N.m.r., i.r. and optical spectra of model compounds were recorded. These were to help in elucidating the structures of the phenylazo derivatives of bilirubin conjugates isolated from human bile. Model compounds included commercial and human bile bilirubin, mesobilirubin, bilirubin dimethyl ester, dimethoxybilirubin dimethyl ester and the corresponding phenylazo derivatives. The phenylazo derivative of vinylneoxanthobilirubinic acid was also investigated. All compounds were of the type IXα, and no other isomer could be detected with the spectroscopic methods employed. The compounds crystallize as the lactams, except for dimethoxybilirubin dimethyl ester and its phenylazo derivative, which are held in the lactim ether configuration. With all other compounds no tautomeric forms other than the lactams could be detected, although small proportions of bilirubin must exist as the lactim. Bilirubin does not form a betaine, a structure that has been proposed by von Dobeneck & Brunner (1965) to explain the bathochromic shift of its optical spectrum as compared with the expected position of the absorption maximum at 420nm. However, this shift to 453nm can be explained on the basis of internal hydrogen bonds occurring between the carboxylic protons and the pyrrole rings of bilirubin, as proposed by Fog & Jellum (1963), and new evidence for such a bonding has been accumulated. The bilirubin sulphate described by Watson (1958), which is formed by treatment of bilirubin with concentrated sulphuric acid and acetic anhydride, was also investigated. The main product of this reaction was isolated as its phenylazo derivative, and was shown to be 3,18-di(ethylidene sulphate)-2,7,13,17-tetramethylbiladiene-ac-8,12-dipropionic acid. The reaction leading to this compound is an addition of sulphuric acid to the vinyl side chains of bilirubin according to Markownikoff's rule.
- © 1970 The Biochemical Society