On the assignment of the carbon-13 NMR spectrum of bilirubin

An individual assignment of certain peaks in the proton and ¹³C NMR spectra of bilirubin and of bilirubin dimethyl ester is described. The assignment was achieved through the spectra of vinylneoxanthobilirubinic acid and by off-resonance decoupled ¹³C spectra of bilirubin dimethyl ester. The signals due to the endo and exo vinyl groups were separately assigned. This assignment allows a rational interpretation of previously obtained spin-lattice relaxation times of the vinyl carbon peaks. The two vinyl groups are found to differ considerably in their motional correlation time.