Carbon‐13 nuclear magnetic resonance study of the motional behaviour of bilirubin and of some of its derivatives

A comparative study of the spin‐lattice relaxation times (T₁) of proton‐bearing carbons in bilirubin and its derivatives is reported. The NT₁ values of the backbone carbons indicate that the overall reorientation of the molecule is isotropic. In dimethyl sulphoxide solutions, the segmental motion of the propionic side chains is very limited in bilirubin, bilirubin dimethyl ester and mesobilirubin dimethyl ester, faster in dimethoxybilirubin dimethyl ester and is very fast in the propionic residue of vinylneoxanthobilirubinic acid methyl ester. The relative mobilities of the propionic residues are interpreted on the basis of intramolecular hydrogen bonding in which the solvent participates. The dimeric form of bilirubin dimethyl ester in chloroform solutions is confirmed by the correlation time of tumbling of the backbone. In this solvent the segmental motion of the propionic side chains is rapid, indicating the probable absence of hydrogen‐bonding. In bilirubin, bilirubin dimethyl ester and dimethoxybilirubin dimethyl ester, internal rotation is rapid for the endo‐vinyl group and slow for the exo group, correlating with reported differences in the chemical behaviour of these residues in bile pigments.