Aqueous pK a values of unconjugated bilirubin are important determinants of its solubility and transport. Published pK a data on an analog, mesobilirubin-XIIIα, studied by 13C-NMR in buffered solutions containing 27 and 64 vol% (C2H3)2SO because of low aqueous solubility of mesobilirubin, were extrapolated to obtain pK a values in water of 4.2 and 4.9. Previous chloroform-water partition data on bilirubin diacid led to higher estimates of its pK a , 8.12 and 8.44, and its aqueous solubility. A thermodynamic analysis, using this solubility and a published solubility in DMSO, suggested that the systems used to measure 13C-NMR shifts were highly supersaturated. This expectation was assessed by measuring the residual concentrations of bilirubin in the supernatants of comparable DMSO-buffer systems, after mild centrifugation to remove microprecipitates.
