A study of the bile pigments and related compounds in solution by 13C nuclear magnetic resonance spectroscopy

Abstract

The natural abundance ¹³C n.m.r. spectra of six bile pigments and several related compounds are reported. Two of the former, namely mesobiliverdin-IXα dimethyl ester (11) and mesobilirubin-IXα dimethyl ester (16), were specifically ¹³C-enriched at several positions in the molecule, which allowed the complete assignment of their quaternary pyrrolic carbon signals and thus aided in the assignment of the latter in the other bile pigments. Substituent changes in the pyrromethen-5(1H)-one model compounds give rise to ¹³C chemical shift changes indicative of considerable delocalisation over the entire conjugative system. The shifts for the bile pigments indicate that the lactam is the preferred tautomeric form in all cases. The magnitudes of the large number of one-bond carbon–carbon spin–spin coupling constants for the enriched compounds correlate with bond lengths derived from crystallographic studies of similar compounds and with the relative percentage of s-character of the carbon–carbon bond involved in the coupling. Consequently the extent of bond delocalisation in (11) and (16) was deduced, and for (11) it is shown to be higher over rings II and III than over rings I and II, or over rings III and IV.