On the Conformation of Glycobilirubin

The first optically active glycine conjugate 1 of a bilirubin was prepared in several steps from (S)-β-methylxanthobilirubic acid glycine conjugate 8. The latter was synthesized by reaction of benzyl glycinate tosylate with the mixed anhydride formed in the reaction of (S)-β-methylxanthobilirubic acid 6 with isobutyl chloroformate. Spectroscopic analysis of the circular dichroism spectra of 1 in various solvents, including aqueous buffer, indicate a conformational preference for the M-helical ridge-tile conformation, thus providing the first spectroscopic evidence on the conformation of glycobilirubins.