On the aggregation of bilirubins using vapor pressure osmometry

Bilirubin and its analogs are carboxylic acids that engage in intramolecula r hydrogen bonding and are thus thought to be monomeric in solution, althou gh the evidence for the molecularity in solution is indirect. Contrastingly , the dimethyl eaters favor intermolecular hydrogen bonding and are thought to be dimeric, yet they, like the bilirubin (acids), exhibit essentially n o concentration dependence of their NH nmr chemical shifts upon dilution fr om 10(-2) to 10(-5) (or even 10(-6)) M in chloroform-d. Vapor phase osmomet ry (vpo) studies of chloroform solutions of eight bilirubins and their dime thyl esters clearly indicate that the former are monomeric, while the latte r are dimeric - except when a beta-methyl group (but not an alpha*-methyl) is present in each methyl propionate chain. Bilirubin mono-eaters might be monomeric or dimeric in solution. Using vpo to study some seven mono-esters or mono acids, we found that the pigments were monomeric in chloroform.