Preparation and properties of crystalline biliverdin IXα. Simple methods for preparing isomerically homogeneous biliverdin and [14C]biliverdin by using 2,3-dichloro-5,6-dicyanobenzoquinone

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Abstract
Amorphous isomerically pure biliverdin IX.alpha. is readily prepared in more than 70% yield by dehydrogenation of bilirubin with 2,3-dichloro-5,6-dicyanobenzoquinone in dimethylsulfoxide under carefully controlled conditions. Crystalline biliverdin IX.alpha. and amorphous [14C]biliverdin can be obtained similarly in more than 40% yield. The pure crystalline pigment was characterized by elemental analysis, methylation, chemical and enzymatic reduction to bilirubin, IR and UV visible-absorption spectroscopy, NMR spectroscopy and field-desorption mass spectrometry and its solubility was determined. Under certain conditions, dehydrogenation gave biliverdin contaminated with III.alpha. and XIII.alpha. isomers as a result of disproportionation of bilirubin. Formation of non-IX.alpha. isomers depends on the concentrations of the reagents and the order in which they are mixed and occurs under neutral anaerobic conditions. Free-radical reactions probably are responsible. The 1st step in the dehydrogenation of bilirubin with 2,3-dichloro-5,6-dichloro-5,6-dicyanobenzoquinone in dimethyl sulfoxide is probably the formation of a bilirubin cation radical, rather than hydride ion abstraction.