Glycine and Sodium Glycinate in Water: Experiment and Molecular Dynamics with Different Model Parameters

Amino acids play a fundamental role in biological processes and have various practical applications. Specifically, they have been considered as promising agents for the preparation of environmentally friendly deep eutectic solvents. To understand the physicochemical behavior of amino acids and their salts on a molecular level, atomistic molecular dynamics (MD) is quite helpful. In this work, glycine and sodium glycinate aqueous solutions were simulated with short-range interactions described by the OPLS-AA force field, using different atomic charge sets for the glycine zwitterion (GLZ) and the glycinate anion (GLA), and different model potentials for Na+. Diffusion in those solutions was measured by the PGSTE NMR method at 298.15 K. A comparison of the MD results with these and other available experimental data shows that to obtain a satisfactory agreement with experiment, one should adjust the model atomic charges in GLZ or GLA as well as the interaction parameters for sodium ions, so that the solute associativity is reproduced properly. For both systems, CNDO charges combined with the Joung-Cheatham model for Na+ work relatively well. MD predicts that in aqueous solutions, GLA anions are less clusterized than GLZ zwitterions, which agrees with the experimental fact that sodium glycinate is much more soluble than glycine.