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Fine effects of the hydration, charge, and conformational structural changes in l-alanyl-l-alanine (Ala-Ala) dipeptide were studied with the aid of Raman and Raman optical activity (ROA) spectra. The spectra were recorded experimentally and analyzed by means of density functional computations. A 15N and 13C isotopically labeled analogue was synthesized and used to verify the vibrational mode assignment. Calculated shifts in vibrational frequencies for isotopically labeled molecule agreed well with the experiment. The assignment made it possible to scale computed vibrational frequencies and extract better structural information from the intensities. Solvent modeling with clusters obtained from molecular dynamics led to a qualitatively correct inhomogeneous broadening of Raman spectral lines but did not bring a convincing improvement of ROA signal when compared to a standard dielectric solvent correction. In comparison with the zwitterionic form, charged anionic and cationic dipeptides provided spectral variations that indicated different conformational behavior. Only minor backbone conformational change occurs in the cation, whereas the results indicate the presence of more anion conformers differing in the rotation of the NH2 group and the backbone ψ-angle. These findings are in agreement with previous electronic circular dichroism (ECD) and NMR studies. The results confirm the large potential of the ROA technique for the determination of final details in molecular structure and conformation.Keywords:
Raman optical activity
Conformational isomerism
Vibrational Circular Dichroism
Cationic polymerization
Vibrational optical activity (VOA) is a collective term applied to two spectroscopic techniques discovered during the early 1970s. The two techniques, which had been predicted on theoretical grounds, are infrared (IR) or vibrational circular dichroism (VCD), and Raman optical activity (ROA). Conceptually, one may view VCD as an extension of the principles of electronic circular dichroism (ECD), normally observed in the UV spectral region, into the domain of vibrational transitions in the IR spectral region. VCD can be observed via dispersive or Fourier transforms (FT) instrumentation. VCD intensity can be produced by the dipolar coupling of (virtually achiral) vibrational transitions, which are in a fixed, dissymmetric geometric pattern, such as a helix. ROA is the differential inelastic scattering of circularly polarized light from chiral molecules. The classical ROA experiment is carried out using a standard Raman instrument to which light modulation optics has been added.
Raman optical activity
Vibrational Circular Dichroism
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Mid-IR vibrational circular dichroism (VCD) and the corresponding Raman optical activity (ROA) spectra of 1-amino-2-propanol and 2-amino-1-propanol in neat solution are compared to yield insight into the dominant structural sensitivity of each technique. The ROA spectra for these isomeric compounds are quite similar while their VCD spectra are substantially different. The contrast between the results with these two techniques can be empirically interpreted to imply that VCD is more sensitive to the overall chirality of a molecule, conformation plus configuration, while ROA is more dependent on the nature of the local environment, or the configuration, of the functional groups. This observation would correlate with VCD having a significant dipolar coupling contribution that is highly dependent on conformation. This distinction between VCD and ROA sensitivities would be expected to be most appropriate for high dipole strength transitions in conformationally unconstrained, open-chain molecules. These observations directly reflect the contrast between current applications of VCD and ROA to biomolecular conformational analyses.
Raman optical activity
Vibrational Circular Dichroism
Chirality
Propanol
Cotton effect
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A comparison of theoretical to experimental results of vibrational circular dichroism (VCD) in the CH stretching region and Raman optical activity (ROA) in selected lower frequency modes is reported for (+)-(3R)-methylcyclohexanone. Calculations were carried out using the fixed partial charge model of VCD and the atom dipole interaction model of ROA. Regions of detailed comparison have been remeasured in both VCD and ROA and confirm earlier results. The principal features of both VCD and ROA spectra are reproduced by the calculations and are interpreted on the basis of their constituent vibrational motions. Correlations and trends in both the VCD and ROA data, as well as between them, are discussed.
Raman optical activity
Vibrational Circular Dichroism
Basis (linear algebra)
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ABSTRACT For three different chiroptical spectroscopic methods, namely, vibrational circular dichroism (VCD), electronic circular dichroism (ECD), and Raman optical activity (ROA), the measures of similarity of the experimental spectra to the corresponding spectra predicted using quantum chemical theories are summarized. In determining the absolute configuration and/or predominant conformations of chiral molecules, these similarity measures provide numerical estimates of agreement between experimental observations and theoretical predictions. Selected applications illustrating the similarity measures for absorption, circular dichroism, and corresponding dissymmetry factor (DF) spectra, in the case of VCD and ECD, and for Raman, ROA, and circular intensity differential (CID) spectra in the case of ROA, are presented. The analysis of similarity in DF or CID spectra is considered to be much more discerning and accurate than that in absorption (or Raman) and circular dichroism (or ROA) spectra, undertaken individually. Chirality 26:539–552, 2014 . © 2014 Wiley Periodicals, Inc.
