Bilirubin in Aqueous Solution. Absorption Spectrum, Aqueous Solubility, and Dissociation Constants

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Abstract This report analyzes the change of both spectrum and solubility of bilirubin with pH in buffer solutions. Bilirubin (AH2) is a dibasic acid which dissociates by two steps (AH2\oversetK1\ightleftharpoonsAH1\oversetK2\ightleftharpoonsA) with increasing pH; the wavelengths at the resulting three isosbestic points are 457 nm (AH2 and AH1), 485 nm (AH1 and A), and 472 nm (A and AH2). The solubility increases with increasing pH, where the total concentration ([AH2]+[AH1]+[A]) changes subject to a second-order curve of 1/aH+. From the spectral change and the three coefficients of the solubility curve, K1 and K2 values were estimated to be (6.7±2.6)×10−7 and (3.4±1.0)×10−8, respectively. These dissociation constants are much smaller than those of aliphatic carboxylic groups, and the reason for it was discussed together with the values already reported and their reliability.

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