Determination of number-average aggregation numbers of bile salts micelles with a special emphasis on their oxo derivatives—The effect of the steroid skeleton

Abstract Background The special geometry of the steroid skeleton causes that bile acid anions, in contrast to aliphatic amphiphiles, form micelles with a small aggregation number. Methods The number-average aggregation numbers ( n ¯ ) are determined using Moroi–Matsuoka–Sugioka thermodynamic method. Also, for analysed bile acid sodium salts functions between spin–lattice relaxation time ( T 1 ) and concentration of monomers ( c B A − ) are determined. Results For 7-oxodeoxicholic (7-ODC) acid and hyodeoxicholic acid (HD) monomers, curve T 1 = f c BA − contains two inflexion points. Mentioned monomers and cholic acid anion (C) are influential observations in relation to a line of linear regression between n ¯ and para\meter of monomer hydrophobicity (ln k, retention capacity from RPHPLC). This suggests that, in micelles of bile acid anions: 7-ODC, HD and C, beside main, hydrophobic interactions, hydrogen bonds are also possible between building units. Conclusion The increase in the number of oxo groups in the molecule is accompanied with a decrease in the hydrophobicity of the convex side of the steroid skeleton of the bile acid anion, resulting in a lower aggregation number. Obtained results indicate that C12 and C7 α-axial OH and oxo groups on the same C atoms of the investigated bile acid molecules have different spatial environment, which is confirmed by conformational analysis. General significance Deviation from the linear model: number-average aggregation numbers with hydrophobicity of monomers, suggests the existence of additional, intermolecular interactions beside hydrophobic in micelles.