EXCITON SPLITTING IN PHYCOERYTHRIN-545

Phycoerythrin 545 is a biliprotein having a polypep tide structure of alpha(2) beta(2) and each alpha and beta polypeptide has chromophores. Circular dichroism (CD) and absorption spectroscopy in the visible re gion together with various biochemical protocols have been used to stu dy these chromophores. The CD spectrum exhibits overlapping positive a nd negative bands. Exciton splitting between closely-spaced pairs of c hromophores produces a CD spectrum that has positive and negative band s of equal rotational strengths, a conservative spectrum. Alternativel y, any positive or negative band could arise from a single chromophore , The results of this study demonstrate that exciton splitting is the likely cause of the negative and corresponding positive bands. The CD spectra of the separated alpha and beta polypeptides, under conditions where the polypeptide structure is denatured, have no negative bands. When the polypeptides are allowed to refold individually, the chromop hores on the beta polypeptide regain a combination of negative and pos itive CD bands. The spectrum of the alpha polypeptide shows no evidenc e of exciton splitting under these refolding conditions. In another ap proach, urea is added to the protein in low concentrations, which resu lt in changes in the conformation and perhaps association of the prote in. A difference CD spectrum of native protein minus protein in 0.8 M urea shows a spectrum characteristic of exciton splitting. Moveover, t he remaining CD spectra in 0.8, 1.6, or 2.4 M urea still show the poss ibility of further exciton splitting, but at slightly different wavele ngths from the spectrum that is deleted by 0.8 M urea. This finding ma y suggest that there are two types of exciton splitting.