Structural analyses Raman spectroscopy

Figure Bl.22.6. Raman spectra in the C-H stretching region from 2-butanol (left frame) and 2-butanethiol (right), each either as bulk liquid (top traces) or adsorbed on a rough silver electrode surface (bottom). An analysis of the relative intensities of the different vibrational modes led to tire proposed adsorption structures depicted in the corresponding panels [53], This example illustrates the usefiilness of Raman spectroscopy for the detennination of adsorption geometries, but also points to its main limitation, namely the need to use rough silver surfaces to achieve adequate signal-to-noise levels.

Solid state NMR is a relatively recent spectroscopic technique that can be used to uniquely identify and quantitate crystalline phases in bulk materials and at surfaces and interfaces. While NMR resembles X-ray diffraction in this capacity, it has the additional advantage of being element-selective and inherently quantitative. Since the signal observed is a direct reflection of the local environment of the element under smdy, NMR can also provide structural insights on a molecularlevel. Thus, information about coordination numbers, local symmetry, and internuclear bond distances is readily available. This feature is particularly usefrd in the structural analysis of highly disordered, amorphous, and compositionally complex systems, where diffraction techniques and other spectroscopies (IR, Raman, EXAFS) often fail. 

Recent developments in Raman equipment has led to a considerable increase in sensitivity. This has enabled the monitoring of reactions of organic monolayers on glassy carbon [4.292] and diamond surfaces and analysis of the structure of Lang-muir-Blodgett monolayers without any enhancement effects. Although this unenhanced surface-Raman spectroscopy is expected to be applicable to a variety of technically or scientifically important surfaces and interfaces, it nevertheless requires careful optimization of the apparatus, data treatment, and sample preparation. 

Principles and Characteristics The prospects of Raman analysis for structural information depend upon many factors, including sample scattering strength, concentration, stability, fluorescence and background scattering/fluorescence from the TLC substrate. Conventional dispersive Raman spectroscopy has been considered as a tool for in situ analysis of TLC spots, since most adsorbents give weak Raman spectra and minimal interference with the spectra of the adsorbed species. Usually both silica and cellulose plates yield good-quality conventional Raman spectra, as opposed to polyamide plates. Detection limits for TLC fractions... 

Pigments in stratigraphic section may be analysed by TXRF (elemental composition), x Raman spectroscopy (functional groups) or XRD (structural analysis). At variance to conventional XRD, SR-XRD allows acquisition of patterns on thin stratigraphic sections... 

In one other example, Raman spectroscopy was employed along with FTIR spectroscopy, XPS, elemental analysis, TGA, SEM and transmission electron microscopy (TEM) to follow the compositional and structure variations of polymethylsilsesquioxane samples pyrolysed at different temperatures in an atmosphere of nitrogen [56]. At 900°C the main product was silica, with formation too of some silica oxycarbide and amorphous carbon, with Raman spectroscopy showing complementary evidence for presence of both the minor species. 

Sirichaisit, J., Brookes, V. L., Young, R.J., and Vollrath, F. (2003). Analysis of structure/ property relationships in silkworm (Bombyx mori) and spider dragline (Nephila edulis) silks using Raman spectroscopy. Biomacromolecules 4, 387-394. 

Aside from the direct techniques of X-ray or electron diffraction, the major possible routes to knowledge of three-dimensional protein structure are prediction from the amino acid sequence and analysis of spectroscopic measurements such as circular dichroism, laser Raman spectroscopy, and nuclear magnetic resonance. With the large data base now available of known three-dimensional protein structures, all of these approaches are making considerable progress, and it seems possible that within a few years some combination of noncrystallo-graphic techniques may be capable of correctly determining new protein structures. Because the problem is inherently quite difficult, it will undoubtedly be essential to make the best possible use of all hints available from the known structures. 

Other excited-state effects besides coordination changes are observed in the transient Raman spectra (10,11). Further analysis of the excited states and dynamics of Ni-porphyrin complexes and Ni-reconstituted heme proteins should benefit from Raman spectroscopy s inherently rich structural information content. Transient Raman methods are now being applied to other metalloporphyrins and metalloporphyrin-based systems. 

In addition, any rational approach to peptide hormone and neurotransmitter design must ultimately depend on the application of physical-chemical principles of conformation and structure, the use of various spectroscopic methods (especially nuclear magnetic resonance, circular dlchrolsm, and Raman spectroscopies. X-ray analysis where possible, etc.), and an understanding of the nature of a hormone-receptor Interaction In physical-chemical terms. Here again the use of conformatlonally restricted peptide structures Is critical (, 2. Recently we have... 

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