On the molecularity of bilirubins and their esters and anions in chloroform solution

Bilirubins with propionic acids at C-8 and C-12 engage in intramolecular hy drogen bonding and are thought to be monomeric in solution, although the la tter is unproven. In contrast, their dimethyl eaters and etiobilirubin anal ogs (with the C-8 and C-12 propionic acids replaced by alkyl residues) favo r intermolecular hydrogen bonding and are thought to be dimeric in nonpolar solvents. There is little information on the molecularity of the bilirubin dianion in solution. In this work, vapor pressure osmometry studies of chl oroform solutions of bilirubins, their dimethyl esters, and etio-analogs cl early indicate that the diacids and dianions are monomeric, whereas the die sters and dialkyls are dimeric. However, the presence of a C-10 gem-dimethy l group causes the ester and the etiobilirubin to become monomeric.