Anisotropic chiral interactions

When instead assemblies of helices are taken into account, it is well known that for many aspects DNA duplexes in solution can be treated as a charged anisotropic particle [2]. Accordingly, steric, electrostatic, and Van der Waals interactions, together with the mechanical properties of the helix (bending and torsional rigidity), play a major role in the formation of DNA mesophases. In addition, all these different kinds of interactions combine in a subtle and still poorly understood way to generate other forces relevant for the case of DNA. A notable example is the helix-specific, chiral interaction, whose importance for DNA assemblies will be discussed below. 

The chirality interaction tensor Wy is responsible for the interaction of the chiral guest, the chirality of which is described by the chirality tensor Cy, with the anisotropic host Wy = ikLkj- Ly covers the anisotropic host properties. W = Tr JT), is different from zero. The gyki are the orientational distribution coefficients of the guest in the molecular ensemble of the guest-host phase. From the two possible representations to avoid non-diagonal elements in (3.19), the representation in the system of principal axes of the order tensor is chosen instead of the principal axes of the chirality interaction tensor Wy. With Saupe s order parameters the HTP is also a sum of three terms... 

One optical feature of helicoidal structures is the ability to rotate the plane of incident polarized light. Since most of the characteristic optical properties of chiral liquid crystals result from the helicoidal structure, it is necessary to understand the origin of the chiral interactions responsible for the twisted structures. The continuum theory of liquid crystals is based on the Frank-Oseen approach to curvature elasticity in anisotropic fluids. It is assumed that the free energy is a quadratic function of curvature elastic strain, and for positive elastic constants the equilibrium state in the absence of surface or external forces is one of zero deformation with a uniform, parallel director. If a term linear in the twist strain is permitted, then spontaneously twisted structures can result, characterized by a pitch p, or wave-vector q=27tp i, where i is the axis of the helicoidal structure. For the simplest case of a nematic, the twist elastic free energy density can be written as ... 

They behave differently only in anisotropic conditions. Chiral-chiral interactions are needed for enantiomeric separations. The fundamental mechanisms for chiral separations are listed along with the commercially available chiral selectors. Two chemometric examples are commented one on quantitative strueture enantioselec-tivity relationship and the second one on linear solvation energy relationships. It is shown that the solvents used in the mobile phase may play the most eritical role in the ehiral mechanism. 

It is interesting to stress that the spin chirality observed in the gadolinium radical chains differs from the more usual one that characterizes anisotropic materials and is solely due to the significant strength of NNN interactions between lanthanide ions that are very far apart. The mechanism responsible for this interaction remains unclear and the complexity of the system has, up to now, hampered an ab initio investigation of the phenomenon. 

The classical cholesteric phase materials show only a weak anisotropic interaction with electric fields and hence are of limited use in electro-optical response applications. Cholesteric phases for these outlets are consequently produced by adding chiral dopants to nematic liquid crystals. 

In the previous section, we reported PMP derivatives bearing chiral side chains having a helicene-like helical structure through intrachain 7r-electron overlap interactions, which lead to a self-assembled whisker morphology due to interchain van der Waals interactions. However, each whisker of the PMP derivative is still randomly oriented, even if it has a self-assembled structure. It is therefore desirable to align the PMP derivative to construct a higher-ordered hierarchical structure and also to evolve an anisotropic nature in its potentially profound op-toelectrical properties. 

Analysis of natural abundance deuterium distribution in organic molecules, an important step in the study of kinetic isotope effects associated with enzyme-catalyzed reactions, by the use of chiral anisotropic media has been explored. An aspect of this analysis is the discrimination of the enantiotopic deuterons in prochiral molecules and the quantification of isotopic fractionation on methylene prostereogenic sites. Towards this an approach has been presented which is based on the use of natural abundance 2-dimensional NMR experiments on solutes oriented by chiral liquid crystalline solvents and the separation of the deuterium signals based on the quadrupolar interaction. The case of 1,1 -bis(phenylthio)hexane derived by cleavage from methyl linoleate of safflower has been used to illustrate the method with (D/H)pro-R and (D/H)pro-S measured at the same methylene position of a fatty acid chain. Enantiomers of water soluble materials can be observed using deuterium NMR spectroscopy in the lyotropic mesophase formed by glucopon/hexanol/buffered water. ... 

Objects like atoms, molecules, and ensembles of molecules are chiral. Thus, properties connected to the chirality of an object may have very different origins. Therefore, it makes sense to introduce a concept in which four levels of chirality exist. The first level of chirality is the chirality of the atoms [30] caused by weak interaction which is of no interest for the discussion of liquid crystal properties. The second level is the chirality of molecules, while the third level is derived from the ordering of atoms, ions, or molecules in isotropic or anisotropic phases by long-range positional and long-range orientational order. The fourth level of chirality is the form of a macroscopic object which can be, e.g., an enantiomorphic crystalline form (habitus of the crystal). 

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