The application of Kirkwood-Buff theory to study hydration properties of $\alpha$-amino acids

TL;DR

This study combines experimental measurements and Kirkwood-Buff theory-based modeling to analyze hydration properties of amino acids across temperatures, providing insights into protein stability.

Contribution

It applies Kirkwood-Buff theory with a coarse-grained model to accurately predict amino acid hydration properties and their temperature dependence.

Findings

Standard molar volume increases with temperature.

Model reproduces experimental molar volumes accurately.

Hydration number decreases at higher temperatures.

Abstract

Protein conformational stability and function depend on non-covalent interactions that are strongly influenced by the surrounding environment. To explore protein properties, amino acids are often utilized as model systems. In this study, we determined the densities of seven $\alpha$-amino acids in aqueous solutions between 278.15 K and 308.15 K and calculated the apparent molar volumes. Linear extrapolation yielded standard molar volumes, which were analyzed to characterize amino-acid hydration. The contributions of side chains to the standard molar volume were determined relative to glycine. The standard molar volume increased with temperature, indicating reduced electrostriction of water around the amino acids, consistent with lower hydration numbers at higher temperatures. We employed the Ornstein-Zernike integral equation with hypernetted-chain closure and a coarse-grained Lennard-Jones bead model to calculate pair correlation functions and Kirkwood-Buff integrals, from which standard molar volumes were obtained. The model reproduced the experimental standard molar volumes very well.