Optical activity natural circular dichroism

Natural circular dichroism (optical activity). Although circular dichroism spectra are most difficult to interpret in terms of electronic structure and stereochemistry, they are so very sensitive to perturbations from the environment that they have provided useful ways of detecting changes in biopolymers and in complexes particularly those remote from the first co-ordination sphere of metal complexes, that are not readily apparent in the absorption spectrum (22). It is useful to distinguish between two origins of the rotational strength of absorption bands. 

In an external magnetic field all matter becomes optically active. This observation was first made by Faraday in 1845 the magnetically induced rotation of the plane of polarized light is therefore referred to as the Faraday effect. In recent years, the common mode of study of this phenomenon has been the measurement of magnetic circular dichroism (MCD). Similarly to natural circular dichroism, magnetic circular dichroism is defined as the difference Af = fi, - of the extinction coefficients for left-handed and right-... 

In the presence of a static magnetic field any sample becomes optically active in a direction parallel to that of the magnetic field, and exhibits a circular dichroism to an extent proportional to the intensity of the applied field. Such an effect has nothing to do with molecular chirality and is called magnetic circular dichroism (MCD). Differently from what discussed for natural circular dichroism, the two... 

The primary motivation for the development and application of vibrational optical activity lies in the enhanced stereochemical sensitivity that it provides in relation to its two parent spectroscopies, electronic optical activity and ordinary vibrational spectroscopy. Over the past 25 years, optical rotatory dispersion and more recently electronic circular dichroism have provided useful stereochemical information regarding the structure of chiral molecules and polymers in solution however, the detail provided by these spectra has been limited by the broad and diffuse nature of the spectral bands and the difficulty of accurately modeling the spectra theoretically. 

Ettinger, M.J., Timasheff, S.N. (1971a). Optical activity of insulin. I. On the nature of the circular dichroism bands. Biochemistry, 10, 824—830. 

Vibrational optical activity (VOA) is a relatively new area of natural optical activity. It consists of the measurement of optical activity in the spectral regions associated with vibrational transitions in chiral molecules. There are two basic manifestations of VOA. The first is simply the extension of electronic circular dichroism (CD) into the infrared region where fundamental one-photon vibrational transitions are located. This form of VOA is referred to as vibrational circular dichroism (VCD). It was first measured as a property of individual molecules in 1974 [1], and was independently confirmed in 1975 [2]. Within the past twelve years, VCD has been reviewed on a number of occasions from a variety of perspectives [3-15], and two more reviews are currently in press [16,17], The second form of VOA has no direct analog in classical forms of optical activity. Optical activity in Raman scattering, known simply as Raman optical activity (ROA), was measured successfully for the first time in 1973 [18], and confirmed independently in 1975 [19], ROA has been described in detail and reviewed several times in the past decade from several points of view [20-24], and two additional reviews [25,26], one with a view toward biological applications [25] and the other from a theoretical perspective [26], are currently in press. In addition, two articles of a pedagogical nature are in press that have been written for a general audience, one on infrared CD [27] and the other on ROA [28],... 

The circular dichroism induced in achiral compounds upon complexation with a chiral material can be used to characterize the nature of the interaction responsible for the association. Studies of this type may be conveniently classified as being the optical activity induced upon dissolution of a solute in a chiral solvent, through association of the species of interest with a co-dissolved chiral solute, or through association of the solute with chiral polymers. 

The highly interesting and now classical monograph of Born contains the theory of the optical properties of matter covering atomic and molecular structural considerations. The fundamentals of molecular optics, presented by Bom, have been developed by Volkenshteyn in tensorial form. Various monographs have been written on the laws of classical optics and the fundamentals of physical optics, including natural optical activity," optical rotatory dispersion and circular dichroism," " i.r. spectroscopy, and other related topics. 

To determine the absolute configuration of optically active organic compounds, there are two nonempirical methods. One is the Bijvoet method in the X-ray crystallographic structure analysis, which is based on the anomalous dispersion effect of heavy atoms. - The X-ray Bijvoet method has been extensively applied to various chiral organic compounds since Bijvoet first succeeded in determination of the absolute stereochemistry of tartaric acid in 1951. The second method is a newer one based on the circular dichroism (CD) spectroscopy. Harada and Nakanishi have developed the CD dibenzoate chirality rule, a powerful method for determination of the absolute configuration of glycols, which was later generalized as the CD exciton chirality method. 8 The absolute stereochemistry of various natural products has been determined by application of this nonempirical method. 

Figure 41. Absorption (A) and circular dichroism (B) spectra of poly-L-glutamic acid, pH 3.85, as a function of particle size, which is achieved by sonication. As particle size increases, the ellipticity and absorption extrema are progressively dampened there is increased light scattering (curve b and c of A), and there is a marked red shifting of the CD extrema and crossover. These are distortions in the spectra due to the particulate nature of the optically active system. Adapted, with permission, from [110].

The chiral atoms of Cl on deoxyribonucleosides and of Cl , C2 , and C3 on ribonucleosides were intact during monomer synthesis and copolymerization polymers 17, 19, 21, and 23 were optically active, which allowed the use of circular dichroism (CD) and optical rotary dispersion (ORD) for investigation of the polymer conformations in aqueous solutions [6, 10, 11]. The CD-curve of 23A is shown in Fig. 1, which is quite similar to that of natural poly(-adenylic acid). 

In copolymers of NVC with various vinyl or vinylidenic optically active monomers it has been found that independent of the nature of the chiral comonomer, composition and distribution of monomeric units, the heteroaromatic moieties are always characterized by a more or less marked induced circular dichroism (F gs- 13 and... 

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