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. 2005 May 10;102(19):6777-82.
doi: 10.1073/pnas.0500225102. Epub 2005 May 2.

Nonpolar solutes enhance water structure within hydration shells while reducing interactions between them

Tanya M Raschke  1 , Michael Levitt
Affiliations

Nonpolar solutes enhance water structure within hydration shells while reducing interactions between them

Tanya M Raschke et al. Proc Natl Acad Sci U S A. .

Abstract

The origins of the hydrophobic effect are widely thought to lie in structural changes of the water molecules surrounding a nonpolar solute. The spatial distribution functions of the water molecules surrounding benzene and cyclohexane computed previously from molecular dynamics simulations show a high density first hydration shell surrounding both solutes. In addition, benzene showed a strong preference for hydrogen bonding with two water molecules, one to each face of the benzene ring. The position data alone, however, do not describe the majority of orientational changes in the water molecules in the first hydration shells surrounding these solutes. In this paper, we measure the changes in orientation of the water molecules with respect to the solute through spatial orientation functions as well as radial/angular distribution functions. These data show that the water molecules hydrogen bonded to benzene have a strong orientation preference, whereas those around cyclohexane show a weaker tendency. In addition, the water-water interactions within and between the first two hydration shells were measured as a function of distance and "best" hydrogen bonding angle. Water molecules within the first hydration shell have increased hydrogen bonding structure; water molecules interacting across shell 1 and shell 2 have reduced hydrogen bonding structure.

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Figures

Fig. 1.
Fig. 1.
SOFs of the water density surrounding benzene and cyclohexane. Slices through the volumes surrounding benzene and cyclohexane are shown. Projections of formula image, the time-averaged water orientation vector (see schematic in Fig. 2 A), into the respective planes are drawn as arrows. The tail of each arrow originates from its corresponding voxel position, and the arrow length is proportional to the projection of formula image. Only the vectors with length greater than one standard deviation above the mean vector length in the dataset are graphed. A unit vector is drawn in the lower left corner for reference. The regions corresponding to the first hydration shells of the SDFs (above the ρO = 1.1 density contour) are highlighted in yellow (18). (A and B) The slice along the z–x plane for benzene (A) and cyclohexane (B). (C and D) The slice along the y–x plane for benzene (C) and cyclohexane (D).
Fig. 2.
Fig. 2.
2D solute centroid to water oxygen radial distribution functions for benzene and cyclohexane. The radial-orientation distribution function, gSol-O(R, cos ω) (where R is the distance to the center of the solute and the tilt angle ω is defined in A) averaged over the axial and equatorial volume regions (as defined in B) surrounding benzene are shown in C and D, respectively. Contours are drawn every 0.1 density unit relative to B (up to 3.0, and every 2.0 units between 4.0 and 20.0). The same functions for the axial (E) and equatorial (F) regions surrounding cyclohexane are also shown. Schematic diagrams show the preferred orientation of water molecules located axial to benzene (G) and equatorial to cyclohexane (H).
Fig. 3.
Fig. 3.
Illustration of hydration shell assignments by the DC and VP methods. Shown is a snapshot of a 5-Å slice through the benzene simulation box. The benzene C atoms are gray, and the surrounding water O atoms are colored by class: S1, blue; S2, green; B, red. (A) Shell assignments based on the DC method. The black “circles” are arcs marking distances to the nearest solute C atom at the cutoff radii of 5.4 Å and 8.9 Å for S1 and S2, respectively. (B) Shell assignments based on the VP method. The 2D Voronoi tessellation of the projection into a plane illustrates how the box volume is divided into cells. Shell labels were based on contacts defined by the polyhedron faces (Computational Methods). The shell labels for individual water molecules may differ between the two methods.
Fig. 4.
Fig. 4.
Water–water contact length/angle distributions within and between the hydration shells of benzene. The distributions of water interaction geometries as a function of O···O distance (r) and minimum O···O–H angle (θ) between contacting water molecules are shown for five sets of inter- and intrashell interactions computed from the benzene simulations. The classes of interactions are S1–S1 (A and F), S1–S2 (B and G), S2–S2 (C and H), S2–B (D and I), and B–B (E and J). (Left) Shells determined by the DC method (A–E) and contours showing gOO(r, θ). (Right) Shells and contacts determined by the VP method (F–J). In this case, the contours show the probability of observing contacts with particular geometries, ρ(r, θ). Angle, θ, measures the deviation from linearity of the least distorted H-bond.
Fig. 5.
Fig. 5.
A water “triangle” that occurs commonly in pure water. It explains both peaks in Fig. 4A. This figure also illustrates how the O···O distance (r) and minimum O···O–H angle (θ) are defined.
Fig. 6.
Fig. 6.
Water–water radial distribution functions and contact angle distributions for the hydration shells of benzene determined by the DC and VP methods. The 2D distributions (Fig. 4) were summed over θ for each contact length, r, to give gOO(r) and ρ(r) (A and B). The 2D distributions were also summed over r for each contact angle, θ, to give gOO(θ) and ρ(θ)(G and H). Shell assignments were determined by the DC method (Left) or the VP method (Right). We distinguish by color five sets of interactions between water molecules: S1–S1, red; S1–S2, blue; S2–S2, green; S2–B, blue; and B–B purple. The Inset in A gives the unnormalized distributions of contact counts vs. r for the DC (solid line) and VP (dashed line) methods for the S1–S1 interactions, confirming the overlap of the two methods. (C and D) Length distributions for shells surrounding a single water molecule from control simulations of pure TIP4P water. (E and F) Benzene length distributions normalized by dividing by the TIP4P results.
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