Thermodynamic studies on the interaction of n-alkyltrimethylammonium bromides with anionic polypeptides in aqueous solution

Abstract

The enthalpies of interaction of a homologous series of n-alkyltrimethylammonium bromides (C12, C10 and C8 TABs) with poly(L-glutamate) and poly(L-aspartate) in water at 25 °C have been measured over a range of TAB concentrations up to approximately the critical micelle concentrations. The interactions are all endothermic and for the C12 and C10 TABs the enthalpies go through maxima as a function of concentration. The maxima are attributed to the onset of an exothermic contribution arising from the aggregation of polypeptide–TAB complexes in the case of the C12 and C10 TABs, evidence for which comes from the onset of turbidity due to the presence of large particles in the suspensions detectable by photon correlation spectroscopy. For the C12 TAB, enthalpy measurements at pH 3.2 show that the enthalpy is greatly reduced, suggesting that a major contribution to the enthalpy arises from interaction between the carboxylate side chains of the polypeptides and the TAB head group. Binding of the C12 and C10 TABs to the polypeptide has been measured by equilibrium dialysis and has been used to calculate the Gibbs energy of binding per TAB molecule. At low levels of binding, in the approximate range 0 to 0.5 TAB molecules per polypeptide residue, the calorimetric data in combination with the Gibbs energies of binding have been used to estimate the entropic contribution to binding. The entropies of binding are positive for all the systems suggesting dehydration and hydrophobic contributions to the interactions between the TABs and the anionic polypeptides.