Helix axis, deformation

Deformed //clical Ferroelectrics (DHF)117 In a thicker cell (ca 5 pm) the surface stabilization no longer dominates and a helical structure is formed with the helix axis parallel to the glass plate. The external electrical d.c. field can influence and modulate the helix. 

Recent reviews from this Laboratory provide an overview of the literature of MD simulations on DNA oligomers through 1993 (27) and theoretical and computational aspects of DNA hydration (28) and counterion atmosphere (29). References to the most recent literature can be found in (30). Experimental data for comparison with MD results are available for crystal structures in the Nucleic acids Data Bank (NDB) (31), and for NMR structure in a review by Ulyanov and James (52). The research described in this article is directed towards understanding the dynamical structure of the various right-handed helical forms of DNA, their deformations and interconversions. The canonical A and B structures of DNA are shown for reference in Figure 1. The A and B forms are distinguishable in three major ways the displacement of nucleotide base pairs from the helix axis, the inclination of base pairs with respect to the helix axis, and sugar puckers. Details on these and other structural features of DNA relevant to MD analysis is readily available (33). 

The Deformed Helix Ferroelectric (DHF) effect was observed in the very first investigations of FLC structures [1, 92], but the first adequate explanation was given in [93-96]. The geometry of the FLC cell with a DHF effect is presented in Fig. 7.18. The polarizer (P) on the first substrate makes an angle / with the helix axis and the analyzer (A) is crossed with... 

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... 

Several cases of dielectric, hydrodynamic, and flexoelectric instabilities and domain structures have been observed and extensively studied in CLCs. Their appearance depends on the initial orientation of molecules, the physical parameters of the material, and the applied electric field. In CLCs with positive dielectric anisotropy Ae > 0, an electric field applied along the helix axis of a planar (Grandjean) texture can induce a two-dimensional spatially periodic deformation which has the form of a square grid [96], The period and threshold voltage of this field-induced instability depend on the elastie constants, the dieleetric anisotropy, and the sample thickness [97],... 

For the distorted structure E the helix axis will have small components in the x direction as before, and therefore in the small deformation approximation we have... 

The periodic deformation in Fig. 34 b cannot be observed in a nonchiral nematic because it does not allow for a space-filling splay-bend structure. Instead, such apattem would require a periodic defect structure. However, we can continually generate such a space-filling structure without defects in a cholesteric by rotating the director everywhere in a plane containing the helix axis. 

Theoretical investigations by Brand [ 135] and Brand and Pleiner [136] predicted that a monodomain liquid-crystalline elastomer exhibiting a cholesteric or a chiral smectic C phase should display piezoelectric properties due to a modification of the pitch of the helix under strain. So, a piezoelectric voltage should be observed across the sample when a mechanical field is applied parallel to the helicoidal axis. In this description, the crosslinking density is supposed to be weak enough to allow the motion of the director, and deformations of the sample (compression, elongation, etc.) are assumed to be much smaller than those that should lead to a suppression of the helix. The possibility of a piezoelectric effect do not only concern cholesteric and chiral smectic C phases, but was also theoretically outlined for more exotic chiral layered systems such as chiral smectic A mesophases [137]. 

Mesophase reveals intermediate order between amorphous and crystaUine phases. In the first studies it was labelled as smectic (Natta Corradini, 1960) or paracrystalline (Miller, 1960). Further studies revealed that mesophase is made up of bundles of parallel chains, which maintain typical for all polymorphic forms of polypropylene three-fold helical conformation. Bundles are terminated in the direction of the chain axis by helix reversals or other conformational defects (Androsch et al., 2010). In the bundles long range ordering maintains only along the chain axes, whereas in lateral packing a large amount of disorder is present (Natta Corradini, 1960). The mesophase is formed by quenching of the molten polypropylene (Miller, 1960 Wyckoff, 1962) or by deformation of the crystalline structure (Saraf Porter, 1988 Qiu, 2007). As for the fibres, the mesophase was observed in fibres taken at low take-up velocity (Spruiell White, 1975 Jinan et al., 1989, Bond Spruiell, 2001) in fibres intensively cooled in water with addition of ice or in the mixture of dry ice... 

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