Influence of temperature on retention parameter of bile acids in normal phase thin-layer chromatography: the role of steroid skeleton.

In this paper, bile acids' retention parameter, R(M), obtained in normal phase thin-layer chromatography is determined as a function of the temperature (293-323 K). Analyzed bile acids belong to congeneric group with two oxygen atoms (OH or oxo groups) on the steroid core and congeneric group with three oxygen atoms. For molecules of both congeneric groups it is found that there is a linear relationship between R(M) and temperature, therefore R(M) decreases with the increase of temperature. In certain congeneric groups, parameters of linear function (R(M) - T) can be linked to the structural characteristics of bile acids, firstly with their spatial orientation (steric position in relation to the steroid mean plane) of the steroid oxygen atom (OH or oxo groups). Absolute values of slope (see text for symbol) of linear function (R(M) - T) increase if steroid oxygen atoms are not in the polar plane, since then the possibility of forming hydrogen bonds with stationary phase decreases. Besides that, absolute value of the parameter (see text for symbol) describes degree of hydrogen bond forming between bile acids and polar stationary phase in each congeneric group as well as hydrophobicity of the steroid skeleton.