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Abstract
Analysis of the N-H 1H-NMR chemical shifts of the monomethyl esters of bilirubin-IIIα, IXα and XIIIα reveals a preference in CDCl3 for folded conformations in which the unesterified propionic acid carboxyl group is intramolecularly H-bonded to the opposing pyrromethenone unit. The remaining pyrromethenone unit may be weakly intramolecularly H-bonded to the propionic ester carbomethoxy group.