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Vibrational Spectroscopy
Published in Grinberg Nelu, Rodriguez Sonia, Ewing’s Analytical Instrumentation Handbook, Fourth Edition, 2019
Peter Fredericks, Llewellyn Rintoul, John Coates
DICHROISM AND OPTICAL ACTIVITY. One of the few methods available for determining the absolute configuration of chiral molecules is a specialist IR spectroscopic technique known as vibrational circular dichroism (VCD). VCD is analogous to traditional CD in that it involves the measurement of the degree of dichroism in the absorption of left and right circularly polarized light. However, VCD operates in the IR region, and involves vibrational transitions; whereas traditional CD operates in the UV–Vis, and involves electronic transitions. The strength and sign of the VCD signal are given by the relative direction (dot product) of the electric and magnetic dipole transition moments, which are not orthogonal for chiral molecules. Among the advantages of VCD is that the absolute configuration of a molecule may be obtained by comparing the measured VCD spectrum with a calculated spectrum of the molecule of known configuration. If the sign and the spectra match then the configuration is the same as used in the calculation. If the spectra match but are of opposite sign then the molecule has the opposite configuration to that used in the calculation. In recent years VCD has matured as a technology to the point where commercial instruments are available either as stand-alone units or as accessory on an external bench. With the availability of accurate molecular orbital calculation programs, such as Gaussian 98/ 03, VCD in combination with ab initio calculations is increasingly adopted as standard practice particularly for small molecules of biological significance. Other applications of VCD include determination of the relative enantiometric purity, and in the study of proteins and nucleic acids. The basic components of a VCD-IR spectrometer are outlined in Figure 7.17.
Theoretical study on spectral differences of polypeptides constituted by L- and D-amino acids
Published in Molecular Physics, 2021
Ren-Hui Zheng, Wen-Mei Wei, Yan-Ying Liu
There are theories and computational methods of studying the IR [28,29], vibrational circular dichroism (VCD) [30], Electronic circular dichroism (ECD) [31–34], Raman [35,36], resonant Raman [37,38], ROA [39,40], and IR-UV doubly resonant SFVS [41,42], which are also important to investigate polypeptides and proteins. IR spectroscopy can be used to detect molecular concentration and study functional groups, dynamics and the secondary structures of polypeptides and proteins [43,44]. Raman is also vibration spectra, which can provide molecular structure information different from IR spectroscopy. Resonant Raman can not only present the molecular vibrations of the ground state but also obtain the structural information of the molecular excited electronic state. Resonant Raman is a selective, sensitive and practical method to investigate the structures, molecular bonding, backbone and side chains for peptides and proteins and environments of protein cofactors. Thus, there are many corresponding theoretical and experimental studies on resonant Raman [45–52].
Molecular dynamics simulations of the chiral recognition mechanism for a polysaccharide chiral stationary phase in enantiomeric chromatographic separations
Published in Molecular Physics, 2019
Xiaoyu Wang, David W. House, Priyanka A. Oroskar, Anil Oroskar, Asha Oroskar, Cynthia J. Jameson, Sohail Murad
In the solvents used in this work, our MD simulations reveal that ADMPC has a left-handed 4/3 helical structure, the glucose ring is regularly arranged along the axis; the carbamate groups are located inside the grooves of the polymer, while the phenyl groups are located outside the polymer chain. This structure is consistent with the reported data from nuclear magnetic resonance (NMR) studies in solution using the 2D Nuclear Overhauser Enhancement (NOESY) technique [6, 44], vibrational circular dichroism (VCD) [37], attenuated total reflection infrared spectroscopy (ATR-IR) [8], and solid-state NMR [71]. The driving hydrogen bonding interactions are buried inside the cavities near the backbone and are flanked by rigid bulky aromatic substituents that are located at the surface and may control the access to the binding site via steric factors. The aromatic moieties may or may not be involved in supportive π-π interactions. We shall consider all these interactions in detail in a later section.
Synthesis of helical branched carbodiimide polymers with liquid crystalline properties
Published in Liquid Crystals, 2022
Enosha Harshani De Silva, Bruce M. Novak
By focusing on the amidine chromophore in the backbone of the polymer, Vibrational Circular Dichroism (VCD) can be a useful technique to probe the helical handedness of polycarbodiimides. The appearance of the ± characteristic bi-signate peak around 1650 cm−1 in the VCD spectra indicates the polymer formed a left-handed or M-helix. Figure 1 displays the VCD spectra of Poly (1B), Poly ((1B)54-(2)46), Poly ((1B)33-(2)67), and Poly ((1B)23-(2)77). The characteristic bisignate peaks observed in the area of interest appear identical, thus demonstrating that helicity remains intact after modification of the sidechains.