Effects of Temperature on the Properties of Glycerol: A Computer Simulation Study of Five Different Force Fields

Abstract

We perform molecular dynamics simulations of glycerol (propane-1,2,3-triol) at normal pressure and a wide range of temperatures (300-460 K) and study the sensitivity of simulation results to the force field (FF) considered. We employ five commonly used FFs: (i) AMBER, (ii) CHARMM22, and (iii) three versions of the OPLS-AA FF (OPLS1, OPLS2, and OPLS3). We study thermodynamic (density ρ(T), thermal expansion coefficient αP(T), isobaric specific heat cP(T)), dynamic (diffusion coefficient D(T)), as well as structural properties (molecular conformations and hydrogen-bond statistics). In comparison with experiments, FFs i and iii provide reasonable estimations of ρ(T) with deviations of ≤4.5%; for FF ii, deviations in density are more pronounced, ≤9%. Values of αP(T) vary considerably among the FFs; e.g., deviations are ≤9% for OPLS1-FF and ≤60% for FF ii. For all models studied, values of cP(T) are approximately twice the corresponding experimental values. Diffusion coefficients are very sensitive to the FFs considered. Specifically, for FFs i and ii and OPLS3, the values of D(T) are remarkably close to the experimental values over the whole range of temperatures studied. Instead, in the cases of OPLS1 and OPLS2-FFs, D(T) is underestimated by approximately 2 orders of magnitude. Interestingly, in all cases, D(T) can be well described by a Vogel-Tamman-Fulcher equation, as observed in experiments. We present a detailed characterization of glycerol backbone conformation based on the traditional classification introduced by Bastiansen, defined in terms of glycerol's OCCC dihedral angles. All FFs indicate that the conformer population varies smoothly with temperature. However, the FFs provide very different conformer distributions. This implies that, from the microscopic point of view, these glycerol models may provide very different liquid environments for, for example, guest biomolecules and hence may play a relevant role in interpreting simulation results involving glycerol-based solutions. We also discuss the statistics of inter- and intramolecular hydrogen bonds (HBs). The FFs are qualitatively comparable regarding HB statistics; however, quantitative differences remain. For example, molecules form a total of 5.5-7 HBs at T = 350 K, depending on the FF considered, including at least one intramolecular HB.