Effects of the translational and rotational degrees of freedom on the hydration of simple solutes

Abstract

Molecular dynamics simulations with separate thermostats for rotational and translational motion were used to study the effect of these degrees of freedom on the structure of water around model solutes. To describe water molecules we used the SPC/E model. The simplest solute studied here, the hydrophobe, was represented as a Lennard-Jones particle. Since direct interaction between the hydrophobe and water molecules has no angular dependence the influence of the increase of the rotational temperature on the solvation of a hydrophobe is only indirect. In the next step the central solute was assumed to be charged with either a positive or a negative charge to mimic an ion in water. Hence, depending on the charge of the ion, the neighboring water molecules assumed different angular distributions. The principal conclusions of this work are: (i) an increase of the translational temperature always decreases the height of the first peak in the solute-water radial distribution function; (ii) an increase of the rotational temperature yields an increase in the first peak in the solute-water radial distribution function for hydrophobes and cations; (iii) in contrast to this, the solvation peak decreases around ions with sufficiently large negative charge; and (iv) an increase of the rotational temperature affects cations in an opposite way to anions. For this reason complex molecules with a small net charge may not be very sensitive to variation of the rotational temperature. © 2014 AIP Publishing LLC. [http://dx

Effects of Translational and Rotational Degrees of Freedom on the Hydration of Ionic Solutes as Seen by Popular Water Models

“…A simple spherical cut-off with a distance 9 Å was used for Lennard-Jones, as well as, for electrostatic interactions. 15 We have verified, that results did not change upon an increase of the cut-off distance.…”

“…Our study also indicated that charge distribution on a solvent molecule could be important. This prompted us to explore to what degree the results obtained in references 14,15 depend on the chosen model of water. The question we wish to answer here is the following: can we expect the same trends as obtained for SPC/E also for other water models, or the results are model-dependent?…”

“…Because the velocity rescaling in every time step provides better control over the temperature values, we adopted the latter approach. 15 We applied the same protocol for the equilibration and production periods.…”

“…Equations of motion were solved separately for translational (centre-ofmass positions) and rotational (orientations) degrees of freedom, which enabled us to couple them to separate thermostats. As before 14,15 we used the velocity rescaling procedure at every time step to keep the two temperatures fixed. We carefully verified that results were not affected by the way how we thermostatted the system: temperature can be rescaled in every time step, or it can be held constant on the average.…”

“…In our previous study we used this approach 14 to simulate the solvation of cations and anions (the latter were not studied before) for a range of conditions. 15 We used the molecular dynamics simulations with separate thermostats to examine effects of translational and rotational degrees of freedom on the structure of water in solution.…”