The authors review the development of extended x-ray absorption fine structure (EXAFS) within the last decade. Advances in experimental techniques have been largely stimulated by the availability of synchrotron radiation. The theory of EXAFS has also matured to the point where quantitative comparison with experiments can be made. The authors review in some detail the analysis of EXAFS data, starting from the treatment of raw data to the extraction of distances and amplitude information, and they also discuss selected examples of applications of EXAFS chosen to illustrate both the strength and limitations of EXAFS as a structural tool.

Extended x-ray absorption fine structure—its strengths and limitations as a structural tool

Publisher Summary Hydration can be considered a process of adding water incrementally to dry protein, until a level of hydration is reached beyond which further addition of water produces no change and only dilutes the protein. The hydration process has several stages and an end point, reflected similarly in different types of measurements. Time-average measurements appear to have a common pattern and most closely fit a single picture of the process. Dynamic measurements sometimes show a dependence on hydration level that lies outside the common pattern of the time-average results. The chapter summarizes the literature that bears on the protein hydration process and the hydration shell, categorized by type of measurement, and provides picture of the hydration process and the hydration shell and an assessment of the ways in which the hydration shell may modulate enzyme and other protein functions. The percolation model is discussed that focuses on the long-range connectivity established at a threshold coverage of a surface or volume with conducting or otherwise functional elements.

Protein hydration and function.

The Raman spectrum from water was obtained for a range of conditions from ambient to above the critical point, 256 bar and 400 °C. A fluorescence-free sapphire high-pressure Raman cell was employed with which the Raman spectra from water were examined between 30 and 4000 cm−1. Computer deconvolution of the Raman OH-stretching contours allowed the hydrogen bond strength to be determined from the integrated component intensity ratios by use of the van’t Hoff relationship. This procedure yielded a hydrogen bond enthalpy of 2.53±0.10 kcal/mol which is in excellent agreement with previously reported values.

Measurement of the Raman spectrum of liquid water

IR and Raman (parallel- and perpendicular-polarized) spectra in the OH stretch region for liquid water were measured some years ago, but their interpretation is still controversial. In part, this is because theoretical calculation of such spectra for a neat liquid presents a formidable challenge due to the coupling between vibrational chromophores and the effects of motional narrowing. Recently we proposed an electronic structure/molecular dynamics method for calculating spectra of dilute HOD in liquid D(2)O, which relied on ab initio calculations on clusters to provide a map from nuclear coordinates of the molecules in the liquid to OH stretch frequencies, transition dipoles, and polarizabilities. Here we extend this approach to the calculation of couplings between chromophores. From the trajectories of the fluctuating local-mode frequencies, transition moments, and couplings, we use our recently developed time-averaging approximation to calculate the line shapes. Our results are in good agreement with experiment for the IR and Raman line shapes, and capture the significant differences among them. Our analysis shows that while the coupling between chromophores is relatively modest, it nevertheless produces delocalization of the vibrational eigenstates over up to 12 chromophores, which has a profound effect on the spectroscopy. In particular, our results demonstrate that the peak in the parallel-polarized Raman spectrum at about 3250 wavenumbers is collective in nature.

IR and Raman spectra of liquid water: theory and interpretation.

Raman spectroscopy is a branch of vibrational spectroscopy in which a sample is exposed to an intense light beam such as a laser, and the spectrum of Raman-active vibrational modes induced in the sample molecules is obtained through analysis of the inelastically scattered photons. The diversity of applications and high content of molecular structure information provided, combined with recent advances in instrumentation, have rekindled interest in this technique in many diverse disciplines, including food science. Suitable analytes cover the entire range of food constituents, including the macro-components (proteins, lipids, carbohydrates and water) as well as minor components such as carotenoid pigments or synthetic dyes, and even microorganisms or packaging materials in contact with foods. Raman spectroscopy may be used as a tool for quality control, for compositional identification or for the detection of adulteration, as well as for basic research in the elucidation of structural or conformational changes that occur during processing of foods.

The applications of Raman spectroscopy in food science

Raman spectra in the O–H stretching band in normal and supercooled water have been investigated. Spectra are taken as a function of temperature in the range +95 to −24°C. Absolute Raman cross sections corrected for instrumental and physical factors are obtained. It is shown that the isotropic O–H stretching bands in the range +20 to +95 °C are due to the contribution of ’’open’’ (or tetrahedrically bonded) and ’’closed’’ water that are temperature independent. The percentage of open water α(T) is connected and interpreted in the frame of the Stanley site percolation model. As a consequence, the isobestic point is obtained. In the supercooled region, a third spectral contribution arises, which is centered near the main peak of the isotropic O–H band of ice. Such a contribution is tentatively explained as the presence in supercooled water of true ice, as a heterophase fluctuation.

Raman scattering and structure of normal and supercooled water

We report polarized Raman spectra for aqueous solutions of NiCl2, CdCl2, ZnCl2, CuCl2, Cu(NO3)2, and SrCl2. Our analysis of the Raman data provides an interpretation for the diffuse low frequency Raman scattering in transition metal halide–water solutions in terms of a vibrational density of states. Such a density corresponds to collective vibrational modes in a solute‐connected middle range lattice. The results are also discussed on the basis of recent measurements of viscosity, thermodynamic quantities, and EXAFS.

Raman spectroscopy and local order in aqueous solutions of strong II–I electrolytes

EXAFS (extended x-ray-absorption fine structure) spectra of Cu${\mathrm{Br}}_{2}$ aqueous solutions at various concentrations have been analyzed both for Br and Cu $K$ edges. They show a local ordering of the Cu and Br ions in planar structure which spreads as the concentration is increased up to saturation. Our data give a direct confirmation of the local-ordering model proposed recently on the basis of Raman and viscosity measurements.

Extended X-Ray-Absorbtion Fine-Structure Studies of Local Ordering in Highly Concentrated Aqueous Solutions of Cu Br 2

Etude des spectres de diffusion Brillouin dans l'etat normal et dans l'etat surfondu metastable jusqu'a −27,0°C. Deduction de l'absorption et de la vitesse d'hypersons

Evidence of anomalous acoustic behavior from Brillouin scattering in supercooled water

Polarized Raman spectra of ZnCl2 were obtained in the liquid phase near the melting point (T = 598 K), and in the glassy phase (T = 293 K). Measurements were performed down to a very low frequency shift (2 cm−1) from the exciting line. Our analysis of the Raman data provides an interpretation of the collision‐like contribution in terms of a structural relaxation time in the picosecond range, while the phonon‐like contribution gives an effective Raman density of states. These results are also discussed in terms of existing structureal models.

F. Wanderlingh

Papers

Raman scattering and structure of normal and supercooled water

Raman spectroscopy and local order in aqueous solutions of strong II–I electrolytes

Extended X-Ray-Absorbtion Fine-Structure Studies of Local Ordering in Highly Concentrated Aqueous Solutions of Cu Br 2

Evidence of anomalous acoustic behavior from Brillouin scattering in supercooled water

Vibrational dynamics of glassy and molten ZnCl2