Simple and Cooperative Electrostatic Binding of Ammonium Ions to Phosphate Polyions: NMR, Infrared, and Theoretical Study

Abstract

Electrostatic binding of methylammonium chloride (MAC), tetramethylammonium chloride (TMAC), and N-(tert-butoxycarbonyl)-1,6-diaminohexane hydrochloride (HAC) with phosphate glass (PG) in D2O was studied using 1H, 2H, 14N, and 23Na NMR spectrometry, relaxometry and pulsed-field-gradient diffusion measurements, Fourier transform infrared spectroscopy, and quantum mechanical calculations. MAC, TMAC, and HAC were used as models of the functional groups of polylysine hydrochloride and its N-dimethyl analogue, PG (polymerization degree 45) as a polyanion with phosphate groups, a very approximate model of DNA. It was found that binding of both MAC and TMAC obeys simple laws of chemical equilibria, without any sign of cooperative behavior. In both cases, binding is slightly exothermic and entropy demanding. Its equilibrium is dynamic, with a fast exchange between bound and free ammonium cation. Inspection of 1H NMR and in particular infrared spectra and comparison with theoretically predicted vibration frequencies shows that the bound states of MAC and TMAC are hydrated ion pairs, in which the complementary ions avoid direct contact. Nevertheless, binding of TMAC is weaker due to steric hindrances, which apparently surpass the effect of a more localized charge in this compound. In the case of HAC, binding becomes cooperative at higher concentrations and temperatures. The cause of this behavior is shown to be hydrophobic interactions of its aliphatic parts. According to 2H quadrupolar relaxation, the driving force of this cooperativeness is the liberation of water molecules into a more disordered state.