Phytochrome Models, I. Isolation, Characterization, and Solution Conformation of Biliverdin Dimethyl Ester and Its XIIIα Isomer

Biliverdin dimethyl ester (1b; IXα) and its XIIIα isomer 2b have been isolated on a preparative scale after esterification of an isomeric mixture obtained by oxidation of bilirubin. Both isomers readily form solid solutions. The substitution patterns and the predominant helical „all‐Z”︁ configuration, „all‐syn”︁ conformation of 1b and 2b are assigned on the basis of an 1H‐NMR study including spin decoupling and nuclear Overhauser effect experiments. The electronic absorption spectra of the two isomers differ significantly. The ethyl (S)‐(‐)‐lactate‐induced circular dichroism of 1b and 2b in the long and short wavelength regions is similar to that found for biliverdin‐peptide complexes.