Spectroscopic changes for C-phycocyanin and phycoerythrin 545 produced by ferric ion

Abstract

Biliproteins are light-harvesting pigments in photosynthesis. Their chromophores are open chain tetrapyrroles, which are not complexed to metal ions. Using visible absorption and circular dichroism (CD) spectroscopy, the effects of ferric and zinc ions were studied for two biliproteins. These biliproteins - C-phycocyanin and phycoerythrin 545 - were selected because they vary in their source, structure and spectroscopic properties. While zinc ions had no effect, ferric ions at pH 5.0 change the absorption and CD spectra in the wavelength range of the first excited state of the chromophores of both biliproteins. Concerning the relationship between chromophore topography and the function of light-harvesting pigments, the possibility of exciton splitting is a topic 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 splitting. Exciton splitting would extend the energy range for light harvesting 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 ferric 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 spectrum was reached at about 1.7 · 10 −4 M ferric ion. C-Phycocyanin was treated with ferric ions at three concentrations of protein (0.20, 0.10, 0.050 g/l). These same protein concentrations were examined without ferric ions by gel filtration chromatography showing that as the concentration 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 were shown to be quite different.