Interactions of Sucrose and Trehalose with Lysozyme in Different Media: A Perspective from Atomistic Molecular Dynamics Simulations

Disaccharides are promising additives for stabilizing proteins in, e.g., pharmaceuticals and cryopreserved biomaterials. However, although many studies have shown that disaccharides exhibit such bioprotective and stabilizing properties, the underlying molecular mechanism is still elusive. In this study, we have tried to reach such an understanding by studying lysozyme in aqueous solutions of sucrose or trehalose and various ions (0.1 M Cl^-, NaCl, and ZnCl₂) by classical atomistic molecular dynamics (MD). The most important finding for understanding the mechanism of protein stabilization is that the disaccharides, in general, and trehalose, in particular, slow down the protein dynamics by reducing the number of internal hydrogen bonds (both with and without bridging water molecules) in the protein molecules. This reduction of internal protein interactions is caused by disaccharides binding to the protein hydration water, and trehalose forms more hydrogen bonds to water than sucrose. Although it is far from obvious that such a reduction of internal hydrogen bonding in the protein should lead to slower protein dynamics and thereby also a stabilization of the protein, the results show that this is clearly the case. The presence of ions also has some effect on the protein dynamics and stability. Particularly, it is discovered that the ability of sucrose to prevent protein aggregation increases substantially if ZnCl₂ is added to the solution. The disaccharide and the salt seem to exhibit a synergistic effect in this case. To summarize, we have obtained a molecular understanding of protein stabilization by disaccharides, and why trehalose is more effective than sucrose for this particular system, and the finding is important for understanding how the protein stability in, e.g., pharmaceuticals should be optimized.