Cations Bind Only Weakly to Amides in Aqueous Solutions

Abstract

We investigated salt interactions with butyramide as a simple mimic of cation interactions with protein backbones. The experiments were performed in aqueous metal chloride solutions using two spectroscopic techniques. In the first, which provided information about contact pair formation, the response of the amide I band to the nature and concentration of salt was monitored in bulk aqueous solutions via attenuated total reflection Fourier transform infrared spectroscopy. It was found that molar concentrations of well-hydrated metal cations (Ca²⁺, Mg²⁺, Li⁺) led to the rise of a peak assigned to metal cation-bound amides (1645 cm^–1) and a decrease in the peak associated with purely water-bound amides (1620 cm^–1). In a complementary set of experiments, the effect of cation identity and concentration was investigated at the air/butyramide/water interface via vibrational sum frequency spectroscopy. In these studies, metal ion–amide binding led to the ordering of the adjacent water layer. Such experiments were sensitive to the interfacial partitioning of cations in either a contact pair with the amide or as a solvent separated pair. In both experiments, the ordering of the interactions of the cations was: Ca²⁺ > Mg²⁺ > Li⁺ > Na⁺ ≈ K⁺. This is a direct cationic Hofmeister series. Even for Ca²⁺, however, the apparent equilibrium dissociation constant of the cation with the amide carbonyl oxygen was no tighter than ∼8.5 M. For Na⁺ and K⁺, no evidence was found for any binding. As such, the interactions of metal cations with amides are far weaker than the analogous binding of weakly hydrated anions