Circular dichroism, chiral properties

Optical properties of cyanines can be usefiil for both chiral substituents/environments and also third-order nonlinear optical properties in polymer films. Methine-chain substituted die arbo cyanines have been prepared from a chiral dialdehyde (S)-(+)-2-j -butylmalonaldehyde [127473-57-8] (79), where the chiral properties are introduced via the chiral j -butyl group on the central methine carbon of the pentamethine (die arbo cyanine) chromophore. For a nonchiral oxadicarbocyanine, the dimeric aggregate form of the dye shows circular dichroism when trapped in y-cyclodextrin (80). Attempts to prepare polymers with carbocyanine repeat units (linked by flexible chains) gave oligomers with only two or three repeat units (81). However, these materials... 

Circular dichroism (CD) is another interesting example of an optical property of the small Au SR clusters. Since the first observation of Schaaff et al. [23,24], several reports have appeared regarding the CD activities of gold clusters protected by chiral thiols such as penicillamine [25] and A-isobutyryl-cysteine [26]. Figure 11 shows the CD spectra of 1-9, which is a good reproduction of the original report by Whetten s group [23,24]. 

The carotenoid family have chiral centres which enable the use of circular dichroism. However, the chirality of carotenoids is not sufficiently characteristic so that the chiroptical properties do not serve as a good analytical tool. 

The inherent difficulty in analyzing enantiomers arises from the well-known fact that apart from their chiroptical characteristics, optical isomers have identical physical and chemical properties in an achiral environment (assuming ideal conditions). Therefore, methods of distinguishing enantiomers must rely on either their chiroptical properties (optical rotation, optical rotatory dispersion, circular dichroism), or must employ a chiral environment via diastereomer formation or interaction. Recently, it has become increasingly clear that such diastereomeric relationships may already exist in nonracemic mixtures of enantiomers via self-association in the absence of a chiral auxiliary (see Section 3.1.4.7.). 

As usual in stereochemical research, four main approaches have been applied to the problem of assigning chiralities to optically active cyclophanes. They are listed in order of their reliabilities i) anomalous X-ray diffraction (Bijvoet method), ii) chemical correlations with compounds of known chiralities (preferably established by the Bijvoet method), iii) kinetic resolutions and/or asymmetric syntheses, iv) interpretation of chiroptical properties (mainly circular dichroism) on the basis of (sector) rules including theoretical methods. 

Polarimetry, circular dichroism (CD) and optical rotatory dispersion (ORD) are the most important tools for the study of properties arising from optical activity. Although many chiral thiophenes have been prepared, there is no secure basis for a systematic discussion of the special effects of thiophene or annelated thiophene rings. For the purpose now at hand it is more expedient to discuss three different areas in which thiophene containing molecules and the related chiroptical techniques are central features. 

Part 2 again relies on a battery of comparative tests to establish the correspondence of the reduced-reoxidized product and the native enzyme (192). Chromatographic behavior, enzymic activity toward RNA, C > p U > p, UV spectra, ORD, viscosity, and reaction to antisera to RNase-A were all identical with or very similar to those of the starting enzyme, while the reduced material was markedly altered in all of these properties for which tests were possible. It was possible to crystallize the reoxidized material. The crystals were identical in space group, lattice constants, and general intensity distribution of the X-ray reflections within the normal limits of variation to the crystals of RNase-A from the same solvent (193). Recent circular dichroism studies show small differences near 240 nm in reoxidized material (194, 19 ) interpreted as arising from change in the chirality of one disulfide or the environment of one tyrosine residue. 

Measurement of circular dichroism can even permit elucidation of relatively small structural changes. CD spectroscopy is also suitable for the solution of specific application-relevant questions. Studies of the sensor properties of chiral dendrimers make use of the fact that complexation of chiral guest molecules induces changes in the CD bands of the host dendrimers. Thus guest-selective chiroptical effects observed in titration experiments with enantiomeric guest molecules give an indication of the potential of the chiral dendrimer to act as an enantioselective sensor [87]. 

An interesting phenomenon whereby achiral compounds occupy chiral cavities has been reported. Steroidal host compounds give rise to the attachment of definite chiral conformations of achiral compounds within cavities, making it possible to observe solid-state circular dichroism spectra. Gdaniec and Polonski reported this type of property for the inclusion compounds of DCA and CA with various aromatic ketones [40a] and benzil [40c], Furthermore, it is possible for the selected conformers to maintain their chiral state temporarily in solution. That is, soon after the inclusion compounds are dissolved, the chirality may be retained for some time. /V-Nitrosopiperidines were found to display this type of dynamic chiral recognition in DCA and CA inclusion compounds [40b], In this case, one can observe the decay of the circular dichroism signal after dissolution of these inclusion compounds in methanol. 

This result demonstrates the tendency of an optically active material to rotate the electric vector as it propagates through the sample. Materials possessing this property are normally composed of molecules having chiral symmetry. This effect leads to circular birefringence and circular dichroism, two optical properties that are frequently used in the characterization of biomaterials. 

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],... 

In a study47 of the ORD and circular dichroism properties of (— )-(8.R,9.fl)-(rcms-octahydrobenzo[c]thiophene and A-nor-2-thia-cholestane it was shown that the sulfur chromophore is useful for making stereochemical correlations. The two compounds show optically active transitions at 244 and 205 nm the signs of these transitions reflect the chirality of the neighboring centers. 

Optical chirality of molecules is a characteristic measure of stereo-chemical property of biological, pharmaceutical, and metal coordination compounds. Choral structures of amino acids, proteins, DNAs, and various drugs in solutions have been determined from the measurement of circular dichroism (CD). However, small amount of molecules at the liquid-liquid interfaces has never been measured before CLM/CD method [19] and SHG/CD method have been reported [20],... 

Chirality is an important topic in chemistry and biochemistry, due to the natural occurrence of chiral molecules in living organisms. In circular dichroism (CD) one measures the differential absorption of left- and right-handed circularly polarized light, which for chiral species are different. Therefore, CD has turned out to be a powerful tool which provides information on the electronic and geometric structure of chiral molecules. Since most CD spectra are measured in solution we extended our DRF/TDDFT method to also calculate such properties. As a first example we studied... 

Keywords Chiroptical properties, Electronic circular dichroisms, Planar chirality, Quantum chemical calculations, Cyclophane... 

When compared to developments associated with the structure and synthesis of chiral structures, less attention has been focused on the electronic and magnetic properties of chiral molecules. Circular dichroism and related optical probes of chirality have been developed mainly as analytical tools and, indeed, they are applied routinely. However, as demonstrated in this volume, the basic physical underpinnings that link structure and chiral properties, including chiro-optical properties, continue to emerge and require the development of physical models and improvement of electronic structure methods before they are fully elucidated. 

When chirality is involved, information on solid-state structures and supra-molecular properties must be obtained by solid-state circular dichroism (CDf spectroscopy, as certain characteristics may be lost upon dissolution. However extreme care is required to obtain artifact-free solid-state CD spectra. This is because CD spectra in the solid state (except for special homogeneous cases [9,10]) are inevitably accompanied by parasitic signals that originate from thd macroscopic anisotropies of a sample such as LD (linear dichroism) and LB (linear birefringence) [11-16]. We have been working in the field of solid-state chirality for the last 30 years and recently developed a novel universal chiroptical spectrophotometer, UCS J-800KCM, for the measurement of true CD and circular birefringence (CB) spectra in the solid state [17]. 

---