Atropisomerism in linear tetrapyrroles

Novel bilirubin and biliverdin congeners with propionic acids replaced by o-carboxyphenyl exhibit diastereomerism due to axial chirality about the carbon–carbon single bond linking the o-carboxyphenyl group to a pyrrole ring. Evidence for atropisomerism was found even in the monopyrrole precursor, ethyl 3,5-dimethyl-4-(o-carboxyphenyl)pyrrole-2-carboxylate. Like bilirubin, o-carboxyphenyl rubin adopts an intramolecularly hydrogen-bonded ridge-tile conformation in nonpolar solvents. In solutions containing optically active amines or human serum albumin exhibits intense bisignate exciton coupling-type induced circular dichroism for its long wavelength absorption near 400 nm.