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Analysis of bilirubin and pyrromethenone N-H 'H-NMR chemical shifts indicates that in CDCI, solvent, where intermolecularly H-bonded dimers or intramolecularly H-bonded conformations are involved, the lactam N-H groups are more deshielded than the broader pyrrole N-H groups. In d,-DMSO, disaggregation and unfolding occurs with H-bonding to the solvent and the sharper lactam N-H resonances becoming more shielded than the pyrrole N-H signals.

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The unnatural bile pigments mesobilirubin‐IVα, mesobilirubin‐XIIIα and etiobilirubin‐IVγ were synthesized following self‐coupling of the pyrromethenones ψ‐xanthobilirubic acid methyl ester, xanthobilirubic acid methyl ester and kryptopyrromethenone, respectively.