Abstract
One of the definitions of hydrophobic interactions is the aggregation of nonpolar particles in a polar solvent, such as water. While this phenomenon appears to be very simple, it is crucial for many complex processes, such as protein folding, to take place. In this work, the hydrophobic association of adamantane and hexane at various temperatures and ionic strengths was studied using molecular dynamics simulations with the AMBER 16.0 program and the GAFF force field. The potentials of mean force of hydrophobic dimer formation, as well as the excess free energy, excess energy, excess entropy, and excess heat capacity corresponding to the formation of the contact minimum, were determined and analyzed. For both systems, the depth of the contact minimum in the potential of mean force was found to increase with both temperature and ionic strength. The excess heat capacity of the association at the contact minimum and T = 298 K was found to be negative and to decrease, while the excess entropy and energy were found to be positive and to increase for both systems, the changes being more pronounced for the hexane dimer. The excess heat capacity is also greater in absolute value for the hexane dimer.
Highlights
One of the definitions of hydrophobic interactions is the propensity of nonpolar particles to aggregate in a polar solvent, such as water.[1]
We used umbrella-sampling molecular dynamics (MD) simulations to determine the potential of mean force (PMF) of pairs of hydrophobic solutes
The PMF plots in the distance between the mass centers of the hydrophobic particles at different ionic strength values at nine temperatures (273, 285, 298, 310, 323, 335, 348, 360, and 373 K) and ionic strength (0, 0.4, 1.0, 1.5, and 2.0 mol/dm3) are plotted as shown in Figures 1 and 2 for adamantane and hexane dimers, respectively, panels A−E of each figure correspond to a different ionic strength value
Summary
One of the definitions of hydrophobic interactions is the propensity of nonpolar particles to aggregate in a polar solvent, such as water.[1]. The solvation-free energy could be considered as the major driving force for the self-assembly of particles.[1,3] Depending on the size of interacting nonpolar molecules, the differences in the water structure near the solute molecules could be observed. In the case of small particles (with a radius less than 1 nm), hydration leads to the reorganization of water molecules into ordered structures around a solute, which is linked with a hydrogen-bond network. The solvation entropy of small hydrophobic molecules is negative and that of larger molecules is positive. In this context, the association of small solutes is entropically driven, while that of larger solutes is driven by enthalpy.[3−5]
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