Molecule nonchiral

Another interesting biooxygenation reaction with alkenes, recently identified, represents an enzymatic equivalent to an ozonolysis. While only studied on nonchiral molecules, so far, this cleavage of an alkene into two aldehydes under scores the diversity of functional group interconversions possible by enzymatic processes [121,122]. 

Recent experimental and theoretical studies on crystal growth, especially in the presence of tailor-made inhibitors, provide a link between macroscopic and microscopic chirality. We shall discuss these principles in some detail for chiral molecules. Furthermore, we shall examine whether it is indeed feasible today to establish the absolute configuration of a chiral crystal from an analysis of solvent-surface interactions. Since these analyses are based on understanding the interactions between a growing crystal and inhibitors present in solution, we shall first illustrate the general mechanism of this effect in various chiral and nonchiral systems. 

Stereoselective polymerization may proceed by ionic or coordination mechanisms. In many cases one admits that in the counterion or in the catalytic complex enantiomeric active centers exist, which give rise to predominantly (R) or (S) chains, respectively. Such centers may exist prior to polymerization or may be formed by reaction of a nonchiral precursor with the enantiomeric mixture of the monomers. Alternatively, one can think that the stereoselectivity depends mainly on the interaction between the entering monomer molecule (which is chiral) and the last unit in the chain (also chiral) according to this hypothesis, the enantiomeric excess inside each chain is generally low, because the occurrence of an accidental error brings about an inversion of the sense of stereoselection. 

The trick used in asyrmnetric inclusion polymerization is to perform the reaction in a rigid and chiral environment. With more specific reference to chirality transmission, the choice between the two extreme hypotheses, influence of the starting radical (which is chiral because it comes from a PHTP molecule), or influence of the chirality of the channel (in which the monomers and the growing chain are included), was made in favor of the second by means of an experiment of block copolymerization. This reaction was conducted so as to interpose between the starting chiral radical and the chiral polypentadiene block a long nonchiral polymer block (formed of isoprene units) (352), 93. The iso-prene-pentadiene block copolymer so obtained is still optically active and the... 

However, in this context CPSs are defined throughout this article as very stable phy-sisorbed (physically absorbed) and/or most often covalently bound chiral selector compounds to a nonchiral (most often silica) surface. To the same category belong the CSPs, which have as their bases beads of polymeric chiral selector material. The strong irreversible adsorption of chiral selector molecules (macromolecules or small molecules onto a plain or premodified surface) depends, of course, on the nature of the mobile phase and whether or not it has some solvation strength for the adsorbed chiral selector moiety. 

It is important to note that not all space groups that can accommodate chiral molecules are necessarily chiral. For example, it is clearly possible to place 2 nonchiral molecules in a monoclinic unit cell in, say P2, and have a nonchiral crystal. On the other hand there are 11 enantiomorphous pairs of space groups that must give chiral crystals because they are inherently chiral, regardless of what is in them. These are the following, which are all based on screw axes, and the pairs simply have axes of the same type spiraling in opposite directions ... 

While a collection of molecules that are all of the same chirality (e.g., a D- or L-amino acid or a naturally occurring protein) must form a chiral crystal, inherently nonchiral molecules are not barred from doing so, if they crystallize in one of the 11 pairs of enantiomorphous space groups. In that event, which is rather rare, there will, of course, be an equal probability of forming either enantiomorph and a batch of crystals will normally contain both. A couple of real examples are (NH4)3Tc2C18 3H20 (P3,21 and P3 >21) and SntTa Cl (P6 22 and P6522). 

These ether lipids are all chiral molecules with an R configuration but are derivatives of the nonchiral glycerol. The carbon atoms of glycerol are numbered using the stereochemical system which is described on p. 470. The ether linkage is to the sn-1 carbon atom. Most phospholipids are derivatives of the sn-3 phosphate ester of glycerol. 

Prochiral molecule. A nonchiral molecule that may react with an enzyme so that two groups that have a mirror image relationship to each other are treated differently. 

Abstract It is well known that spontaneous deracemization or spontaneous chiral resolution occasionally occurs when racemic molecules are crystallized. However, it is not easy to believe such phenomenon will occur when forming liquid crystal phases. Spontaneous chiral domain formation is introduced, when molecules form particular liquid crystal phases. Such molecules possess no chiral carbon but may have axial chirality. However, the potential barrier between two chiral states is low enough to allow mutual transformation even at room temperature. Therefore the systems are essentially not racemic but nonchiral or achiral. First, enhanced chirality by doping chiral nematic liquid crystals with nonchiral molecules is described. Emphasis is made on ester molecules for their anomalous behavior. Second, spontaneous chiral resolution is discussed. Three examples with rod-, bent-, and diskshaped molecules are shown to give such phenomena. Particular attention will be paid to controlling enantiomeric excess (ee). Actually, almost 100% ee was obtained by applying some external chiral stimuli. This is very noteworthy in the sense that we can create chiral molecules (chiral field) without using any chiral species. 

