H. Lehner

ABSORPTION, LUMINESCENCE, SOLVENT‐INDUCED CIRCULAR DICHROISM AND1H NMR STUDY OF BILIRUBIN DIMETHYL ESTER: OBSERVATION OF DIFFERENT FORMS IN SOLUTION*

Bilirubin dimethyl ester (I) has been studied by absorption, fluorescence and fluorescence excitation, solvent‐induced circular dichroism, and 1H NMR techniques. The existence of two forms of the solute involving either or both syn‐anti dipyrromethenone and lactam‐lactim equilibria was established, and the occurrence of even more forms remains possible. The relative population is strongly influenced by the nature of the solvent and by temperature. Thus, the population of one species predominates in ethanol. What is presumed to be the same species increases on cooling in all solvents, while the population of another species is strongly preferred in dichloromethane. In other solvents appreciable populations of both forms persist over the temperature range 77–300 K.

Luminescence and solvent-induced circular dichroism of bilirubin dimethyl ester: evidence for heterogeneous composition of the solute

Abstract BiLirubin dimethyl ester (1) has been studied by fluorescence and fluorescence excitation and by solvent-induced circular dichroism. The existence of two or more forms of the solute involving either or both syn-anti dipyrromethenone and lactamlactim equilibria, was established. The relative population is strongly influenced by the nature of the solvent and by temperature. Thus, the population of one species predominates in ethanol. Presumably the same species generally increases on cooling. The population of another species is strongly preferred in dichloromethane. In other solvents appreciable populations of both persist at all temperatures. Comparison with data on bilirubin diacid and dianion suggests that the composition of the solute species is mainly governed by properties common to all three compounds independent of the specific nature of the propionic side chains.