Vibrational circular dichroism sampling techniques

However, the improved sensitivity of FT-IR allows one to obtain better sensitivity using the conventional sampling accessories and expand the range of sampling techniques. Emission, diffuse reflectance and photoacoustic spectroscopy represent new areas where FT-IR reduces the difficulty of the techniques considerably. Greatly improved results are also achievable from reflection spectroscopy. Special effects such as vibrational circular dichroism can be observed using FT-IR instrumentation. 

Chemical compounds absorb infrared radiation when there is a dipole moment change (in direction and/or magnitude) during a molecular vibration, molecular rotation, or molecular rotation-vibration. Absorptions are also observed with combinations, differences or overtones of molecular vibrations. A specific type of molecule is limited in the number of vibrations and rotations it is allowed to undergo. Therefore, each chemical compound has its own specific set of absorption frequencies and thus exhibits its own characteristic IR spectrum. This unique property of a compound allows the organic chemist to identify and quantify an unknown sample. (A special infrared technique called vibrational circular dichroism (VCD) is required to distinguish optical isomers). 

Optical measurements (/) such as Raman Scattering, Fluorescence techniques. Vibrational Circular Dichroism, (VCD), Optical Rotational Dispersion (ORD), Raman Optical Activity (ROA) and infrared absorption spectroscopy can overcome many of the obstacles mentioned above due to the fact that optical techniques are non-invasive and can monitor proteins in their native environment and with accurate time resolution. One disadvantage is the low sensitivity. However, the use of Surface Enhanced Raman Scattering (SERS), techniques (2-4) means that proteins can be observed down to the single molecule level. Thus, optical teclmiques hold great promise for the future investigation of protein dynamics processes provided that proteins can be maintained in a suitable and controllable sample cell. 

Yet another spectroscopic method that can be used to help determine molecular structure is vibrational circular dichroism (VCD). This technique detects differences in attenuation of left and right circularly polarized hght passing through the sample. VCD is sensitive to the mutual orientation of groups of atoms in a molecule and provides three-dimensional structural information. It is especially important in the study of chirality and molecular conformation. Only chiral molecules have a VCD spectrum. In particular, molecules that have either a plane of symmetry or a center of symmetry are VCD inactive. 

Vibrational spectroscopy is an important tool to obtain information about the secondary structure of proteins [827]. The ability to relate protein conformations to infrared vibrational bands was established very early in the pioneering work of Elhot and Ambrose before any detailed X-ray results were available [828]. Vibrational circular dichroism (VCD) provides sensitive data about the main chain conformation [829, 830]. The Raman optical activity (ROA) signal results from sampling of different modes but is especially sensitive to aromatic side chains [831, 832]. A theoretical prediction for the ROA phenomenon was developed by Barron and Buckingham [833, 834], and the first ROA spectra were measured by Barron, Bogaard and Buckingham [835, 836]. First ab initio predictions were provided by Polavarapu [837]. In 2003, Jalkanen et al. showed that DPT approaches in combination with explicit water molecules and a continuum model reproduce the experimental spectra much better [838]. DFT-based approaches to VCD spectra were, for example, pioneered by Stephens et al. [839]. To extract the local structural information provided by ROA, Hudecova et al. [721] developed multiscale QM/MM simulation techniques. 

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