Microcalorimetry study of the interactions between Poly(N-isopropylacrylamide) microgels and amino acids

Here, we present the first direct measurement of the binding enthalpy between two model amino acids, valine and aspartic acid, and poly(N-isopropylacrylamide)(PNIPA) microgels. Equilibrium binding isotherms of the two acids showed enhanced binding with a temperature elevation from 25 to 37°C. The binding of aspartic acid was somewhat higher than valine at 25°C, whereas at 37°C, the binding of valine was higher because of its more hydrophobic nature. Isothermal titration calorimetry measurements showed that the binding of valine was endothermic, which implies an entropy-driven binding mechanism. Contrary, the binding of aspartic acid was exothermic, suggesting a binding mechanism based on interactions such as hydrogen bonding. Differential scanning calorimetry measurements detected a 0.4°C lowering of the gel phase transition temperature in dilute solutions of either valine or aspartic acid relative to buffer. Our results show that a direct measurement of the binding enthalpy gives further insight into the balance of binding mechanisms between biochemicals and PNIPA gel.