components in the deconvolution allowed analysis of the discrete steps in the transfer of excitation energy within the biliprotein. By comparison of the deconvolution of the absorption spectrum to the published results from ultrafast fluorescence kinetics of phycocyanin 612, the times for s‐to‐s and s‐to‐f excitation energy transfer are established. This is the first evidence of excitation energy transfer between two types of sensitizing (s) chromophores in a biliprotein. The analysis of the ultrafast kinetics and the deconvolution of the absorption spectrum allowed the assignments of 7–10 ps to the transfer of excitons from an s to a fluorescing (f) chromophore and a time faster than 7 ps to the transfer of excitons between two spectrally distinct s chromophores. The steady‐state fluorescence polarization spectrum of this biliprotein supports the hypothesis that excitation energy transfer occurs between s chromophores.