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 (n̄) are determined using Moroi-Matsuoka-Sugioka thermodynamic method. Also, for analysed bile acid sodium salts functions between spin-lattice relaxation time (T1) and concentration of monomers (cBA-) are determined. Results For 7-oxodeoxicholic (7-ODC) acid and hyodeoxicholic acid (HD) monomers, curve T1=fcBA- contains two inflexion points. Mentioned monomers and cholic acid anion (C) are influential observations in relation to a line of linear regression between n̄ and para\meter of monomer hydrophobicity (lnk, 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. © 2013 Elsevier B.V.