Microcalorimetric study of the interaction of 1-hexanol with dimyristoylphosphatidylcholine vesicles

Abstract

Partitioning of 1-hexanol into dimyristoylphosphatidylcholine (DMPC) small unilamellar vesicles (SUVs) has been evaluated by isothermal titration calorimetry (ITC) at various temperatures, the so-called solvent-null method has been used for this purpose. Complementary measurements with differential scanning calorimetry on multilamellar vesicles (MLVs) show that the pre-transition disappears and the main-transition temperature is lowered by the addition of hexanol. Cooling scans show hysteresis which increases rapidly over 5 mM hexanol (molar ratio hexanol to lipid ≥1.5). There are indications for the presence of an interdigitated phase over this concentration in the system, however, this has not been confirmed in this work. The corresponding phenomena in SUVs is not as explicit but it is clear that the behaviour of the two types of vesicles is different. Implications of analysing data in terms of bulk concentrations of alcohol or ratios have been discussed. Light scattering does not indicate any significant change in the size of the SUVs by the action of hexanol in the range of molar ratio hexanol to DMPC 0.08–0.35. Molal and mole-fraction based partition coefficients have been calculated from the solvent-null experiments and compared with the literature values. Differences in the values have been attributed primarily to the type of vesicles used in the measurements since the SUVs present the lipids in a highly curved, defect-dense, enthalpically higher state as compared with the MLVs. The free energy, enthalpy and entropy of transfer of hexanol to lipid phase has been calculated, the entropy of transfer indicates a hydrophobic interaction driven process. These transfer parameters have been compared with dissolution and show that transfer from aqueous to the lipid phase is favoured over transfer to a bulk hydrocarbon phase. A van't Hoff analysis suggests that the interaction of hexanol to SUVs above 25°C is co-operative while a simple binding interaction can be used to describe the interaction below this temperature. The solvent-null method has been found to be useful in the study of drug-membrane interactions for non-saturable and non-specific interactions.