Induced helical structures

A chiral compound, dissolved in a nematic liquid crystal phase, transforms this phase into a chiral phase that is very often a chiral nematic - cholesteric -phase. Under the same condition of concentration and temperature two enantiomers induce helical structures with the same pitch but of opposite sign. The helical pitch p is for low concentrations of the dopant a linear function of mole fraction x. The molecular measure for the chiral induction is the helical twisting power (ITTP) ... 

A sufficient amount of oriented chiral molecules can be obtained in an induced cholesteric liquid crystal phase if the induced helical structure has been untwisted by an electric field. In the following description tensors are needed for the sake of simplicity (At least there are three tensors required the transition moment tensor (absorption tensor ,y), the rotational strength tensor (circular dichroism tensor A ,y), and the order tensor g,y33 (i,j= 1,2,3). If the molecules do not possess any symmetry, the principal axes of all of these tensors are differently oriented with respect to the molecular frame (the coordinate system in which only the three diagonal elements of a tensor are different from zero).) The only tensorial property, needed here explicitly, is the existence of three coordinates (components) of a tensor with respect to three specially chosen mutually perpendicular axes. This means that three information instead of one information about a molecule are needed instead of one CD value, namely Ae, three CD values, namely As, (i=l, 2, 3), have to be introduced. Ac is then one-third of a sum of the three so-called tensor coordinates of the CD tensor ... 

Although many peptides can have a restricted conformation about certain peptide bonds, it is generally accepted that most peptides in an aqueous benign environment do not have a unique conformation. However, it has now been well established that a nonpolar environment such as that characteristic of RPC separations may induce helical structure in potentially helical molecules [200-205]. Hence, when applied to an RPC column, peptides in a peptide mixture may be expected to be bound to the hydrophobic stationary phase either as random coils or as a-helices, depending on both their length and a-helical potential. Such a situation would certainly be characteristic of peptide mixtures derived from, e.g., proteolytic digests of proteins. 

When a chiral substance is dissolved in a nematic liquid crystal, a cholesteric mesophase is obtained(8). The cholesteric structure is characterized by its handedness (P-or M-helix) and pitch. Equal amounts of enantiomeric solutes of equal optical purity induce helical structures with identical pitch and opposite handedness... 

Imidazole derivatives have been used with metal ions to form noncentrosymmetric MOFs as they have strong tendency to induce helical structures of axial chirality. The im-N M donor-acceptor systems in these MOFs are expected to enhance the hyperpolarizabiUty p and thus the second-order NLO susceptibility. By introducing polar substituents such as - CN into the imidazole ring, the p value... 

G.S. Chilaya, The Physical Properties and Use of Liquid Crystals with an Induced Helical Structure [in Russian], Metsineieba, Tbilisi (1985). 

Nanotubes from Hydrogen Bonding-Induced Helical Structures... 

The discovery of the base-paired, double-helical structure of deoxyribonucleic acid (DNA) provides the theoretic framework for determining how the information coded into DNA sequences is replicated and how these sequences direct the synthesis of ribonucleic acid (RNA) and proteins. Already clinical medicine has taken advantage of many of these discoveries, and the future promises much more. For example, the biochemistry of the nucleic acids is central to an understanding of virus-induced diseases, the immune re-sponse, the mechanism of action of drugs and antibiotics, and the spectrum of inherited diseases. 

Analyses by native polyacrylamide gel electrophoresis and circular dichroism (CD) spectroscopy revealed that spontaneous coiled-coil associations between EGF-E5-His and EGF-K5-His promoted heterodimer (dEGF-His) formation. The CD spectroscopic analysis suggested that the E5 peptide in monomeric EGF-E5-His had a disordered structure. However, the ot-helical structure was induced in the E5 peptide when it associated with EGF-K5-His. These findings are shown schematically in Fig. 6. 

Urry and coworkers 4 O 1 1 proposed a left-handed helical structure for gramicidin A. This conformation can undergo ion induced relaxations which provides a mechanism for the movement of the ion along the channel. These workers confirmed this proposed structure by nuclear magnetic resonance spectrometry1 2. 

The structural characterization of this assembly has revealed that chloride coordination (via hydrogen bonding to the protonated pyridyl groups of the strands) induce the strands to adopt a double-helical structure in the solid state. 

Fig. 15 Representation of the metal-induced formation of helical structures. The metal ions (Ag+) are shown as spheres...

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