Circularly polarized light , absorption

In general, it may be said that enantiomers have identical properties in a symmetrical environment, but their properties may differ in an unsymmetrical environment. Besides the important differences previously noted, enantiomers may react at different rates with achiral molecules if an optically active catalyst is present they may have different solubilities in an optically active solvent., they may have different indexes of refraction or absorption spectra when examined with circularly polarized light, and so on. In most cases these differences are too small to be useful and are often too small to be measured. 

Circular dichroism (c.d.) spectroscopy measures the difference in absorption between left- and right-circularly polarized light by an asymmetric molecule. The spectrum results from the interaction between neighboring groups, and is thus extremely sensitive to the conformation of a molecule. Because the method may be applied to molecules in solution, it has become popular for monitoring the structure of biological molecules as a function of solvent conditions. 

Ever since Pasteur s work with enantiomers of sodium ammonium tartrate, the interaction of polarized light has provided a powerful, physical probe of molecular chirality [18]. What we may consider to be conventional circular dichroism (CD) arises from the different absorption of left- and right-circularly polarized light by target molecules of a specific handedness [19, 20]. However, absorption measurements made with randomly oriented samples provide a dichroism difference signal that is typically rather small. The chirally induced asymmetry or dichroism can be expressed as a Kuhn g-factor [21] defined as ... 

Optically active chromophores show different absorption for left and right circular polarized light (where the orientation of the polarized light changes periodically). These substances modify a circular polarized beam in such a way that the light is elliptically polarized after leaving the sample, an effect called circular dichroism. 

In the case of synthetic optically active polymers, the intuitive meaning of a CD signal intensity is very similar to that of UV spectroscopy, with the additional dimension of the subtracted absorption between left and right circularly polarized light.37 Absorption of light obeys the Beer-Lambert law, and thus CD intensity is defined as Ae = eL - eR = (AL - AR)/cl, where Ae is the... 

Photoluminescence (PL) in the polysilanes is well documented,34b,34c and for the poly(diarylsilane)s occurs typically with a small Stokes shift and almost mirror image profile of the UV absorption.59 This is due to the similarity of the chromophore and fluorophore structures in the ground and excited states, respectively, which is a result of the fact that little structural change occurs on excitation of the electrons from the a to the a orbitals. As PL is the emissive counterpart to UV, the emissive counterpart to CD is circularly polarized pho-toluminescence (CPPL). Where the fluorophore is chiral, then the photoexcited state can return to the ground state with emission of circularly polarized light, the direction of polarization of which depends on the relative intensities of the right-handed and left-handed emissions (/R and /l, respectively), which in turn depends on the chirality of the material, or more accurately, the chirality... 

Several direct methods are available to analyze the tertiary structure of ODNs like nuclear magnetic resonance (NMR) and X-ray crystallographic (XRC) techniques, which needs a sophisticated setup and infrastructure. An alternative but indirect method to study the structure and conformations of nucleic acids is circular dichroism spectroscopy (CD spectroscopy) (25, 26), where circular dichroism refers to the differential absorption of left and right circularly polarized light (27). 

Enantiomers can be distinguished by their rotation of plane-polarized light at a specific wavelength, or over a range of wavelengths (optical rotatory dispersion, ORD), as well as by the difference in absorption of right and left circularly polarized light (circular dichroism. Cotton effect, CD). 

However, (8.25) is not restricted to spheres but holds for particles of arbitrary shape. Thus, circular dichroism in particulate media includes a component that is the result of differential scattering, in contrast with circular dichroism in homogeneous media, which arises solely from differential absorption of left-circularly and right-circularly polarized light. 

In later work, Stephens rederived Eq. (1) starting from the difference of absorption coefficients of left and right circularly polarized light and applying the theory of radiation absorption in the presence of a magnetic field (29). In this approach the leading contribution to MCD is... 

If we make the dipole approximation the following expression is obtained for the difference of absorption coefficients of left and right circularly polarized light ... 

As Difference in absorption coefficients of left and right circularly polarized light. 

Circular dichroism (CD) is a spectroscopic analytical technique used for conformational analysis of peptides and proteins (Johson 1988). It uses the principles of chirality and absorption specifically the different absorption profiles demonstrated by a system for left as opposed to right circularly polarized light. For a system to exhibit CD activity, it must contain a chiral (asymmetric) center that is linked in some way to the chromophore responsible for the absorption. 

Polarimetric instruments arc operative with asymmetric molecules in the direct measurement of circular clichrosm (i.e., the difference of absorption of the left and right circularly polarized light as it passes through the sample), The technique is analogous to absorptiometry. 

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. 

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