Micellization parameters (number average, aggregation number and critical micellar concentration) of bile salt 3 and 7 ethylidene derivatives: Role of the steroidal skeleton II

• C3 and C7 ethylidene derivatives of bile salts have aggregation numbers in the range from 2.4 to 14.8.
• There is a linear expression between n¯ and ln k, deviation indicates GE (H-bonds).
• Relative increase of n¯ is stronger when ethylidene group is introduced in C7 position.
• Bile salts with GE (H-bonds) must have C3-α-(e)-OH and C12-α-(a)-OH groups.

BackgroundBile salts are steroidal biosurfactants. Micellar systems of bile salts are not only important for solubilization of cholesterol, but they also interact with certain drugs thus changing their bioavailability.MethodsThe number-average aggregation numbers (n¯) are determined using the Moroi–Matsuoka–Sugioka thermodynamic method. Critical micellar concentrations were determined by spectrofluorometric method using pyren and by surface tension measurements.ResultsMicelles of ethylidene derivatives possess the following values for n¯: 7-Eth-D (n¯=11 (50 mM)–n¯=14.8 (100 mM)); 12-Ox-7-Eth-L (n¯≈8.8, without concentration dependence) and 7,12-diOx-3-Eth-Ch (n¯≈2.9, without concentration dependence). In the planes n¯–ln k and ln CMC–ln k derivative 7-Eth-D is outlier in respect to hydrophobic linear congeneric groups.ConclusionGibbs energy of formation for 7-Eth-D anion micelles in addition to the Gibbs energy of hydrophobic interactions consists excess Gibbs energy (GE) from hydrogen bond formation between building blocks of micelles. Gibbs energy of formation for 7,12-diOx-3-Eth-Ch and 12-Ox-7-Eth-L anion micelle is determined by the Gibbs energy of hydrophobic interactions. Relative increase in hydrophobicity and aggregation number for ethylidene derivatives is larger when ethylidene group is introduced from the C7 lateral side of steroidal skeleton then it is when ethylidene group is on C3 carbon.General significancePosition of outlier towards hydrophobic congeneric groups from n¯–ln k and ln CMC–ln k planes indicates the existence of excess Gibbs energy (GE) which is not of hydrophobic nature (formation of hydrogen bonds). For the bile salt micelles to have GE (formation of secondary micelles) it is necessary that steroidal skeleton possesses C3-α-(e)-OH and C12-α-(a)-OH groups.

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