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.