Circular Dichroism of Chromopeptides from Phycocyanin

Abstract

The circular dichroism of bilipeptides from Spirulina geitleri phycocyanin is strongly solvent and pH dependent. Maximum optical activity has been observed in aqueous solutions containing urea (8 ᴍ). In aqueous buffer, a sign reversal occurred upon the change from neutral to acidic pH; in methanolic solutions shows the optical activity a strong pH dependence both with respect to sign and magnitude. These findings have been rationalized by the presence of chrom ophorepeptide interactions, which are minimized in the presence of urea. M olecular orbital calculations indicate that the observed sign reversal is not necessarily due to a reversal of the chirality of the entire chromophore, but may also result from more localized conform ational changes