pThe i.r. spectra of bilirubin isomers that differ in number and position of vinyl groups were examined to verify the assignment of the 988 cm?1 peak of bilirubin (991 cm?1 peak in calcium bilirubinate) to its pendant vinyl groups. There were only small changes in this peak with changes in position of vinyl groups (exo-2- and ?18-vinyl versus endo-3- and ?17-vinyl), but progressive loss of peaks in this region was observed when vinyl groups were reduced to ethyl groups (dihydrobilirubin and mesobilirubin). Methylvinylmaleimide, a monopyrrole derived from the outer (A and D) rings of bilirubin, has a pendant vinyl group and exhibits a prominent peak at 986 cm?1, but haematinic acid methyl ester derived from the inner (B and C) rings has no vinyl group and shows no peak near 988 cm?1. These observations verify the assignment of the 988 cm?1 peak of bilirubin to its pendant vinyl groups. This supports our previous proposal that a decrease in the peak at 985-995 cm?1 in the i.r. spectra of pigment gallstones, as compared with unconjugated bilirubin or calcium bilirubinate, indicates a consumption of vinyl groups in the process of formation of the polymer in the pigment stones./p
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