Bile salt micellar electrokinetic chromatography of bilirubin and related compounds

The interaction of bilirubin, biliverdin, bilirubin dimethyl ester, biliverdin dimethyl ester, xanthobilirubic acid, and xanthobilirubin methyl ester with trihydroxy and dihydroxy bile salt solutions is investigated by micellar electrokinetic chromatography (MEKC). The capacity factor of each compound is measured in solutions of the different bile salts over the pH range of 6.5-9.0. The capacity factor of bilirubin increases with pH below 7 in all bile salt solutions. Biliverdin and xanthobilirubin show essentially identical capacity factors for all bile salts. Biliverdin dimethyl ester and xanthobilirubin methyl ester also have very similar capacity factors, which are greater than those of the carboxy analogs, in trihydroxy bile salts. The capacity factors of these esters are higher in the dihydroxy bile salts, with the capacity factor of biliverdin dimethyl ester being twice that of xanthobilirubin methyl ester. Factors involved in the MEKC analysis of these compounds are discussed.