Hydrophobic self-association of sodium taurochenodeoxycholate and tauroursodeoxycholate

The aggregation patterns of sodium taurochenodeoxycholate (TCDC) and taurou rsodeoxycholate (TUDC) in an isotonic sodium chloride solution are investig ated by frontal chromatography on Sephadex G-10 columns at 298.2 K and are quantitatively analyzed on the basis of stepwise aggregation models. The mi cellar aggregation numbers, critical micelle concentrations (cmc's), stepwi se aggregation constants, and micelle size distributions of TCDC and TUDC a re determined and compared to those of sodium taurocholate and taurodeoxych olate. The logarithms of gel-water partition coefficients, dimerization con stants, and reciprocal cmc values for the four bile salts exhibit linearity with their hydrophobic molecular surface areas, and the slopes are close t o that of the logarithms of reciprocal cmc values against the hydrophobic m olecular surface areas of lecithins. This fact shows that the gel adsorptio n, dimerization, and micellization of bile salts are driven mainly by hydro phobic interactions and that the Small model for the micellization of bile salts is better than models based on hydrophilic interactions. The micelle of TUDC grows less cooperatively than that of TCDC, an epimer of TUDC. This result suggests that the 7 beta-hydroxyl group of TUDC inhibits micellar g rowth because of increased intermolecular mismatching contact with the hydr ophobic surface and decreased intermolecular hydrogen bonds in the micelle.