SPECTROSCOPIC CHANGES FOR C-PHYCOCYANIN AND PHYCOERYTHRIN-545 PRODUCED BY FERRIC ION

Biliproteins are light-harvesting pigments in photosynthesis. Their ch romophores are open chain tetrapyrroles, which are not complexed to me tal ions. Using visible absorption and circular dichroism (CD) spectro scopy, the effects of ferric and zinc ions were studied for two bilipr oteins. These biliproteins - C-phycocyanin and phycoerythrin 545 - wer e selected because they vary in their source, structure and spectrosco pic properties. While zinc ions had no effect, ferric ions at pH 5.0 c hange the absorption and CD spectra in the wavelength range of the fir st excited state of the chromophores of bath biliproteins. Concerning the relationship between chromophore topography and the function of li ght-harvesting pigments, the possibility of exciton splitting is a top ic of current debate. For phycoerythrin 545, the effect of ferric ions reveals that two different pairs of chromophores are linked in their behavior, and this may mean that the pairs are involved in exciton spl itting. Exciton splitting would extend the energy range for light harv esting and establish exciton-migration routes within the protein. For C-phycocyanin, the effect of ferric ions on the absorption spectra was primarily focused on the high-energy chromophores, but CD at higher f erric ion to protein ratios showed that the lower-energy chromophores were also affected. At a C-phycocyanin concentration of 0.050 g/l, the end point of the ferric effect in the high-energy region of the spect rum was reached at about 1.7.10(-4) M ferric ion. C-Phycocyanin was tr eated with ferric ions at three concentrations of protein (0.20, 0.10, 0.050 g/l). These same protein concentrations were examined without f erric ions by gel filtration chromatography showing that as the concen tration of C-phycocyanin was lowered the trimeric protein dissociated to more monomer. The effect of the protein denaturant, urea, was also investigated with C-phycocyanin. The effects of urea and ferric ion we re shown to be quite different.