title: 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 (T1) and concentration of monomers ( [Formula: see text] ) are determined.
  
  
  RESULTS
  For 7-oxodeoxicholic (7-ODC) acid and hyodeoxicholic acid (HD) monomers, curve [Formula: see text] 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.
  
  
  CONCLUSIONS
  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.
  
  
  CONCLUSIONS
  Deviation from the linear model: number-average aggregation numbers with hydrophobicity of monomers, suggests the existence of additional, intermolecular interactions beside hydrophobic in micelles.
authors.affiliation: Department of Pharmacy, Faculty of Medicine, University of Novi Sad, Hajduk Veljkova 3, 21000 Novi Sad, Serbia. Electronic address: mihaljp@uns.ac.rs.
authors.forename: Mihalj
authors.initials: M
authors.lastname: Poša
authors.affiliation: Department of Pharmacy, Faculty of Medicine, University of Novi Sad, Hajduk Veljkova 3, 21000 Novi Sad, Serbia.
authors.forename: Ana
authors.initials: A
authors.lastname: Sebenji
mesh_terms: D001647:Bile Acids and Salts / Q000737:chemistry*; D006860:Hydrogen Bonding; D057927:Hydrophobic and Hydrophilic Interactions; D009682:Magnetic Resonance Spectroscopy; D008823:Micelles; D008968:Molecular Conformation; D013256:Steroids / Q000737:chemistry*
chemical_list: D001647:Bile Acids and Salts; D008823:Micelles; D013256:Steroids
keywords: aggregation number; conformational analysis; micelle of bile salt; spin–lattice relaxation time; steroid skeleton