The influence of NaCl on hydrophobicity of selected, pharmacologically active bile acids expressed with chromatographic retention index and critical micellar concentration

Many of bile acids’ (BA) physiological properties, as receptor binding, activation of ionic channels, binding to blood proteins, etc. are due to their hydrophobicity. On the other hand, hydrophobicity determines BAs’ physico-chemical characteristics as micelle forming and adsorption (surface activity). However, BA hydrophobicity is not determined solely by their structure. Medium composition, especially the concentration of electrolytes has influence on BA hydrophobicity. Thus, the objective of this work was to examine the effect of NaCl on hydrophobicity of selected bile acids. This influence is specified with the retention factor k (reversed phase high pressure liquid chromatography (RPHPLC)) and critical micellar concentration (CMC) determined by non-invasive NMR method. The value of ln k elevates with the increase in mobile phase NaCl concentration i.e. Δ ln k/ΔcNaCl depends on the number of water molecules not stabilised by hydrogen bonds in bile acid hydration sheath. For bile acids that contain hydroxyl groups (except those with β equatorial hydroxyl groups) the value of |Δ ln CMC/ΔcNaCl| rises with the increase in the number of non-stabilized water molecules in their hydration sheath. Even though oxo derivatives of cholic acid have similar chromatographic parameters they behave differently when it comes to CMC. In fact with the introduction of oxo groups the value of its |Δ ln CMC/ΔcNaCl| elevates but it results in a decrease in the number of non-stabilized water molecules i.e. hydrophobicity falls. Different behaviour of oxo derivatives implicate that, besides “hydrophobic interactions” in their micelles, there are also hydrogen bonds i.e. fiord effect exists.