Investigation on binding between cations and amides using UV Raman spectroscopy

The interaction of proteins with salt ions plays an important role in life activities. We used butyramide as a model molecule to investigate the interaction of protein backbones with cations. The experiment was performed in an aqueous solution of metal chloride using UV Raman spectroscopy. It was found that well-hydrated metal cations (Ca²⁺, Mg²⁺) tend to bind to C=O in the amide bond, resulting in redistribution of the amide I band peaks. Specifically, the peak intensity ratio of 1655 cm^-1 to 1610 cm^-1 increases significantly with increasing concentrations. However, this phenomenon is not obviously observed in NaCl solution. Furthermore, we studied the effect of salt ions on the water structures. The addition of Ca²⁺ and Mg²⁺ is beneficial to the enhancement of the water signal at the 3400 cm⁻¹ position, while the Na⁺ at the same concentration is not obvious. The results have shown that the interaction between cations and amides satisfies the following order: Ca²⁺>Mg²⁺>Na⁺, which conforms to the Hofmeister series.