ABSTRACT
Bilirubin isomers vary considerably with respect to solubility of the acids in water. The main isomer, bilirubin-IXα, is nearly insoluble in water at pH-values below 7. The solubility of bilirubin acid as such in water is too low for direct measurement. Since, however, the disodium salt is soluble, and since a small amount of the acid must be present in equilibrium with the dianion, the acid must be soluble to some extent. Bilirubin, besides being an acid, is a weak base, as seen from the fact that it forms a red cation with light absorption maximum at 512 to 517 nm. Accurate determination of pK-values of bilirubin cannot be based on determinations in aqueous media, due to insolubility of the acid and instability of the tetraanion. Light absorption spectra of bilirubin-IXα acid, dissolved in organic solvents, show two maxima, one weak band at about 290 nm and another, intense maximum at 450 to 460 nm.
