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. 2011 Oct 25;108(43):17678-83.
doi: 10.1073/pnas.1110703108. Epub 2011 Oct 10.

Extended surfaces modulate hydrophobic interactions of neighboring solutes

Amish J Patel  1 , Patrick Varilly, Sumanth N Jamadagni, Hari Acharya, Shekhar Garde, David Chandler
Affiliations

Extended surfaces modulate hydrophobic interactions of neighboring solutes

Amish J Patel et al. Proc Natl Acad Sci U S A. .

Abstract

Interfaces are a most common motif in complex systems. To understand how the presence of interfaces affects hydrophobic phenomena, we use molecular simulations and theory to study hydration of solutes at interfaces. The solutes range in size from subnanometer to a few nanometers. The interfaces are self-assembled monolayers with a range of chemistries, from hydrophilic to hydrophobic. We show that the driving force for assembly in the vicinity of a hydrophobic surface is weaker than that in bulk water and decreases with increasing temperature, in contrast to that in the bulk. We explain these distinct features in terms of an interplay between interfacial fluctuations and excluded volume effects--the physics encoded in Lum-Chandler-Weeks theory [Lum K, Chandler D, Weeks JD (1999) J Phys Chem B 103:4570-4577]. Our results suggest a catalytic role for hydrophobic interfaces in the unfolding of proteins, for example, in the interior of chaperonins and in amyloid formation.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
Size-dependent hydrophobic hydration at interfaces. (A) A schematic of a cuboidal cavity (green) at the SAM–water interface. The SAM head groups (black and white), alkane tails (gray), and water (red and white, partially cut out for clarity) are shown. (B) A typical configuration of the model membrane, color coded by its distance from the model surface (gray). (C) Important volumes in estimating the free energy, μex, of emptying the probe volume V (green) using the theoretical model. The region above the membrane is the volume B (blue), and the intersection of V and B is v (dark green). (D) Length-scale dependence of the cavity hydration free energy per unit area, μex/A, in bulk water and at interfaces, at T = 300 K, obtained from molecular dynamics simulations. (E) Theoretical model estimates of μex/A, near surfaces with different attraction strengths, η. (F) Connecting the microscopic binding free energy of a cavity to an interface, to the macroscopic surface wettability. The cos θ values were obtained from molecular dynamics simulations of a water droplet on SAM surfaces (10). Lines are predictions using Eq. 1 with size-dependent γLV taken from D.
Fig. 2.
Fig. 2.
Temperature dependence of μex in bulk water and at SAM–water interfaces for large (L = 3.0 nm) cavities (A and B) and for small (L = 0.5 nm, L = 0.75 nm) cavities (C and D), obtained from simulations (A, C) and from the model (B, D) of Eq. 5.
Fig. 3.
Fig. 3.
Length-scale dependence of the excess solvation entropy per unit surface area for (A) cavities in bulk water and at the -CH3 and -OH SAM–water interfaces, and (B) cavities in the model of Eq. 5 near surfaces of different attraction strengths, η.
Fig. 4.
Fig. 4.
Schematic illustrating the thermodynamics of binding and assembly. The points represent free energies of solvating small objects individually (Left) and in the assembled state (Right), in bulk (Top) and at a hydrophobic interface (Bottom), at a lower (blue, TL) and a higher (red, TH) temperature near ambient conditions. Assembly: The driving force for assembly at hydrophobic interfaces is smaller than that in bulk, and is enthalpic, decreasing with increasing temperature, unlike in bulk. Binding: The driving force for binding small objects to a hydrophobic surface increases with temperature, so it is entropic, whereas for large objects, it is enthalpic.
Fig. 5.
Fig. 5.
Power spectrum of the instantaneous liquid–vapor interface at T = 300 K. A liquid–vapor interface was simulated using a 24 × 24 × 3 nm3 slab of SPC/E water in a periodic box of size 24 × 24 × 9 nm3 and the instantaneous interface configuration, h(x,y), and its Fourier transform, formula image, were evaluated as in ref. . The power spectrum of our simulated instantaneous interface is in good agreement with the capillary-wave theory prediction (formula image) for wavevectors smaller than approximately 2π/9 Å. For larger wavevectors, the power spectrum is sensitive to molecular detail (i.e., the coarse-graining length, ξ, used to define the intrinsic interface), as expected (54). Fitting the ξ = 2.0 Å data in the range 0.01 Å-1 < k < 0.3 Å-1 yields γ = 62.0 ± 0.5 mJ/m2, in reasonable agreement with the experimental value of 72 mJ/m2 and some simulated values of the SPC/E surface tension (e.g., 63.6 ± 1.5 mJ/m2; ref. 55), but not others (e.g., 52.9 mJ/m2; ref. 54).
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