Local structure of dilute aqueous DMSO solutions, as seen from molecular dynamics simulations

Abstract

The information about the structure of dimethyl sulfoxide (DMSO)-water mixtures at relatively low DMSO mole fractions is an important step in order to understand their cryoprotective properties as well as the solvation process of proteins and amino acids. Classical MD simulations, using the potential model combination that best reproduces the free energy of mixing of these compounds, are used to analyze the local structure of DMSO-water mixtures at DMSO mole fractions below 0.2. Significant changes in the local structure of DMSO are observed around the DMSO mole fraction of 0.1. The array of evidence, based on the cluster and the metric and topological parameters of the Voronoi polyhedra distributions, indicates that these changes are associated with the simultaneous increase of the number of DMSO-water and decrease of water-water hydrogen bonds with increasing DMSO concentration. The inversion between the dominance of these two types of H-bonds occurs around X = 0.1, above which the DMSO-DMSO interactions also start playing an important role. In other words, below the DMSO mole fraction of 0.1, DMSO molecules are mainly solvated by water molecules, while above it, their solvation shell consists of a mixture of water and DMSO. The trigonal, tetrahedral, and trigonal bipyramidal distributions of water shift to lower corresponding order parameter values indicating the loosening of these orientations. Adding DMSO does not affect the hydrogen bonding between a reference water molecule and its first neighbor hydrogen bonded water molecules, while it increases the bent hydrogen bond geometry involving the second ones. The close-packed local structure of the third, fourth, and fifth water neighbors also is reinforced. In accordance with previous theoretical and experimental data, the hydrogen bonding between water and the first, the second, and the third DMSO neighbors is stronger than that with its corresponding water neighbors. At a given DMSO mole fraction, the behavior of the intensity of the high orientational order parameter values indicates that water molecules are more ordered in the vicinity of the hydrophilic group while their structure is close-packed near the hydrophobic group of DMSO.

Microstructural free volume and dynamics of cryoprotective DMSO–water mixtures at low DMSO concentration

“…Based on modeling, the congurations of different types of clusters have been shown. [9][10][11][12] The expanded range of the glass transition of water was found via the molecular dynamics annealing simulations. This would correspond to the formation of a stronger glass in water-DMSO mixtures 13 and it brings some new insights into the mechanism of cryoprotection at the molecular level.…”

Is Dissociation of HCl in DMSO Clusters Bistable?

“…Binary mixtures of DMSO and other co-solvents are also known to be heterogeneous. [10][11][12][13] In the case of DMSO-water mixture, the maximum deviation from the ideal mixing behavior occurs around 33% mole fraction of DMSO. 10 The existence of different types of aggregates in the DMSO-water mixture is believed to be the reason behind this heterogeneity.…”

“…It has been reported that acid will get dissociated in a particular solvent if the stability of the dissociated acidsolvent cluster is greater than the stability of the undissociated acid-solvent cluster. [11][12][13][14][15][16] However, it was shown that the acid dissociation mechanism of HCl in water and reported that the stabilization energy of the HCl-water cluster is not the only criteria for the acid to be dissociated in water, and the orientation of the water molecules around the HCl molecule also plays a pivotal role in the acid dissociation process. 14 It was also reported that in some HCl-water clusters the HCl remains undissociated, even though in those structures the number of water molecules and their stabilization energy were almost identical to the dissociated HCl-water clusters.…”

“…Using a combination of infrared absorption spectroscopy, molecular dynamics, and quantum chemistry models, it has been shown that in a DMSO/water mixture, water prefers to form hydrogen bonds with the aggregated species over free DMSO . Further, the binary mixtures of DMSO and other co-solvents such as water, methanol, and glycerol are known to be heterogeneous. − …”

Is Dissociation of HCl in DMSO Clusters Bistable?

“…Binary mixtures of DMSO and other co-solvents are also known to be heterogeneous. [10][11][12][13] In the case of DMSO-water mixture, the maximum deviation from the ideal mixing behavior occurs around 33% mole fraction of DMSO. 10 The existence of different types of aggregates in the DMSO-water mixture is believed to be the reason behind this heterogeneity.…”