THERMODYNAMIC STUDIES ON THE INTERACTION OF N-ALKYLTRIMETHYLAMMONIUM BROMIDES WITH ANIONIC POLYPEPTIDES IN AQUEOUS-SOLUTION

The enthalpies of interaction of a homologous series of n-alkyltrimeth ylammonium bromides (C12, C10 and C8 TABs) with poly(L-glutamate) and poly(L-aspartate) in water at 25-degrees-C have been measured over a r ange of TAB concentrations up to approximately the critical micelle co ncentrations. The interactions are all endothermic and for the C12 and C10 TABs the enthalpies go through maxima as a function of concentrat ion. The maxima are attributed to the onset of an exothermic contribut ion 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, entha lpy measurements at pH 3.2 show that the enthalpy is greatly reduced, suggesting that a major contribution to the enthalpy arises from inter action between the carboxylate side chains of the polypeptides and the TAB head group. Binding of the C12 and C10 TABs to the polypeptide ha s been measured by equilibrium dialysis and has been used to calculate the Gibbs energy of binding per TAB molecule. At low levels of bindin g, in the approximate range 0 to 0.5 TAB molecules per polypeptide res idue, the calorimetric data in combination with the Gibbs energies of binding have been used to estimate the entropic contribution to bindin g. The entropies of binding are positive for all the systems suggestin g dehydration and hydrophobic contributions to the interactions betwee n the TABs and the anionic polypeptides.