Chiroptical spectroscopy

Chiral molecules possess the unique property of interacting differentially with left and right circularly polarised light. A differential absorption is known as 

Online CD detectors are now commercially available for use with HPLC that are inherently more sensitive than corresponding OR detectors and not affected by solvent changes to the same extent and are thus more gradient compatible [121]. Provided Ae and the concentration of an analyte are known with good precision/accuracy, the measurement of CD will allow the determination of enantiomeric purity. In addition, with CD-based detection systems, both chiroptical and ordinary absorbance can be determined simultaneously allowing the measurement of the g-factor (or dissymmetry factor), which is defined as the ratio of the CD to the absorbance (AA/A) [122]. The g-factor is concentration independent and its measurement allows a more reliable determination of enantiomeric purity (without using a CSP) with reference to standards of known enantiomeric composition irrespective of their concentration [123]. A small number of recent literature examples have suggested the potential use of achiral HPLC with online CD detection for the determination of extreme enantiomeric ratios [121, 124-126] however, chiral separation techniques currently provide a more reliable measurement of enantiomeric purity. 

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Bochicchio, B. and Tamburro, A.M., Polyproline II structure in proteins Identification by chiroptical spectroscopies, stability, and functions. Chirality, 14(10), 782-792, 2002. 

Levai, A., Utilization of the chiroptical spectroscopies for the structure elucidation of flavonoids and related benzopyran derivatives, Acta Chim. Slav., 45, 267, 1998. 

Another open area is the assignment of the various electronic transitions observable in the absorption, CD, and ORD spectra. Owing to the fact that bands in chiroptical spectroscopy have opposite signs, maxima can be more easily identified in CD and ORD spectra than in absorption spectra. Also, such information as the rotatory strength can be used for band assignment, a necessary condition for a widespread use of chiroptical spectroscopy for correlation of configurations. 

In this book, a variety of topics will be presented which are centered upon applications for which chiroptical spectroscopy has played an essential role. Neil Purdie has placed the field into perspective by contributing a comprehensive summary of the analytical applications of CD spectroscopy to forensic, pharmaceutical, clinical, and food science areas, focussing strongly on compounds of natural origin. 

As instrumental advances continue and computational methods become more powerful, the frontiers of chiroptical spectroscopy are being continually pushed back. One important area has been in the characterization of biopolymers. Mark Manning and John Towell have contributed a chapter covering the use of ultraviolet-CD in the analysis of protein structure, and Max Diem has described the application of infrared-CD to the study of similar systems. Finally, the range of chiroptical investigations which can be performed on molecules in electronically excited states has been summarized by James Riehl. 

The author would like to acknowledge the support and assistance of his colleagues in the field of chiroptical spectroscopy, especially Dr. Harry P. J. M. Dekkers, Dr. Roel B. Rexwinkel, and Stefan C. J. Meskers of the University of Leiden, The Netherlands Dr. David H. Metcalf and Dr. Frederick S. Richardson of the University of Virginia, and former students Dr. Nursen Coruh and Dr. Gary L. Hilmes. 

H.G. Brittain, N. Grinberg, Techniques of chiroptical spectroscopy. Chapter 10, in J. Cazes (Ed.), Handbook of Analytical Instrumentation, third ed., Marcel Dekker, New York, 2005, pp. 271-294. 

Tab. 8.1. Quantities and units used in conventional and chiroptical spectroscopy. The spectral dependence of these quantities can be expressed as a function of wavelength A, frequency v, or wave-number...

Chiroptical spectroscopies are based on the concept of chirality, the signals are exactly zero for non-chiral samples. In terms of molecular symmetry, this means that the studied system must not contain a rotation-reflection axis of symmetry. This lapidary definition implies that the more known symmetry elements (symmetry plane - equivalent to the one-fold rotation-reflection axis and the center of symmetry - equivalent to the two fold rotation-reflection axis) must also be absent and that the system must be able to exist at least formally in two mirror image-like forms. At first glance this limitation seems to be a disadvantage, however, this direct relation to molecular geometry gives chiroptical properties their enormous sensitivity to even minor and detailed changes in the three-dimensional structure. 

This property is absent in the parent non-chiral spectroscopies. Chiroptical methods sometimes provide enhanced resolution, because of the simple fact that di-chroic bands can be positive and negative. Chiral spectroscopies give also a new dimension to the intensity parameter. The information about structure is also encoded in the sign, the absolute value and the width of spectral bands. Not only the positions of bands, but also the entire shape of the spectral pattern carries structural information on the sample. While parent spectroscopies are more oriented toward the positions of the spectral bands, chiroptical spectroscopies are primarily intensity oriented, although band positions are just as important as in the parent methods. Chiroptical spectroscopies can draw on substantial knowledge on electronic and vibrational molecular transitions that has been collected throughout the years of analytical use of the parent spectroscopies. 

In chiroptical spectroscopy, an important part of structural information is provided by the intensity parameters of the spectrum and also by its overall shape. Primarily... 

Since polypeptides are chiral polymers, the chromophore arrangement shows some chirality. Therefore, additional information on the chromophore arrangement in the ground state, as well as in the excited state, may be obtained from chiroptical spectroscopy, such as circular dichroism (CD), circularly polarized fluorescence (CPF) [43], and fluorescence-detected circular dichroism (FDCD) [44). 

CD and ORD are the two most familiar and widely used examples of chiroptical spectroscopy. However, there are other forms of chiroptical spectroscopy which can provide useful information about biological molecules. Excited states of chiral molecules emit rcpl and Icpl with different probabilities, thus giving rise to circularly polarized emission (CPE) or... 

Kuball H-G and Hofer T (1999) Chiroptical spectroscopy oriented molecules and anisotropic systems. In Lindon JC, Tranter GE, and Holmes JL (eds.) Encyclopedia of Spectroscopy and Spectrometry, pp. 267-281. London Academic Press. 

The notion of handedness in physical science arose from John Frederick William Herschel s descriptions (2) of quartz. (Parenthetically, it is worth noting here his discovery of the complexation of silver ions by thiosulfate and his comment that the resulting solutions tasted sweet. The eutropic [Au(S203)2] is also bioactive in "myochrysin", the chrysotherapeutic agent for rheumatoid arthritis.) The now well developed relationship between shape and chiroptical spectroscopy has roots in the... 

Circular dichroism (CD) is a chiroptical spectroscopy that measures the differential absorption of left versus right circularly polarized light. Its higher sensitivity to molecular conformation and configuration has made CD spectroscopy a more powerful tool in the structural analysis of various chiral supramolecular systems than its parent achiral absorption spectroscopies such as ultraviolet (UV), visible (vis), and infrared (IR) spectra (Figure 1). CD measurements in the UV and vis regions are the most widely... 

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