Reflection Cotton effect

Figure 4.6-9 Induced cholesteric solutions Schematic outline of experiment and evaluation of the optical rotation p(A) related to the selective reflection band (reflection Cotton effect, RCE, centred at the wavelength A/ ) in order to characterize the chirality of the solute molecules by the helical twisting power.

Because of its correlation with the selective reflection we refer to this feature as reflection Cotton effect or, in short, RCE (Korte and Schrader, 1981). All together, a positive RCE indicates a left-handed cholesteric helical structure also called M helix and, vice versa,a negative RCE indicates a right-handed P helix. The evaluation is summarized in Fig. 4.6-9. 

Both the pitch and handedness of the cholesteric helices can be detected by chiroptical techniques, optical rotatory dispersion (O.R.D.) and circular dichroism (C.D.), by characterizing the selective reflection band. A right-handed helix (P-helix) originates a negative reflection Cotton-effect and vice versa (A = np where is the wavelength of the reflection band, n the main refractive index and p the pitch). 

The wavelength dependence of specific rotation and/or molecular ellipticity is called the Cotton effect. The Cotton effect can provide a wealth of information on relative or absolute configurations. The sign of the Cotton effect reflects the stereochemistry of the environment of the chromophore. By comparing the Cotton effect of a compound of known absolute configuration with that of a structurally similar compound, it is possible to deduce the absolute configuration or conformation of the latter. 

The signs of the exciton-split Cotton effect reflect the absolute configuration of the molecule if the direction of the transition moment in the chromophore is established. For example, chiral 9,9 -spirobifluorene derivative 1 has R configuration as indicated by a strong positive CD couplet (A = +1111.7) due to the LBb band of the anthracene chromophores, the electric transition moment of which is polarized along the long axis of the chromophore (Figure 14)100... 

If preparative or instrumental artifact is ruled out, the universal occurrence of red-shifted Cotton effects with a-helical character in all the membranes studied points to a common property of the proteins in biological membranes. The ORD results from lipid-free mitochondrial structural protein and erythrocyte ghost protein are consistent with assigning the red shift in these membranes to aggregated protein. It is, therefore, reasonable that similar protein-protein association may occur in all membranes. Ionic requirements for membrane stability could then reflect in part the requirements for protein-protein association. To some extent the molecular associations which stabilize membranes, therefore, may be protein-protein as well as lipid-lipid in nature. 

Cathou et al. (459) found that the Cotton effect near 270 nm in the ORD spectrum of RNase disappeared on interaction with either 2 -CMP or 3 -CMP. The X-ray studies (120) (see Fig. 23) clearly show that no tyrosine residues are in close contact with the substrate. Thus the change in rotatory behavior must reflect either (1) a shift in protein structure on association of the nucleotide or (2) the induction of a Cotton effect of the opposite sign in the bound nucleotide. In the independent spectral and chemical studies of Irie and Sawada (480), the reduced nucleotide 5,6-dihydrouridine-2 (3 )-phosphate, known to interact with the enzyme, showed no difference spectrum. With nucleotides containing... 

The Cotton effect at 350 nm reflects the configuration at C-5. The ORD and CD curves are opposite in sign in this region, thus confirming that the two types of alkaloids differ in configuration at C-5 (24). 

Rigorously, ORD and CD spectra are related through the Kronig-Kramers theorem, a well-known general relationship between refraction and absorption, i.e. nL - nR is determined by eL - % for A from zero to infinity [128], (The analogous relationship between refraction and reflection applies to cholesteric liquid crystals.) Hence, in order to maximize ORD in the transparent region, Cotton effects, associated with exciton coupling (both intramolecular and intermolecular), have... 

Unfortunately, many of the observations of Cotton effects in proteins must, in the absence of spectrophotometric details, be viewed with some skepticism. This caution arises from the ease with which rotatory artifacts may be produced in regions of high absorbance. Winkler and Markus (1959) found that the unusual rotatory dispersions observed in azo-dye complexes of human serum albumin between 550 and 750 m/i (Markus and Karush, 1958) were attributable to stray light polarized by reflection within the cell housing and which the opacity of dye solutions allowed to predominate. It has more recently been found (Urnes et al., 1961b) that stray light in the optical path can give rise to artifacts at absorption bands that simulate... 

A. After all, the CD spectrum of diester 60 reflects a positive chirality of conformer 60A. The CD spectrum of (2. V,3. V)-2,3-butanedioI bis(/>-Br-benzoate) 64 shows a positive exciton couplet, from which the AC of this compound could be assigned (Figure 28).72 If an internal 1,2-glycol has the opposite AC, the opposite CD Cotton effects are observed as shown in 61. 

Comparison of the NMR data and the circular dichroism of toxlsterol C2 with those of the now known Cl suggests that C2 should have formula II. The circular dichroism curve of C2 is opposite in sign (Ae227 -7.09) to that of Cl (Ae233 +4.55), The sign of the Cotton effect, associated with the absorption band at 227 to 229 nm, reflects the configuration in the chromophore concerned—that Is, the relative orientation of the cyclopropane ring and the double bond. This rules out formula... 

Dichroic Behavior of Carboxyl and Amide Chromophores in Polysaccharide It is evident that the configurational differences of the uronic acid moiety are well reflected in the dichroic behavior of the polymer, and the carboxyl chromophore thus plays a significant role in the chiroptical properties of the molecules. Certain facts regarding carboxyl chromophores are apparent from this study a) monomeric CD properties are well reflected in the Cotton effect of the polymer b) similarities in A0 values (Table 11) between monomers and polymers containing similar uronic acids indicate the additivity of monomer contributions to polymer CD c) iduronic acid shows considerably larger CD than glucuronic acid, which may have some origin in the observed difference in the NMR behavior (29) of methyl a-D-idopyranosiduronic acid from uronic acid with normal C-1 conformation. This has been interpreted in terms of either an equilibrium between the C-1 and 1-C chair forms or adoption of a hybrid skew boat structure. 

Although the Cotton effects of a-amino acids can be measured directly with up-to-date ORD/CD equipment, more often it is advantageous to form chromophoric derivatives or complexes whose chiroptical properties reflect the absolute configuration of the a-carbon atom. This is especially important when adequate instrumentation is not available or chromophoric substituents other than COOH or NH2 are present. For instance, in the case of aromatic amino acids, more complex chirospectral patterns have to be considered which might prevent the unequivocal assignment of absolute stereochemistry. Also, minor optically active impurities might interfere with spectral data in the 200-250-nm area. 

Heller, 1960, 1972). One is the possible presence of residual optical activity in cell windows, and this may be either inherent or introduced by strain. The other possible source of cell error is reflection from the cell walls due to the use of a cell whose diameter is too small. The temperature effect is variable, and often T or 2°C control is adequate. Also variable are concentration and solvent effects. A wavelength error of 1 nm leads to an error in rotation of about 1%, relative in positions remote from a Cotton effect. 

Cotton effect was observed at about 185 nm. S. F. Mason pointed out that this might reflect interactions of the isolated methyl groups with the geminal pair, which should be approximately twice as strong as those of the isolated methyl groups with one another, but opposite in sign, A similar interpretation of the corresponding dichotomy in the visible rotations of the trans acids, 32 and 33, was proposed earlier ... 

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