Molar circular dichroism

This [Ae] is the specific, not molar, circular dichroism. Molar values are not given because the molecular weight of the diastereomer is not known. Since the molar circular dichroism of the diastereomer is assumed to be of little interest, the specific CD should suffice for following a resolution. [Ae] = (Ae)(100)/MW. 

Figure 5.32 Circular dichroism spectra of tubules made from mixtures of opposite enantiomers of DCg.gPC in 80 20 methanol-water. Spectra shown correspond to mixtures of (a) 100 0, (b) 75 25, (c) 50 50, (d) 25 75, and (e) 0 100 L-/D-DC89PC. Inset shows dependence of peak molar ellipticity on enantiomeric excess along with linear fit of data. Reprinted with permission from Ref. 125. Copyright 1998 by the American Chemical Society.

The circular dichroism spectra (Figure 1) was obtained on a 0.0155 mg/ml solution in methanol with a Cary Model 61 Circular Dichroism Spectrophotometer. The following molar ellipticity values were obtained ... 

Optical activity also manifests itself in small differences in the molar extinction coefficients el and er of an enantiomer toward the right and left circularly polarized light. The small differences in e are expressed by the term molecular ellipticity [9 J = 3300(el — r). As a result of the differences in molar extinction coefficients, a circularly polarized beam in one direction is absorbed more than the other. Molecular ellipticity is dependent on temperature, solvent, and wavelength. The wavelength dependence of ellipticity is called circular dichroism (CD). CD spectroscopy is a powerful method for studying the three-dimensional structures of optically active chiral compounds, for example, for studying their absolute configurations or preferred conformations.57... 

A mixture consisting of aniline ( 0.2 g) and (lS)-(+) camphorsulfonic acid (3.48 g) was dissolved in 10 ml of water and then treated with five separate portions of 0.1 g of ammonium peroxydisulfate dissolved in 1 ml water. Each successive portion was added when the solution turned from blue to green while the reaction mixture was maintained at 20°C. After the additions were completed the mixture was centrifuged and the product washed with water. The circular dichroism spectrum of the product suspensed in water indicated a molar ellipticity of about 90 x 103 deg-cm2/dmol. Transmission electron micrographs showed that the product had a nanofibrous structure with fiber diameters from 30 to 70 nm and had a length of several hundred nanometers. 

Transmission electron micrographs showed that the precipitate was in the form of particles, although some nanofibrous structures were observed. The circular dichroism indicated a molar ellipticity of 5 x 103 deg-cm2/dmol. 

Fig. 28a and b. Circular dichroism spectra of calf thymus DNA at pH 7 (27 °C) in aqueous solutions of varying concentrations of LiCl (a) and NH4C1 (b) 72). The molar concentrations are printed above each spectrum. The decrease in the CD magnitude of the positive band at 275 nm demonstrates the transformation from the B to C form... 

The circular dichroism of a sample is the difference between the molar extinction coefficients for left-handed and right-handed polarized light (Eq. 23-7) and is observed only for chiral molecules.7 95-97... 

The dichrograph gives a direct measure of Ae. A circular dichroism (CD) spectrum often resembles an absorption spectrum, the peaks coming at the same positions as the peaks in the absorption spectrum of the same sample. However, the CD can be either positive or negative and may be positive for one transition and negative for another (Fig. 23-8). It is most convenient to plot Ae directly as a function of wavelength or wave number. However, much of the literature makes use of the molar ellipticity (Eq. 23-8) ... 

This means that the molar extinction coefficients of the two enantiomers (e, and er) are unequal in circularly polarized light. These differences in absorption (e, and er) can be measured as a function of wavelength, and the curves obtained are called circular dichroism curves. They have positive or negative signs (Cotton effect) just as for optical rotatory dispersion curves. 

Fig. 18 Circular dichroism spectra comparing (a) Trl and (b) Tr4. Titration of pDNA with Trl results in minimal change in molar ellipticity representative of B-form DNA. Tr4 elicits a shift in ellipticity to a modified B-form, suggesting interaction with DNA base pairs by the polymer. Figure adapted with permission from [156], 2008 American Chemical Society...

Circular dichroism is the differential absorption of left- and right-circularly polarized light by a sample with excess of one enantiomer. The effect is usually expressed as the difference between the molar extinction coefficients for left- and right-circularly polarized light (eL(A) and eR(A))... 

Conformational changes are easily followed by optical rotation (Hui and Neukom, 1964). Circular dichroism spectroscopy (CD) of polysaccharides (Morris, 1994) exploits optical anisotropy. In a CD instrumental design, the clockwise and counterclockwise rotation of two polarized beams of equal intensity, traversing a 180° path through a chiroptical medium, display a molar ellipticity maximum and minimum. CD is the differential measurement as a function of X. By CD spectroscopy, mixed interchain association rather than nonspecific incompatibility or exclusion was identified as the molecular basis of alginate-polyguluronate interaction (Thom et al., 1982). 

Spectroscopic Studies. All measurements were made at 25° in 0.05M or 0.1M potassium phosphate buffer, pH 7.0. Each spectrum was recorded with several samples, which generally differed in concentration, and was scanned two or three times. Circular dichroism is expressed as molar ellipticity [9] in units of degrees cm2 dmole 1. 

Fig. 1 Top Behavior of the electronic linear chiroptical response in the vicinity of an excitation frequency. Re = real part (e.g., molar rotation [< ]), Im = imaginary part (e.g., molar ellipticity [0]). Without absorption line broadening, the imaginary part is a line-spectrum (5-functions) with corresponding singularities in the real part at coex. A broadened imaginary part is accompanied by a nonsingular anomalous OR dispersion (real part). A Gaussian broadening was used for this figure [37]. Bottom Several excitations. Electronic absorptions shown as a circular dichroism spectrum with well separated bands. The molar rotation exhibits regions of anomalous dispersion in the vicinity of the excitations [34, 36, 37]. See text for further details...

Both A A and cis trans equilibria of siderophore complexes can exist in solution. The chirality of the ligand can impose a preferred metal-center chirality. In addition, the degree of this preference depends on the stereochemical rigidity of the ligand. In principle, the magnitude of the molar circular dichroism can be used as a measure for diastereoisomeric equilibria based on a comparison of the solid-state and solution ellipticity. Nevertheless, predictions of metal-center chiralities require theoretical calculations. For example, empirical-force-field calculations of iron(III) enterobactin show that the A orientation at the metal center is more stable than the A by 0.5 kcalmoH, which is consistent with the CD spectra. ... 

Ae In circular dichroism, amplitude of the CD maximum difference in molar absorption coefficients for right and left circularly polarised light. May be positive or negative. 

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