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

• Published in: Biochimica et biophysica acta Vol. 1840; no. 3; pp. 1072 - 1082
• Main Authors: Poša, Mihalj, Sebenji, Ana
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier B.V 01.03.2014
• Subjects: aggregation number; anions; Bile Acids and Salts - chemistry; bile salts; cholic acid; conformational analysis; geometry; Hydrogen Bonding; Hydrophobic and Hydrophilic Interactions; hydrophobic bonding; hydrophobicity; linear models; Magnetic Resonance Spectroscopy; micelle of bile salt; Micelles; Molecular Conformation; regression analysis; sodium; spin–lattice relaxation time; steroid skeleton; steroids; Steroids - chemistry; thermodynamics; steroid skeleton; conformational analysis; micelle of bile salt; spin–lattice relaxation time; aggregation number
• ISSN: 0304-4165; 0006-3002; 1872-8006
• DOI: 10.1016/j.bbagen.2013.11.008

The special geometry of the steroid skeleton causes that bile acid anions, in contrast to aliphatic amphiphiles, form micelles with a small aggregation number. 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 (T1) and concentration of monomers (cBA−) are determined. 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 n¯ 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. 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. Deviation from the linear model: number-average aggregation numbers with hydrophobicity of monomers, suggests the existence of additional, intermolecular interactions beside hydrophobic in micelles. [Display omitted] •The bile acid (BA) anions aggregation numbers were in the range of 3.28 to 12.60.•There is a linear expression between n¯ and lnk, deviation: H-bonds.•Increase in the number of the oxo groups in BA, results in a lower n¯.•The BAs C12 and C7 α-axial OH or oxo groups have a different steric environment.