Chiral induction in biliverdin covalently bound to amino acids
Amidation of the carboxy groups of biliverdin-IXα(2) with the appropriate (S)-amino acid methyl ester yields the optically active bis[(S)-amino acid] derivatives (3)–(8) which exhibit exceedingly strong Cotton effects in their visible and near-u.v. absorption bands. This is due to the influence of the chiral centre on the population of helical conformers of the bilatriene moiety across five bond lengths. The magnitude of the c.d. observed depends on the steric requirements of the amino acid and the polarity of the solvent. The chiral induction is interpreted in terms of intramolecular interactions of both side chains exerted from different sides of the bilatriene helix by hydrogen bonds. Both the donor and the acceptor sites of the amino acid residues are involved in the chiral association with the bilatriene backbone.