Abstract
Lipophilicity of 15 derivatives of sodium cholate, defined by the octan-1-ol/water partition coefficient (log P), has been theoretically determined by the Virtual log P method. These derivatives bear highly hydrophobic or highly hydrophilic substituents at the C3 position of the steroid nucleus, being linked to it through an amide bond. The difference between the maximum value of log P and the minimum one is enlarged to 3.5. The partition coefficient and the critical micelle concentration (cmc) are tightly related by a double-logarithm relationship (), meaning that the Gibbs free energies for the transfer of a bile anion from water to either a micelle or to octan-1-ol differ by a constant. The equation also means that cmc can be used as a measurement of lipophilicity. The demicellization of the aggregates formed by three derivatives of sodium cholate bearing bulky hydrophobic substituents has been studied by surface tension and isothermal titration calorimetry. Aggregation numbers, enthalpies, free energies, entropies, and heat capacities, ΔCP,demic, were obtained. ΔCP,demic, being positive, means that the interior of the aggregates is hydrophobic.
Highlights
Natural bile salts, having a rigid skeleton with a peculiar distribution of their polar and apolar regions in a back-belly way, are surface active agents that self-aggregate in water solutions [1]
Depending on experimental conditions, their aggregates are usually small in size [1,2,3], and the monomers are packed in a back-to-back way with the hydrophobic surface of the steroid towards the aggregate interior and the hydrophilic one facing the solvent [2,4]
The virtual logP [49] is obtained by the molecular lipophilicity potential (MLP), which is calculated by projecting the Broto–Moreau lipophilicity atomic constants on the molecular surface
Summary
Natural bile salts, having a rigid skeleton with a peculiar distribution of their polar and apolar regions in a back-belly way, are surface active agents that self-aggregate in water solutions [1]. Roda et al [8,9] have experimentally determined the log P values for natural bile acids and their taurine and glycine conjugates, showing that it depends on the number, position, and orientation of hydroxyl groups. This dependence is demonstrated by the retention parameters of reverse-phase chromatography [10,11].
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