Distortion helical axis

A careful study of the NMR spectra followed by conformational analysis suggests a rather small distortion of the double helix of this decanucleotide upon platination, which has been described as a kink of about 40° in the helical axis at or around the GG lesion. Later studies of Lippard (72) have determined the kink more accurately at 33°, with an unwinding at 13°. 

Careful analysis of the chemical shifts and coupling constants after platination, suggests a distortion of the double helix which is rather small and which can - at the moment - best be described as a kink in the helical axis of about 40-70°, without a large change in overall helical unwinding. 

Figure 9.40. Distortion of the Recognition Site. The DNA is represented as a ball-and-stick model. The path of the DNA helical axis, shown in red, is substantially distorted on binding to the enzyme. For the B form of DNA, the axis is straight (not shown).

When an electric field E is applied normal to the helical axis, the helix gets distorted in a manner somewhat analogous to that depicted in fig. 4.6.1 for the cholesteric case. Above a critical field given by... 

Fig. 7.3. The deviation of the optic axis in a cholesteric (hard-twisted chiral nematic, p <, where p is the cholesteric pitch and A is the wavelength of light) when an electric field E is applied perpendicular to the helical axis. The cholesteric geometry allows a fiexoelectric polarization to be induced in the direction of E. The plane containing the director, which is perpendicular to the page in the middle figure and is shown in the lower figure, illustrates the splay-bend distortion and the corresponding polarization that arises. (After Rudquist, inspired by Meyer and Patel.

It is well known that nematic liquid crystals are nonpolar. However, for a certain asymmetrical shape of the molecules, splay or bend deformations of the director field lead to an electrical polarization [87]. This feature is known as the flexoelectric effect. Theoretically, the influence of an electric field on CLCs for the case where the helical axis is oriented parallel to the plane of the sample was first considered by Goossens [88]. Experimentally, the flexoelectric electro-optic effect in CLCs can be observed in conventional sandwich cells with transparent electrodes when the helix axis of the CLC lies parallel to the glass surfaces [89]. In the absence of an electric field, the CLC behaves as a uniaxial material with its optic axis perpendicular to the director and parallel to the helix axis. When an electric field is applied normal to the pitch axis, the helix distorts, as shown in Figure 6.6. Thus, the optical axis is reoriented and the medium becomes biaxial. The deviation direction... 

An ultrasonic field applied parallel to the helical axis k (k ) can cause a square gridlike pattern deformation of the planar texture [17, 58, 62]. This can be observed in nematic/cholesteric mixtures with a helix of large pitch. According to experimental data, the spatial period of distortion follows the equation L= p dy and tends to decrease slightly with frequency. The threshold particle velocity in the wave i is practically... 

Plotting the z-component of the local director, riz = sinO(x), as a function of position along the helical axis (x- axis) reveals that the sinusoidal pattern for riz(x) at H = 0 becomes distorted at finite values of H as shown in Fig. 11. The distortion makes riz(x) more square-wave-like and lengthens the pitch of the helix. Both of these effects can be understood on the basis that alignment along the magnetic... 

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