Influence of Ionic Strength on Hydrophobic Interactions in Water: Dependence on Solute Size and Shape

Abstract

Hydrophobicity is a phenomenon of great importance in biology, chemistry, and biochemistry. It is defined as the interaction between nonpolar molecules or groups in water and their low solubility. Hydrophobic interactions affect many processes in water, for example, complexation, surfactant aggregation, and coagulation. These interactions play a pivotal role in the formation and stability of proteins or biological membranes. In the present study, we assessed the effect of ionic strength, solute size, and shape on hydrophobic interactions between pairs of nonpolar particles. Pairs of methane, neopentane, adamantane, fullerene, ethane, propane, butane, hexane, octane, and decane were simulated by molecular dynamics in AMBER 16.0 force field. As a solvent, TIP3P and TIP4PEW water models were used. Potential of mean force (PMF) plots of these dimers were determined at four values of ionic strength, 0, 0.04, 0.08, and 0.40 mol/dm³, to observe its impact on hydrophobic interactions. The characteristic shape of PMFs with three extrema (contact minimum, solvent-separated minimum, and desolvation maximum) was observed for most of the compounds for hydrophobic interactions. Ionic strength affected hydrophobic interactions. We observed a tendency to deepen contact minima with an increase in ionic strength value in the case of spherical and spheroidal molecules. Additionally, two-dimensional distribution functions describing water density and average number of hydrogen bonds between water molecules were calculated in both water models for adamantane and hexane. It was observed that the density of water did not significantly change with the increase in ionic strength, but the average number of hydrogen bonds changed. The latter tendency strongly depends on the water model used for simulations.

Figure 1

Overlap PMF of adamantane at ionic strength 0 mol/dm³ for 6250 (25%), 12,500 (50%), 18,750 (75%), and 25,000 (100%) configurations in (a) TIP3P and (b) TIP4PEW water models.

Figure 2

PMFs at different ionic strength values for adamantane dimer in (a) TIP3P and (b) TIP4PEW water models and for hexane dimer in (c) TIP3P and (d) TIP4PEW water models.

Figure 3

Dependence of depth of contact minima (with standard deviations as error bars) in PMF at different values of ionic strength on the number of carbon atoms in spherical molecules in (a) TIP3P and (b) TIP4PEW water models and for spheroidal molecules in (c) TIP3P and (d) TIP4PEW water models.

Figure 4

Dependence of desolvation energy barrier (with standard deviations as error bars) calculated as a height of desolvation maximum (counting from baseline y = 0) on the number of carbon atoms in spherical molecules in (a) TIP3P and (b) TIP4PEW water models and calculated as difference between the CM depth and the height of desolvation maximum on the number of carbon atoms in spherical molecules in (c) TIP3P and (d) TIP4PEW water models.

Figure 5

Distribution of average number of hydrogen bonds between water molecules in the vicinity of adamantane dimer at IS = 0 mol/dm³ at distances between solute particles of (a) 6.6 Å (CM) and (b) 10.0 Å (SSM) and at IS = 0.40 mol/dm³ at distances between solute particles (c) 6.6 Å (CM) and (d) 10.1 Å (SSM) in TIP3P water model. The color scale is shown above the maps, and the average number of H-bonds for bulk water is displayed as white.

Figure 6

Distribution of average number of hydrogen bonds between water molecules in the vicinity of adamantane dimer at IS = 0 mol/dm³ at distances between solute particles of (a) 6.7 Å (CM) and (b) 9.8 Å (SSM) and at IS = 0.40 mol/dm³ at distances between solute particles of (c) 6.7 Å (CM) and (d) 9.7 Å (SSM) in TIP4PEW water model. The color scale is shown above the maps, and the average number of H-bonds for bulk water is displayed as white.

Figure 7

Distribution of average number of hydrogen bonds between water molecules in the vicinity of hexane dimer at IS = 0 mol/dm³ at distances between solute particles of (a) 4.8 Å (CM) and (b) 7.8 Å (SSM) and at IS = 0.40 mol/dm³ at distances between solute particles of (c) 4.7 Å (CM) and (d) 7.7 Å (SSM) in TIP3P water model. The color scale is shown above the maps, and the average number of H-bonds for bulk water is displayed as white.

Figure 8

Distribution of average number of hydrogen bonds between water molecules in the vicinity of hexane dimer at IS = 0 mol/dm³ at distances between solute particles of (a) 4.9 Å (CM) and (b) 7.9 Å (SSM) and at IS = 0.40 mol/dm³ at distances between solute particles of (c) 4.9 Å (CM) and (d) 7.9 Å (SSM) in TIP4PEW water model. The color scale is shown above the maps, and the average number of H-bonds for bulk water is displayed as white.