TAUTOMERISM AND HYDROGEN BONDING IN BILIRUBIN AND BILIVERDIN

1. 1. Infrared and NMR spectrometric studies on bilirubin and biliverdin and their derivatives suggest that whereas bilirubin occurs as the enol-imine tautomer, biliverdin and its dimethyl ester are the lactam form. 2. 2. Methylation of bilirubin results in a shift in the absorption maximum from 450 mμ to 405 mμ due to assumption of the lactam form. The change is caused by loss of hydrogen bonding between the propionic acid side chains which hold the molecule in a conformation suitable for hydrogen bonding between the two terminal hydroxyl groups. 3. 3. The failure of bilirubin to form stable zinc complexes and its weakly basic character are discussed in terms of the data leading to the conclusions above. © 1969.