Handedness

EC (ethyl cellulose)/glacial acetic acid + dichloro acetic acid [53] 

CDC (cellulosediphenylcarbamate)/pyridine (byconcentration) [52] CTC/diethylene glycol monoethyl ether (DEME) [54, 55] 

Acetoxypropyl-aceto cellulose/acetic acid, dibutyl phthalate [75, 62] 3C1-CTC/DEME or other solvents [57, 11] 

2- Ethyloxypropyl cellulose/acetonitrile, dioxane, methanol [59] (also thermotropic) Acetoacetoxypropyl cellulose/acetic acid [62] 

Phenylacetoxy cellulose (PAC DS = 1.9), 4-methoxyphenylacetoxy cellulose (4MPAC DS= 1.8)/CH2Cl2 [63] (also thermotropic) 

how do we study and talk about this important property of our world in an unambiguous way We first need a term to apply to a static or d)mamic system in which this type of mirror-image symmetry is present. We could and will use the term handed, but for the reasons given above, we will more often use the slightly more general term chiral (pronounced kai-ral) 

FIGURE 1.1. A left hand and a right hand as mirror images. 

FIGURE 1.4. Mirror-image cubes with inversion symmetry. See color insert. 

A major focus of this book is the connection between the microscopic molecular world and the macroscopic world that we live in. With this in mind, we need to describe the nomenclature used for molecular structures that are chiral. It is sometimes useful (but not universally applied) to classify chiral structures that have no symmetry elements as asymmetric and those that have some symmetry elements but are still chiral as dissymmetric Using this distinction. 

FIGURE 1.5. A three-bladed right-handed propeller. 

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Consideration of stereochem-iitry. The parity or handedness - R/S or chjirans - of a stcreoccnter can be obtained by considering the sequence of the Morgan numbers of die atoms, similarly to CIP. Then the number of pairwise interclianges is counted until the numbers arc in ascending order (see Section 2,8,5). 

Chirality (handedness) is older than life on tliis planet. Still it was not until 1848 when Pasteur manually separated enantiomeric crystals that chirality in chemistry was first appreciated ". The independent work of Van t Hoff and Le Bel revealed the molecirlar origin behind this phenomenon. 

There are many other examples of interrelationship. Symmetry, for example, is of fundamental importance in the sciences and arts alike. It plays a key role in our understanding of the atomic world as well as the cosmos. The handedness of molecules, with nature selecting one... 

Mixtures containing equal quantities of enantiomers are called racemic mixtures Racemic mixtures are optically inactive Conversely when one enantiomer is present m excess a net rotation of the plane of polarization is observed At the limit where all the molecules are of the same handedness we say the substance is optically pure Optical purity or percent enantiomeric excess is defined as... 

Figure 1.3 The "handedness" of amino acids Looking down the H-Ca bond from the hydrogen atom, the L-form has CO, R, and N substituents from Cq going In a clockwise direction. There is a mnemonic to remember this for the L-form fhe groups read CORN in clockwise direcfion.

As they occur in known protein structures, almost all P sheets—parallel, antiparallel, and mixed—have twisted strands. This twist always has the same handedness as that shown in Figure 2.7, which is defined as a right-handed twist. 

Richardson, J.S. Handedness of crossover connections in p sheets. Proc. Natl. Acad. Sci. USA 73 2619-2623, 1976. 

The handedness (righthanded rotation of the electric vector describes clockwise rotation when looking into the beam)... 

In ellipsometry only quantities 1 and 2 (and sometimes 3) are determined. The absolute intensity or phase of the light doesn t need to be measured, which simplifies the instrumentation enormously. The handedness information is normally not critical. 

This may be the first demonstration that a natural L-enan-tiomer enrichment occurs in certain cosmological environments. Could these observations be relevant to the emergence of L-enan-tiomers as the dominant amino acids on the earth And, if so, could there be life elsewhere in the universe that is based upon the same amino acid handedness ... 

Like helical gears, worms and worm gears have handedness, which is determined by the direction of the angle of the teeth. The worm and worm gear must be the same hand in order to mesh correctly. 

Handedness is also important in organic and biological chemistry, where it arises primarily as a consequence of the tetrahedral stereochemistry of 5p3-hybridized carbon atoms. Many drugs and almost all the molecules in our bodies, for instance, are handed. Furthermore, it is molecular handedness that makes possible the specific interactions between enzymes and their substrates that are so crucial to enzyme function. We ll look at handedness and its consequences in this chapter. 

Achiral (Section 9.2) Having a lack of handedness. A molecule is achiral if it has a plane of symmetry and is thus superimposable on its mirror image. 

Chiral (Section 9.2) Having handedness. Chiral molecules are those that do not have a plane of symmetry and are therefore not superimposable on their mirror image. A chiral molecule thus exists in two forms, one right-handed and one left-handed. The most common cause of chirality in a molecule is the presence of a carbon atom that is bonded to four different substituents. 

Fig. 9. The right-handed (R) and left-handed (L) three-fold helices of i-PP. For each handedness, the two different orientations (up or down) with respect to the reference axis are shown. The heights of the methyl groups are expressed ic c/6 units...

Differences in the. solid-state NMR signals of crystalline forms having identical conformations have been also observed. For instance, well-crystallized a form samples of i-PP show splittings for the methyl (22.6, 22.1 ppm) and methylene resonances (45.2, 44.2 ppm) into two lines with relative intensities 2 1 [117,118]. These splittings have been interpreted in terms of the known crystalline packing of the a form, which is characterized by pairs of 3/1 helices of opposite handedness at closer distances (Fig. 10). This generates inequivalence between the carbons indicated as A and those indicated as B in Fig. 10 [117,118]. 

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