Effects of pH on the absorption, emission and light scattering spectroscopy of bilirubin and xanthobilirubic acid in sodium taurocholate solution

Abstract

Electronic absorption, differential absorption and emission spectra, as well as light scattering intensities, have been obtained for solutions of the bile pigment bilirubin in the presence of the bile salt sodium taurocholate over the pH range 5.0–8.0. Similar measurements were performed on the dipyrrinone analog compound xanthobilirubic acid. The results are interpreted in terms of hydrophobic solubilization of monomeric bilirubin at alkaline pH, and formation of hydrophobic mixed aggregates of pigment and bile salt at lower pH, with the size of the bilirubin aggregates increasing as the pH decreases. The data are consistent with a model of co-micellization of pigment and bile salt, and also suggest that the chemistry of these complexes is determined mainly by the structural properties of the tetrapyrrole.