Tetrahedrality and structural order for hydrophobic interactions in a coarse-grained water model

Abstract

The hydrophobic interaction manifests two separate regimes in terms of size: Small nonpolar bodies exhibit a weak oscillatory force (versus distance) while large nonpolar surfaces exhibit a strong monotonic one. This crossover in hydrophobic behavior is typically explained in terms of water's tetrahedral structure: Its tetrahedrality is enhanced near small solutes and diminished near large planar ones. Here, we demonstrate that water's tetrahedral correlations signal this switch even in a highly simplified, isotropic, "core-softened" water model. For this task, we introduce measures of tetrahedrality based on the angular distribution of water's nearest neighbors. On a quantitative basis, the coarse-grained model of course is only approximate: (1) While greater than simple Lennard-Jones liquids, its bulk tetrahedrality remains lower than that of fully atomic models; and (2) the decay length of the large-scale hydrophobic interaction is less than has been found in experiments. Even so, the qualitative behavior of the model is surprisingly rich and exhibits numerous waterlike hydrophobic behaviors, despite its simplicity. We offer several arguments for the manner in which it should be able to (at least partially) reproduce tetrahedral correlations underlying these effects.

Unraveling Hydrophobic Interactions at the Molecular Scale Using Force Spectroscopy and Molecular Dynamics Simulations

“…To seek molecular signatures that may delineate distinct regimes of HI physics, we thus characterize hydration water structure at the initial and final equilibrium states of the pulling process (Figure ). Longstanding views have sought to understand HIs in terms of water’s unique tetrahedral correlations, − and to quantify these, we calculate distributions of the angle formed by triplets of coneighboring water molecules within a 5 Å hydration shell of both the peptide and SAM surface. In bulk water this distribution peaks near the tetrahedral angle (109.5°), but surfaces and molecules can either induce enrichment or depletion of tetrahedral, random (∼90°, ideal gas-like), or icosahedral (∼60°, simple liquid-like) populations .…”

Analytical theory of the hydrophobic effect of solutes in water

“…A similar definition of this angle was used by Chaimovich and Shell in their work [47,48]. Integration over all possible orientations of water (over all three Euler angles) is equal to 8 π 2 .…”

Atomistic water models: Aqueous thermodynamic properties from ambient to supercritical conditions

“…The emphasis is solely on models that provide an atomistic description of water. We note that some success has been reported recently (Chaimovich and Shell, 2014) for coarse grain models that lack any atomistic detail. Indeed, some unlikely models (Mausbach and Sadus, 2011) can qualitatively reproduce some of the water anomalies but an atomistic model is usually essential for quantitatively accurate predictions.…”