Parameters of micellization and hydrophobicity of sodium salts of 7-buthyl (butylidene) and 7-octyl (octylidene) derivatives of the cholic and the deoxycholic acid in a water solution: Pattern recognition — Linear hydrophobic congeneric groups

Abstract In this paper the influence of hydrophobicity is analysed (expressed with the reversed phase HPLC capacity factor, k ) of the steroid skeleton anion derivatives with next groups placed in a position C7 of the steroid skeleton: octyl, octylidene, butyl, and butylidene group; the influence on their micellization parameters: aggregation number ( n ) and critical micelle concentration ( CMC ). If the examined derivative is more hydrophobic (higher values of k ), CMC values are lower. Examined molecules have the same length of C7 hydrocarbon chain but different position of a double bond in the ring B (endocyclic between C6 and C7, and exocyclic on the C7) and have a different hydrophobicity as well as different CMC values. This can be explained using conformation analysis according to which conformation of the B ring of the steroid skeleton determines conformation of the C7 hydrocarbon chain, i.e. the degree of their departuring from steroid skeleton mean plane and approaching to convex surface of the molecule. In the plane ln k – n new derivatives with octyl/octylidene i.e. buthyl/butylidene group form two linear congeneric group: G E -LCC and H-LCC. Bile acid anions that form the H-LCC group, due the steric reasons, have a low possibility for formation of hydrogen bonds between different building units of a micelle. In the plane ln k –ln CMC only one linear hydrophobic congeneric group is formed which suggests that in micelles, on CMC, hydrogen bonds between building units are not present. Values of partition coefficients of nitrazepan for analysed derivatives micelles suggest that, in formation of mixed micelles of nitrazepan C7 alkyl chain from twisted conformation switches to elongated conformation and takes part in ecraning of the guest molecule.