Raman optical activity
Vibrational Circular Dichroism
Chirality
Similarity (geometry)
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Infrared vibrational circular dichroism (VCD) and vibrational Raman optical activity (ROA) have been measured and compared quantitatively over the frequency range from 835 to 1345 cm −1 for trans-pinane, cis-pinane, α-pinene, and β-pinene. For these molecules in this region of spectral overlap between VCD and ROA, the average ratio of VCD or ROA to its parent vibrational intensity favors ROA by a factor of two to three. Several vibrational modes in each molecule yield both large VCD and large ROA, while several other modes show little propensity toward significant VCD or ROA intensity. Beyond this general property of a few strongly chiral and strongly achiral vibrational modes, little additional correlation between VCD and ROA intensity is found. This quantitative compilation of VCD, infrared, ROA, and Raman intensities provides an experimental basis for computational intensity studies of VCD, ROA, and their theoretical comparison.
Raman optical activity
Vibrational Circular Dichroism
Intensity
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Abstract Vibrational Circular Dichroism (VCD) spectra often differ strongly from one conformer to another, even within the same absolute configuration of a molecule. Simulated molecular VCD spectra typically require expensive quantum chemical calculations for all conformers to generate a Boltzmann averaged total spectrum. This paper reports whether machine learning (ML) can partly replace these quantum chemical calculations by capturing the intricate connection between a conformer geometry and its VCD spectrum. Three hypotheses concerning the added value of ML are tested. First, it is shown that for a single stereoisomer, ML can predict the VCD spectrum of a conformer from solely the conformer geometry. Second, it is found that the ML approach results in important time savings. Third, the ML model produced is unfortunately hardly transferable from one stereoisomer to another.
Conformational isomerism
Vibrational Circular Dichroism
Quantum chemical
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Natural vibrational optical activity consists of two principal forms. The IR form is known as vibrational circular dichroism (VCD) and is simply the extension of electronic circular dichroism into the IR vibrational region of the spectrum. The Raman form, known as Raman optical activity (ROA), is a new form of optical activity that has no counterpart in the classical forms of optical activity. In this paper, the similarities and differences of the IR and Raman forms of vibrational optical activity will be examined. Although both VCD and ROA were discovered and confirmed in the period from 1973 to 1975, each field has evolved independently with key advances in theoretical description, instrumentation and application coming at different times over the past 20 years. The current relative strengths and weaknesses of VCD and ROA will be discussed, and specific examples of VOA spectra of (—)-α-pinene and the amino acid L-alanine, for which overlapping VCD and ROA data are available, will be presented.
Raman optical activity
Vibrational Circular Dichroism
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Raman optical activity
Vibrational Circular Dichroism
Optical Rotation
Chirality
Absolute Configuration
Enantiomeric excess
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The use of circularly polarized radiation to obtain new information about the structure of chiral molecules will be explored. This area of spectroscopy is known as optical activity. The classical forms of optical activity, typically measured in the visible and ultraviolet regions of the spectrum, are called optical rotatory dispersion (ORD) and electronic circular dichroism (CD). In recent years, new forms of optical activity have been developed and applied to the study of molecular structure. These include two forms of fluorescence optical activity and two forms of vibrational optical activity. The vibrational forms are called infrared vibrational circular dichroism (VCD) and vibrational Raman optical activity (ROA). The principles of optical activity will be explained and the application of optical activity to problems of chemical and biological significance will be discussed.
Raman optical activity
Vibrational Circular Dichroism
Optical rotatory dispersion
Ultraviolet
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Abstract This article describes recent progress in the field of vibrational optical activity (VOA) as a probe of the structural properties of pharmaceutical and biological molecules. A strong emphasis is placed on vibrational circular dichroism (VCD) owing to its more advanced state of development. Raman optical activity (ROA) is included in the first two sections for completeness; other reviews give additional information about ROA. 1–3,8 The article focuses on the practical aspects of VCD spectral measurement and interpretation. This is supplemented by examples that serve to illustrate the principal areas of VOA application. VCD is defined as the difference in the absorbance of the left circularly polarized (LCP) versus right circularly polarized (RCP) infrared (IR) radiation for a chiral molecule undergoing a vibrational transition. A pair of enantiomers will produce VCD spectra that are equal and opposite in sign and a racemic mixture will have a null VCD signal. VCD can be measured for all kinds of chiral molecules, irrespective of their size. In practice, measurements are often carried out in solution but, with the new advances in instrumentation, it is now possible to measure spectra of solids and mulls. Compared to other optical techniques such as electronic circular dichroism (CD) and IR absorption, VCD is unique because it combines the optical activity property of CD with the rich structural fingerprint region of IR. The discovery and first measurements of VCD occurred in the early 1970s. Although over a dozen practitioners have published close to a 1000 papers since then, it is only recently that VCD instrumentation has become available commercially for nonspecialists. Applications span a variety of fields from chemical and pharmaceutical to biological. The VCD technique can be used for the determination of absolute configuration of small chiral molecules or larger natural products. It can also be used to follow a chiral synthesis both for stereochemistry and for optical purity, and to study the secondary structure of large proteins and small peptides, and the conformation of nucleic acids and sugars. Some recent reports have also shown the unique sensitivity of ROA in the study of viruses and in protein‐folding experiments. VOA is now fulfilling its promise of becoming a technique that is broadly used for stereochemical and conformational studies of all varieties of chiral molecules, both natural and synthetic.
Raman optical activity
Vibrational Circular Dichroism
Absorbance
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