Hydrophobic hydration is considered to have a key role in biological processes ranging from membrane formation to protein folding and ligand binding. Historically, hydrophobic hydration shells were thought to resemble solid clathrate hydrates, with solutes surrounded by polyhedral cages composed of tetrahedrally hydrogen-bonded water molecules. But more recent experimental and theoretical studies have challenged this view and emphasized the importance of the length scales involved. Here we report combined polarized, isotopic and temperature-dependent Raman scattering measurements with multivariate curve resolution (Raman-MCR) that explore hydrophobic hydration by mapping the vibrational spectroscopic features arising from the hydrophobic hydration shells of linear alcohols ranging from methanol to heptanol. Our data, covering the entire 0-100 °C temperature range, show clear evidence that at low temperatures the hydration shells have a hydrophobically enhanced water structure with greater tetrahedral order and fewer weak hydrogen bonds than the surrounding bulk water. This structure disappears with increasing temperature and is then, for hydrophobic chains longer than ~1 nm, replaced by a more disordered structure with weaker hydrogen bonds than bulk water. These observations support our current understanding of hydrophobic hydration, including the thermally induced water structural transformation that is suggestive of the hydrophobic crossover predicted to occur at lengths of ~1 nm (refs 5, 9, 10, 14).

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The compatibility of nonenhanced Raman spectroscopy with chromatographic and mass spectroscopic proteomic sensing is demonstrated for the first time. High-quality normal Raman spectra are derived from protein solutions with concentrations down to 1 microM and 1 fmol of protein nondestructively probed within the excitation laser beam. These results are obtained using a drop coating deposition Raman (DCDR) method in which the solution of interest is microdeposited (or microprinted) on a compatible substrate, followed by solvent evaporation and backscattering detection. Representative applications include the DCDR detection of insulin derived from an HPLC fraction, nondestructive DCDR followed by MALDI-TOF of lysozyme, the DCDR detection of protein spots deposited using an ink-jet microprinter, and the identification of spectral differences between glycan isomers of equal mass (such as those derived from posttranslationally modified proteins).

Potential distribution and coupling parameter theories are combined to interrelate previous solvation thermodynamic results and derive several new expressions for the solvent reorganization energy at both constant volume and constant pressure. We further demonstrate that the usual decomposition of the chemical potential into noncompensating energetic and entropic contributions may be extended to obtain a Gaussian fluctuation approximation for the chemical potential plus an exact cumulant expansion for the remainder. These exact expressions are further related to approximate first-order thermodynamic perturbation theory predictions and used to obtain a coupling-parameter integral expression for the sum of all higher-order terms in the perturbation series. The results are compared with the experimental global solvation thermodynamic functions for xenon dissolved in n-hexane and water (under ambient conditions). These comparisons imply that the constant-volume solvent reorganization energy has a magnitude of at most approximately kT in both experimental solutions. The results are used to extract numerical values of the solute-solvent mean interaction energy and associated fluctuation entropy directly from experimental solvation thermodynamic measurements.

The unique structural, dynamical and chemical properties of air/water and oil/water interfaces are thought to play a key role in various biological, geological and environmental processes. For example, non-hydrogen-bonded ('dangling') OH groups--which create surface defects in water's hydrogen bonding network and are experimentally detected at both macroscopic (air/water or oil/water) and microscopic (dissolved hydrophobic molecule) interfaces--are thought to catalyse some chemical reactions. However, how the size, curvature or charge of the exposed hydrophobic surface influences water's propensity to form dangling OH defects has not yet been established quantitatively. Here we use Raman multivariate curve resolution to probe spectroscopically the hydrophobic hydration shell and, using a statistical multisite analysis, we show that such interfacial dangling OH structures are entropically stabilized and their formation is cooperative (the probability that a non-hydrogen-bonded OH group will form depends nonlinearly on the hydrophobic surface area). We thus expose an important difference between the chemical properties of molecular and macroscopic oil/water interfaces.

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...

Experimental MethodsRaman Spectral Measurements: Raman spectra were obtained using a home-built, micro-Raman system similar to that used in previous studies (1). The system used in the present studies includes an Ar-Ion laser source (514.5nm, ~ 50 mW power at the sample) and a thermoelectrically cooled CCD detector (Princeton Instruments Inc., Pixis 400, 1340x400 pixel) mounted to a 300 mm focal length imaging spectrograph (SpectraPro300i, Acton Research Inc.), with a 300 g/mm grating, such that the dispersion is approximately 5 cm -1 per CCD pixel. Liquid samples were analyzed in spectroscopic 1 cm glass cuvettes contained within a thermoelectric, temperature-controlled, translating cell holder (Quantum Northwest). The solution temperature was controlled to within ± 0.01 °C over a 0°C to 60°C temperature range. Such temperature control was required in order to avoid spurious spectral features in the solute-correlated (SC) spectra resulting from small differences in temperature between the solution and pure water samples. A helium lamp was placed behind the sample so that two He lines at 587.562 nm and 667.815 nm were visible in each Raman spectrum (on either side of the OH stretch band). The He lines were used to correct for small wavelength drifts (resulting from small changes in ambient temperature and pressure) which, if uncorrected, would also produce spurious features in the SC spectra within the OH stretch band. The frequency shifts were corrected by introducing a sub-pixel shift the wavelength axis of the solution spectra so as to precisely match the He peak positions in the solution and pure water spectra. The latter shifts were performed using IgorPro (Wavemetrics Inc.) which facilitates duplication of waves with sub-pixel shifts, using a command such as wave1=wave0(p+d), where p is the pixel number (variable) d is a real constant whose magnitude is typically less than 0.1, and wave1 and wave0 are the shifted and un-shifted solution spectra, respectively.Benzene: Spectrophotometry grade benzene (99.93 % assay, EMD Chemicals Inc., Germany) was used without further purification. Water was ultra-purified (Milli-Q UF plus, Millipore Inc.) to an electrical resistance of 18.2 MΩ •cm. Saturated benzene in water was prepared by thoroughly stirring a sample consisting of water with a small excess benzene for 5 min. The solution was then allowed to equilibrate at the desired temperature for at least one day (in equilibrium with the excess benzene phase). Raman spectra were collected from the aqueous phase of the resulting solution (with an excess benzene layer at the top) as well as from a pure water sample (at the same temperature). All samples were equilibrated for at least 5 min in the temperature controlled sample cell holder, and measurements were made in the same cell position. Total (un-polarized) Raman scattering spectra of each solution of benzene in water

Hydrophobic interactions are driven by the combined influence of the direct attraction between oily solutes and an additional water-mediated interaction whose magnitude (and sign) depends sensitively on both solute size and attraction. The resulting delicate balance can lead to a slightly repulsive water-mediated interaction that drives oily molecules apart rather than pushing them together and thus opposes their direct (van der Waals) attraction for each other. As a consequence, competing solute size-dependent crossovers weaken hydrophobic interactions sufficiently that they are only expected to significantly exceed random thermal energy fluctuations for processes that bury more than ∼1 nm(2) of water-exposed area.