Mixed Micelle Formation of Bile Salt Mixtures

The formation of mixed micelles was investigated as a function of the mole fraction of each surfactant mixture for two bile salt mixtures of sodium chenodeoxycholate (3α, 7α-dihydroxy-5β-cholanoic acid, sodium salt : NaCDC) with the 7β-epimer of NaCDC, sodium ursodeoxycholate (3α, 7β-dihydroxy-5β-cholanoic acid, sodium salt : NaUDC) (in borate buffer solution of pH= 9. 94 at 30°C) and sodium taurochenodeoxycholate (NaTCDC) with sodium tauroursodeoxycholate (NaTUDC) (in physiological saline solution at 37°C). The results obtained were based primarily on surface tension and ESR measurements. Critical micellization concentrations (cmc) for the NaTCDC-NaTUDC mixed system were determined from solubilization data on monoolein using the bile salt mixture. The following conclusions were drawn. A mixed system of NaCDC with NaUDC shows slightly positive deviation from an ideal mixing, while the NaTCDC-NaTUDC system constitutes an ideal mixture as indicated by the curve of cmc vs. mole fraction. From the ESR data a 1 : 1 mixing ratio of CDC and UDC appears to lead to a more favorable structure of mixed micelle than the other mixing ratios. It thus follows that CDC and UDC form a coupled dimer as an aggregate unit in water.