Circular Dichroism of Phytochromobilin and Related Compounds

Abstract

AbstractAbsorption and circular dichroism (CD) spectra are recorded for partially denatured phytochrome, Pfr chromopeptide as well as (ZZZ)‐ and (ZZE)‐configurated phycocyanin chromopeptides. Solvent induced circular dichroism (SICD) spectra are included for (ZZZ)‐ and (ZZE)‐octaethylbilindione. Irradiation of Pr in 4 M urea yields a bleached chromophore with weak absorption and Δϵ values. On acidification in the dark it shows the known absorption and CD spectra of the Pfr chromophore. Subsequent irradiation of the acidified solution leads to absorption and CD spectra of the Pr chromophore. The red shift of absorption and CD bands obtained during this Pfr ŕ Pr chromophore transformation is observed in both absorption and CD or SICD spectra of Pfr peptides, (E)‐configurated phycocyanin peptides and (E)‐configurated octaethylbilindione during irradiation in acid solution. The shift may be coupled with inversion of the sign of CD or SICD bands, e.g., in fraction 5, phycocyanin peptide and octaethylbilindione. Several fractions of phycocyanin peptides with almost identical absorption spectra show significant differences in their CD spectra. Differences are also found in the pH dependency of CD or SICD spectra of the various fractions of phycocyanin peptides and of octaethylbilindione.