Hydrophobicity and self-association (micellization) of bile salts with a lactone or lactam group in a steroid skeleton

Bile acid salts are amphiphiles that have planar polarity: the convex surface (the beta side) of their steroid skeleton is hydrophobic, whereas the concave surface (the alpha side) is hydrophilic. Lactone and lactam derivatives of bile acid salts have reduced hydrophobicity on the convex surface (beta side) of the steroid skeleton, but they are still able to build micelles. In the vicinity of their critical micellar concentration (CMC), they probably form Small's primary micelles. In this work, it is shown that retention capacities (reverse-phase high-performance liquid chromatography) are linearly dependent on temperature, for each tested bile acid. The parameters of this linear dependence (intercept and slope) can be used to describe the hydrophobicity of the analyzed bile acid salts. The first and the second principal components, derived from the covariance matrix of temperature dependence of retention capacity, can also be used as hydrophobicity parameters of the analyzed bile acid salts. There is a strong correlation between theCMCvalues and the values of the hydrophobicity parameters of the convex surface (the beta side) of the steroid skeleton: theCMCvalues decrease as the hydrophobicity of the beta side of the steroid skeleton decreases.