Cooperative interaction of a highly hydrophilic pluronic with bile salts of different hydrophobicity

Abstract

In this study, we investigated the association of bile salts with pluronic F127 (F127) as a function of their hydrophobicity. Sodium cholate (NaC), sodium deoxycholate (NaDC) and sodium taurodeoxycholate (NaTDC) were used as bile salt. Our interpretations are based on the observations of isothermal titration calorimetry (ITC), high sensitivity differential scanning calorimetry (HSDSC), dynamic light scattering (DLS) and small angle neutron scattering (SANS) experiments. Bile salts affected the aggregation of F127 in accordance to their hydrophobicity, i.e. NaC<NaDC<NaTDC. Thermodynamically stable mixed aggregates were formed with NaC which remained confined at the interface even at high concentration (∼10 mM). On the contrary, hydrophobic NaTDC engaged more profoundly with F127 which manifested into dissociation of aggregates. With 4 mM NaTDC, the number density of aggregates increased more than ten-fold. We argue that dissociation would have occurred because of development of electrostatic repulsive forces in the presence of NaTDC molecules. This could be verified from bile salt driven changes in size distribution, aggregation number and thermodynamic parameters. For instance, upshift in critical micelle concentration and micellization temperature of F127 was recorded depending upon concentration of NaDC and NaTDC. Inspite of this, neutron scattering data ruled out the possibility of morphological transitions during these associations.