Monolayer Properties of Bilirubin and Its Binary Mixtures with Cholesterol and Lecithin

Abstract

The monolayer properties of bilirubin, known as one of bile conponents, and its binary mixtures with Cholesterol and lecithin were studied on the basis of their surface pressure-area isotherms obtained by varying pH values of substrate solutions with various concentrations of NaC1 in the presence and absence of 0.1 molls CaCl₂. The limiting area for bilirubin monolayer on 25 w/v% NaCl increased up to 1.3 nm2/molec. in the absence of CaCl₂, but by adding O.1 mol/l CaCl₂ to the substrate, it was reduced to 0.67 nm2/molec. Thiss uggests that both of the carboxyl groups of bilirubin molecule are anchored to the substrates and then bilirubin molecules orient in the condensed state to the water surface due to formation of monocalcium salt. In the case of binary mixture of bilirubin and cholesterol, the mean limiting area-mole fraction of bilirubin curve deviated positively from an ideal one independently from the presence or absence of calcium ion. This may be due to the repulsion between cholesterol and pyrrole rings. In the case of binary mixture of bilirubin and lecithin, the mean limiting area-mole fraction curve deviated negatively in the absence of calcium ion, but, by adding CaCl₂, the areas increased and the curve deviated positively. This may be due to an electrical repulsion force among calcium ions and lecithin molecules.