Dissymmetry factor

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. 

In the determination of M , the intensity of scattered light is measured at different concentrations and at different angles 6). The incident light sends out a scattering envelope that has four equal quadrants (Figure 3.11a) for small particles. The ratio of scattering at 45° compared with that at 135° is called the dissymmetry factor or dissymmetry ratio Z. The reduced dissymmetry factor Z is the intercept of the plot of Z as a function of concentration extrapolated to zero concentration. 

Further studies reported similar CPL spectra for [Yb(28a)]3+, as compared to [Yb(28b)]3+ (Dickins et al., 1999), with dissymmetry factor gium for the peak corresponding to the maximum of luminescence (at 995 nm) of —0.18 (Maupin et al., 2000). A quantitative comparison with the CD spectrum for which the absorption dissymmetry factor gabs = —0.11 is difficult in view of the spectral overlap of various transitions from differently populated LF sub-levels, as demonstrated in a previously cited study (Di Bari et al., 2000a). However, such a comparison is useful to verily the sign of the CPL emission ... 

Breaking of the internal PLP-lysine aldimine bond by reaction with carbonyl reagents, hydroxylamine and aminooxyacetate was also accompanied by a strong diminution of the positive CD and by a reduction of the dissymmetry factor.44 It has been inferred that the... 

A positive CD was found in the 500-nm quinonoid band which is formed on reaction of tryptophanase with L-alanine and oxindolyl-L-alanine (Fig. 9.10). The dissymmetry factor in this band is much smaller than in the absorption bands of the unliganded enzyme (Table 9.2). A negative 315-nm peak, which appears in the presence of L-alanine (Fig. 9.10), may be caused by interaction of an aromatic amino acid residue with the quinonoid coenzyme ring. 

Table 9.2 1 The dissymmetry factor and reduced linear dichroism values in the coenzyme absorption bands of tryptophanase and its complexes with inhibitory amino acidst2...

Enzyme form Ligand concentration mM Absorption maximum nm Dissymmetry factor X 104 Reduced linear dichroism X 103... 

The extra factor of 1/2 in this equation results from the fact that the equivalent expression for the absorption dissymmetry factor (or ratio), gabs > employed in CD spectroscopy is defined in terms of the difference in extinction coefficients, Ae, divided by the average extinction coefficient, e. 

If the g-factor is on the order of 10 3 or higher, this detection system can be used to determine the (ee) of chiral compounds using nonchiral HPLC columns. The absorbance gives the concentration of the eluate and the value of the CD gives its enantiomeric excess. The (ee) can be directly evaluated by the dissymmetry factor, because it does not depend upon concentration. 

Equation (3.26) is adapted to nonspherical particles by multiplying D by a dissymmetry factor (Geddes, 1949). 

Molecular mechanics calculations and temperature-dependent CD spectra may reveal details of the conformational map not attainable by DNMR. Thus, double minima within each of the anti and syn forms were inferred from molecular mechanics calculations and were also supported by the temperature dependence of the CD spectra. Rotational strength calculations for the stable conformations predicted by the molecular mechanics calculations (or for any other arrangement) are in fair agreement with respect to both the sign and the dissymmetry factor for many of the studied compounds and may also allow for tentative assignments of absolute configuration in cases where this is not known (87JA492). 

In addition, chiral oligofluorenes have exhibited circularly polarized electroluminescence with the highest polarization to date, as characterized by the dissymmetry factor, without addition of optical elements external to the OLED device. This device had an efficiency of 0.94 cd A-1 in the deep blue,... 

The ratio of the circular dichroism signal and the corresponding absorption is called dissymmetry factor g ... 

The dissymmetry factor g can be expressed using dipole and rotational strengths. When SI units are used ... 

CPF spectroscopy measures a difference in intensities of left-and right-circularly polarized components of a fluorescence emitted from a chiral fluorophore and reflects the chirality of conformations and interactions in the excited state (2 ). Figure 1 shows CPF spectra of poly(l- and 2-napAla)s measured in DMF solution (12). The ordinate of the CPF spectra is the Kuhn s emission dissymmetry factor g, ... 

Since the circular dichroism is generally associated with coupled groups in the near-zone range of distances, the explicit results for the Kuhn dissymmetry factors can be obtained from the leading /jo terms in the rate expressions, and are as follows ... 

Finally, consideration is given to the results appearing in column 7 of Table VII, which apply to the situation in which the two laser beams have circular polarizations of opposite handedness. The results here differ from those in either columns 5 or 6 in each value of j. This represents the fact that changing the helicity of one beam produces a dichroism associated with a discrimination of the handedness of the Aj—A2 pair dressed by the chirality of the other circularly polarized beam. This again is a known feature of two-photon absorption (Thirunamachandran 1979), and one which persists even when the pair is not held in a fixed mutual orientation. In this case the dissymmetry factor has both numerator and denominator given by the leading jo terms, and the explicit result is as follows ... 

Circular dichroism employs standard dispersive or interferometric instrumentation, but uses a thermal source that is rapidly modulated between circular polarization states using a photoelastic or electro-optic modulator. Using phase-sensitive detection, a difference signal proportional to the absorption difference between left- and right-polarized light, A A = AL — AR, is recorded as a function of wavenumber. Relative differential absorptions (dissymmetry factors) AA/A at absorption maxima are typically 0.1—0.01 for uv—vis electronic transitions and 10-4 —10-5 for vibrational modes in the ir. 

CPL is the emissive pendent of CD and therefore probes the excited state chirality it also reflects the molecular motions taking place between absorption and emission. In this case, the parameter of interest is the luminescence dissymmetry factor ... 

One characteristic feature of excimers in the polypeptide I-O and 1-2 is that they emit circularly polarized light [43,51]. Circularly polarized fluorescence (CPF) spectroscopy measures the difference in fluorescence intensity of the left and right circularly polarized component. Usually the CPF spectrum is plotted with Kuhn s circular dissymmetry factor, as the... 

The Pfeiffer effect, the outer-sphere interaction of a chiral substrate with a rapidly interconverting racemic solution of a chiral lanthanide complex, can be investigated by measurement of the luminescence dissymmetry factor (the ratio of circularly polarized luminescence to total luminescence) for Eu or Tb " complexes. Thus the racemic D chiral complexes [M(dpa)3], where M = Eu or Tb, interact in an outer-sphere manner with the following optically active spiecies cationic chiral transition metal complexes, ascorbic acid, aminocarboxylates, tartrates, amines and phenols. Association constants can be obtained from limiting values of the dissymmetry factors. In some cases, inner-sphere complexation can be demonstrated, as judged by changes in the general nature of the circularly polarized luminescence spectrum and pH irreversibility of the complexation. 

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