There are two different kinds of sources of molecular chirality central chirality and axial chirality (Fig. 1). Central chirality is due to the existence of chiral carbon, whereas axial chirality originates from twisted structures of molecules, between which a sufficiently high energy barrier exists, preventing the chiral conformational interconversion in ambient conditions. Surprisingly, however, the introduction of nonchiral molecules to chiral liquid crystalline environments sometimes enhances the chirality of the systems [3-5]. This means that inherently nonchiral molecules act as chiral molecules in chiral environments. This occurs in the following way. Molecules with axial chirality behave as nonchiral molecules when the potential barrier is low enough for chiral conformational interconversion. But when such... 

Essentially, the origin of spontaneous chiral resolution is the same as the previous example. When molecules with the same chiral conformation form small chiral domains due to packing entropy effects, the same chiral conformation of molecules is stabilized when they approach the chiral domain. Thus both chiral domains with different chiral conformations grow, resulting in spontaneous chiral resolution [6-8]. Chirality enhancement occurs even in such chiral domains. For instance, chirality in both segregated chiral domains is enhanced by doping nonchiral bent-shaped molecules (BSMs) with nonchiral rod-shaped molecules (RSMs), as observed by circular dichroism (CD) or optical rotatory power (ORP) [9],... 

In this chapter very unusual phenomena are described, i.e., (1) chirality enhanced by achiral or nonchiral molecules, (2) spontaneous chiral resolution in apparently nonchiral molecular systems composed of rod-, bent-, or disk-shaped... 

Extensive studies have been conducted to investigate the formation of chiral columns or helical superstructures in chiral and nonchiral disk- [53], star- [54, 55], and board-shaped [56] molecules. However, spontaneous deracemization has never been unambiguously demonstrated in discotic columnar phases consisting of nonchiral or racemic molecules. We recently observed clear evidence showing chiral resolution in a disk-like molecules with a dibenzo[g,p]chrysene core [57]. 

As nonchiral molecules enhance the chirality of chiral phases, as described in Sects. 2.1 and 2.4, the chirality of spontaneously resolved chiral domains can also be enhanced by adding nonchiral rod-shaped molecules. We introduce such an example observed in mixtures of bent- (P8-0-PIMB) and rod-shaped molecules... 

CB) [9]. The enhancement occurs in both chiral domains, although the dopant is nonchiral. The phase diagram with chemical structures of the two components are shown in Fig. 17. The B2 and B3 phases readily disappear by the introduction of a small amount of 5CB, and the B4 phase is stabilized. In the mixtures with more than 50 wt% 5CB, we observe a new phase transition at an almost constant temperature of 32 °C, corresponding to the Iso-N transition of 5CB. This transition point can be detectable by DSC [58] and texture observation, but not by X-ray analysis [59]. Based on the experimental fact of no layer structure changes, we can infer that P8-O-PIMB and 5CB molecules are nanosegregated. Namely helical nanofilaments are in the isotropic and nematic sea of 5CB in the B4 and lower phases, respectively. Hereafter we call these phases B4/Iso and B4/N phases, respectively. 

Fig. 19 Possible model to show enhanced optical activity by nonchiral 5CB molecules, (a) 5CB molecules form twisted nematic structure between helical nanofilaments, (b) 5CB molecules form superhelical structure around a helical nanofilament [9]...

This large optical activity can be used as a device with electric-field-controllable optical activity. The device performance using a mixture with 90% 5CB is shown in Fig. 20 [60]. By applying an electric field, 5CB molecules reorient toward the field direction, resulting in nonchiral N phase and rotatory power becoming almost zero. Here the contribution of the helical nanofilaments to the optical activity is very low in the transparent wavelength region. 

In this chapter, chirality-related topics driven by achiral molecules are described. Such studies have been accelerated by the revolutionary observation of chirality in nonchiral BSMs. They include spontaneous chiral resolution in several banana phases, and enhanced chirality by doping chiral host with achiral BSMs. These works triggered similar works in rod-shaped and even disk-shaped LCs, as... 

Jakli A, Nair GG, Lee CK, Su R, Chen LC (2001) Macroscopic chirality of a liquid crystal formed nonchiral molecules. Phys Rev E 63 061710-1-4. 

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