Microscopic Heterogeneity Driven by Molecular Aggregation and Water Dynamics in Osmolyte-Water Mixtures

Abstract

Living organisms have small organic compounds called osmolytes to modulate protein stability and structure. Numerous experimental and computational works have been performed to understand the mode of action of osmolyte, but their operating mechanism is still inconclusive. The direct mechanism, one of the two primary hypotheses, suggests that protecting osmolytes are likely to be excluded from the protein surface, while the destabilizing osmolytes preferentially form intermolecular interaction with protein molecules. The indirect mechanism focuses on the modulation of water hydrogen bond structure and dynamics in the presence of osmolytes. Herein, we employed a combination of molecular dynamics simulation with graph theory and microheterogeneity measurement in four osmolyte-water binary solutions of dimethylsulfoxide (DMSO), trimethylamine-N-oxide (TMAO), tetramethylurea (TMU), and urea to clarify the operating mechanism of osmolyte on protein stability. The protecting osmolyte, TMAO does not form noticeable self-association aggregates and uniformly distributed even at the microscopic level. On the other and, the destabilizing osmolyte TMU readily associates with neighboring osmolyte molecules, resulting in microheterogeneity of TMU-water solutions. Furthermore, we investigated the water dynamical properties, translational and rotational motion and H-bond lifetime of water in the two representative binary mixtures, TMAO-water and TMU-water mixtures. The translational and rotational motions of water are more significantly retarded in TMAO-water mixtures, exhibiting fully homogeneous distribution, than microscopically heterogeneous TMU-water mixtures. The H-bond lifetime of TMAO-water is much longer than that of TMU-water, which means the protecting osmolyte TMAO forms more tight binding with water molecules. As a result, TMAO has less chance to interact with protein, and TMU is more likely to make hydrophobic interaction around protein surfaces. Taken together, we suggest that the both direct and indirect mechanisms work in combination that encompasses the direct intermolecular interaction between osmolyte and protein and indirect interaction through the alteration of water H-bond network and dynamical properties in osmolyte aqueous solutions.