We report the experimental observation of water dangling OH bonds in the hydration shells around dissolved nonpolar (hydrocarbon) groups. The results are obtained by combining vibrational (Raman) spectroscopy and multivariate curve resolution (MCR), to reveal a high-frequency OH stretch peak arising from the hydration shell around nonpolar (hydrocarbon) solute groups. The frequency and width of the observed peak is similar to that of dangling OH bonds previously detected at macroscopic air-water and oil-water interfaces. The area of the observed peak is used to quantify the number of water dangling bonds around hydrocarbon chains of different length. Molecular dynamics simulation of the vibrational spectra of water molecules in the hydration shell around neopentane and benzene reveals high-frequency OH features that closely resemble the experimentally observed dangling OH vibrational bands around neopentyl alcohol and benzyl alcohol. The red-shift of Ϸ50 cm ؊1 induced by aromatic solutes is similar to that previously observed upon formation of a -H bond (in low-temperature benzene-water clusters).hydrophobic ͉ interface ͉ vibration ͉ Raman C hanges in the structure and dynamics of water induced by nonpolar groups have long been considered to play a key role in protein folding, ligand binding, and the formation of biological cell membranes (1). Early thermodynamic evidence suggested that water may form an ''iceberg'' or clathrate-like structure around nonpolar molecules (2). Although no such rigid structures are currently thought to form (3), recent experimental (4, 5) and theoretical (6) results indicate that the rotational mobility of water molecules is reduced around nonpolar solutes (relative to bulk water). Moreover, fundamental theoretical arguments (and simulation measurements) imply that the size of a hydrophobic group may play a critical role in dictating water structure (7). This has led to the provocative suggestion that the structure of water around hydrophobic groups of nanometer (or greater) size may bear some resemblance to that at a macroscopic air-water interface (8). Although cohesive interactions between water and nonpolar groups tend to suppress the formation of an interfacial vapor layer (8-11), recent experimental (and molecular dynamics) studies indicate that the nonpolar binding cavity in bovine ␤-lactoglobulin is completely dehydrated in liquid water (12). Moreover, both experimental and simulation evidence suggests that water at nonpolar interfaces experiences significantly larger fluctuations than either bulk water or water at hydrophilic interfaces (13,14). Here, we present experimental evidence that reveals a similarity between the structure of water around dissolved hydrocarbon groups and that at macroscopic oil-water interfaces (15-18), in the sense that both interfaces induce the formation of dangling OH bonds.We have detected water dangling OH bonds by combining vibrational Raman spectroscopy with multivariate curve resolution (MCR). This procedure is used to decompose solution sp...

Molecular dynamics and electric field strength simulations are performed in order to quantify the structural, dynamic, and vibrational properties of non-H-bonded (dangling) OH groups in the hydration shell of neopentane, as well as in bulk water. The results are found to be in good agreement with the experimentally observed high-frequency (∼3660 cm(-1)) OH band arising from the hydration shell of neopentanol dissolved in HOD/D(2)O, obtained by analyzing variable concentration Raman spectra using multivariate curve resolution (Raman-MCR). The simulation results further indicate that hydration shell dangling OH groups preferentially point toward the central carbon atom of neopentane to a degree that increases with the lifetime of the dangling OH.

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Although short n-alkane chains are classic examples of hydrophobic solutes, mounting evidence points to a hydrophilic crossover for the hydration free energies (DeltaG) of sufficiently long n-alkane chains. Experimental and simulation results for the hydration of n-alkanes from methane (C1) to docosane (C22) are combined with fundamental thermodynamic relations to elucidate intermolecular contributions to DeltaG. Theoretical bounds on the influence of solute conformation on DeltaG are inferred by considering the hydration of idealized linear (all-trans) and globular (spherical) model solutes. More detailed theoretical extrapolations of experimental and simulation results imply that the water-mediated free energy change associated with collapsing an all-trans C100 chain is on the order of -100 kJ/mol and thus that n-alkane chains of this length and longer may be hydrophilic (DeltaG < 0).

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