Effect of salt on the micellisation of bile salts in aqueous medium

Abstract

Bile salts are typical biosurfactants, biosynthesized in liver by the cytochrome P450 mediated oxidation of cholesterol and act as solubilizer and emulsifier for cholesterol and other lipids. Their unique biological importance lies in the fact that they form mixed micelles with otherwise insoluble biological lipids (e.g. lecithin, monoglyceride, fatty acid and cholesterol). Bile salts self-assemble in aqueous solution, similar to classical amphiphiles. In this present study we aim to investigate the effect of different salts on the micellisation of bile salts by UV-Vis. Pyrene has been employed as the probe and two bile salts sodium deoxycholate (NaDC) and sodium cholate (NaC) have been used. The salts used were NaCl, KCl, KBr, CaCl2 and MgCl2. The CMC was calculated to be 6 mM for NaDC and 16 mM for NaC. However the CMC of NaDC shifted from 6 mM to 3 mM in presence of 0.05 M NaCl and to 2 mM in presence of 0.1 M NaCl. Similar observation was obtained for NaC where the CMC shifted from 16 mM in water to 12 mM in presence of 0.05 M NaCl and 10 mM in presence of 0.1 M NaCl. The absorption spectral behavior and CMC change were found to be very similar in case of KCl. Even the effect of KBr on the absorption behavior of pyrene in NaDC solutions was very similar to that of KCl and NaCl. However even a small amount (0.001 M) of CaCl2 or MgCl2 induced turbidity in the bile salt solution. So we confirmed that it is the charge that plays a major role and not the size of the cation or anion.