Salting-Out of Methane in the Aqueous Solutions of Urea and Glycine–Betaine

Abstract

We have studied the hydrophobic association and solvation of methane molecules in aqueous solutions of urea and glycine betaine (GB). We have calculated the potentials of mean force (PMFs) between methane molecules in water, aqueous GB, aqueous urea and aqueous urea-GB mixtures. The PMFs and equilibrium constants indicate that both urea and GB increase the hydrophobic association of methane. Calculation of thermodynamic parameters shows that the association of methane is stabilized by entropy whereas solvation is favored by enthalpy. In the case of the water-urea-GB mixture, both hydrophobic association and solvation are stabilized by entropy. From the investigation of radial distribution functions, running coordination numbers and excess coordination numbers, we infer that both urea and GB are preferentially excluded from methane surface in the mixtures of osmolytes and methane is preferentially solvated by water molecules in all the mixtures. The favorable exclusion of both urea and GB from the methane surface suggests that both urea and GB increase the interaction between methane molecules, i.e., salting-out of methane. We observe that addition of both urea and GB to water enhances local water structure. The calculated values of diffusion constants of water also suggest enhanced water-water interactions in the presence of urea and GB. The calculated free energies of methane in these mixtures show that methane is less soluble in the mixtures of urea and GB than in water. The data on solvation free energies support the observations obtained from the PMFs of methane molecules.

Salting-out of methane in the aqueous solutions of urea and sarcosine

“…However sarcosine decreases the depth of the first minimum while glycine-betaine increases the depth. 66 The location of peaks and heights in the RDFs of water match with the earlier reported values. 38 This synergistic effect of sarcosine can be further explained by analyzing hydrogen bond properties in the watersarcosine mixture.…”

“…Glycine-betaine also enhances the association of methane i.e., saltingout of methane. 66 The preferential solvation of methane by water over sarcosine is supported by the RCN values. The preferential binding constants for methane in water-sarcosine further support this observation.…”

“…A similar trend have been observed in the water-glycine betaine solutions. 66 As we go from water to water-urea mixture, the depth of CM changes from −2.3 to −3.2 kJ mol −1 and the height of transition state barrier with respect to CM changes from 3.7 to 4.5 kJ mol −1 . The depth of SSM changes from −0.4 to −0.3 kJ mol −1 .…”

Ammonium based stabilizers effectively counteract urea-induced denaturation in a small protein: insights from molecular dynamics simulations

“…82 The hydrophobic association of methane molecules in presence of urea and glycine betaine (GB) studied by Dixit et al had OPLS derived parameters for urea in it. 83 The synergistic behaviour shown by glycine betaine-urea mixture on a small model peptide by Kumar and Kishore 84 indicates exclusion of GB and decrease in interaction of the peptide surface with the urea molecules that leads to the structural stability of the peptide. The OPLS model of urea has been adopted in this study.…”

Unzipping of Double-Stranded Ribonucleic Acids by Graphene and Single-Walled Carbon Nanotube: Helix Geometry versus Surface Curvature

“…This qualitatively illustrates a relatively higher propensity of neucleobase stacking to the graphene in the case of XNA as compared to that for RNA. We mention here that peaks other than contact pair minima having negative values in a typical potential of mean force (PMF) curve usually indicate the presence of solvent separated minima for association between two hydrophobic molecules 78 in water and for ion pairs in solutions containing more than one kind of solvent. 79 In our case, however, the sticky ends and the unzipped nucleobases interact with graphene and these give rise to very rugged free energy profile of the duplex nucleic acids during the simulation.…”