Hydrogen Bonding of Bilirubins and Pyrromethenones in Solution

Analysis of bilirubin and pyrromethenone N—H 1H‐NMR chemical shifts indicates that in CDCl3 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 d6‐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.