CONFORMATIONAL-ANALYSIS OF AN OPTICALLY-ACTIVE BILIRUBIN DIMETHYL ESTER AND BIS-N,N-DIMETHYL AMIDE BY CIRCULAR-DICHROISM, NMR AND MOLECULAR-DYNAMICS

The dimethyl ester and bis-N,N-dimethylamide of an optically active analog of bilirubin (beta S,beta'S-dimethylmesobilirubin-XIII alpha) is stabilized in a ridge-tile conformation by intramolecular hydrogen bonding, as detected by H-1-NMR and CD spectroscopy. The ridge-tile shape is most evident in nonpolar solvents, where very strong exciton coupling CD is evident: e.g., in benzene, Delta epsilon(412)(max) -199, Delta epsilon(372)(max)+87 for the dimethyl ester and Delta epsilon(421)(max)-165, Delta epsilon(384)(max)+46 for the bis-N,N-dimethylamide.