Mirror-image property

Specific interactions based on this mirror-image property of the interacting polymers have been observed in other enantiomorphic polymer mbctures. Precipitation has been reported when enantiomorphic solutions of the polypeptide poly(y-methyl d- and L-glutamate) (known to be or -helical) at 1% concentration in dimethylformamide (DP = 1700) were mixed at 100°C. Mixing in different proportions gave a precipitate, the weight of which corresponded to twice that of... 

Fig. 17. An illustration of the near mirror-image property of alginate and pectin molecules in the two-fold helical conformation.

An example of a chiral compound is lactic acid. Two different forms of lactic acid that are mirror images of each other can be defined (Figure 2-69). These two different molecules are called enantiomers. They can be separated, isolated, and characterized experimentally. They are different chemical entities, and some of their properties arc different (c.g., their optical rotation),... 

Because diastereomers are not mirror images of each other they can have quite different physical and chemical properties For example the (2R 3R) stereoisomer of 3 ammo 2 butanol is a liquid but the (2R 3S) diastereomer is a crystalline solid... 

Enantiomers. Two nonsuperimposable structures that are mirror images of each other are known as enantiomers. Enantiomers are related to each other in the same way that a right hand is related to a left hand. Except for the direction in which they rotate the plane of polarized light, enantiomers are identical in all physical properties. Enantiomers have identical chemical properties except in their reactivity toward optically active reagents. 

Multiple Chiral Centers. The number of stereoisomers increases rapidly with an increase in the number of chiral centers in a molecule. A molecule possessing two chiral atoms should have four optical isomers, that is, four structures consisting of two pairs of enantiomers. However, if a compound has two chiral centers but both centers have the same four substituents attached, the total number of isomers is three rather than four. One isomer of such a compound is not chiral because it is identical with its mirror image it has an internal mirror plane. This is an example of a diaster-eomer. The achiral structure is denoted as a meso compound. Diastereomers have different physical and chemical properties from the optically active enantiomers. Recognition of a plane of symmetry is usually the easiest way to detect a meso compound. The stereoisomers of tartaric acid are examples of compounds with multiple chiral centers (see Fig. 1.14), and one of its isomers is a meso compound. 

In considering whether a molecule is superimposable on its mirror image you may sense that the symmetry properties of the molecule should be able to give this information. This is, in fact, the case, and the symmetry-related mle for chirality is a very simple one ... 

Antisymmetry of a laminate requires (1) symmetry about the middle surface of geometry (i.e., consider a pair of equal-thickness laminae, one some distance above the middle surface and the other the same distance below the middle surface), but (2i some kind of a reversal or mirror image of the material properties [Qjjlk- In fact, the orthotropic material properties [Qjj], are symmetric, but the orientations of the laminae principal material directions are not symmetric about the middle surface. Those orientations are reversed from 0° to 90° (or vice versa) or from + a to - a (a mirror image about the laminate x-axis). Because the [Qjj]k are not symmetric, bending-extension coupling exists. 

The optical activity of quartz and certain other materials was first discovered by Jean-Baptiste Biot in 1815 in France, and in 1848 a young chemist in Paris named Louis Pasteur made a related and remarkable discovery. Pasteur noticed that preparations of optically inactive sodium ammonium tartrate contained two visibly different kinds of crystals that were mirror images of each other. Pasteur carefully separated the two types of crystals, dissolved them each in water, and found that each solution was optically active. Even more intriguing, the specific rotations of these two solutions were equal in magnitude and of opposite sign. Because these differences in optical rotation were apparent properties of the dissolved molecules, Pasteur eventually proposed that the molecules themselves were mirror images of each other, just like their respective crystals. Based on this and other related evidence, in 1847 van t Hoff and LeBel proposed the tetrahedral arrangement of valence bonds to carbon. 

Some physical properties of the three stereoisomers are listed in Table 9.3. The (+)- and (-j-tartaric acids have identical melting points, solubilities, and densities but differ in the sign of their rotation of plane-polarized light. The meso isomer, by contrast, is diastereomeric with the (+) and (-) forms. As such, it has no mirror-image relationship to (+)- and (-)-tartaric acids, is a different compound altogether, and has different physical properties. 

A chiral complex is one that is not identical to its mirror image. Thus, all optical isomers are chiral. The cis isomers of [CoCl2(en)2 + are chiral, and a chiral complex and its mirror image form a pair of enantiomers. The trans isomer is superimposable on its mirror image complexes with this property are called achiral. Enantiomers differ in one physical property chiral molecules display... 

Enantiomers differ in one physical property chiral molecules display optical activity, the ability to rotate the plane of polarization of light (Section 16.7 and Box 16.2). If a chiral molecule rotates the plane of polarization clockwise, then its mirror-image partner rotates it through the same angle in the opposite direction. 

If a molecule is nonsuperimposable on its miixor image, the mirror image must be a different molecule, since superimposability is the same as identity. In each case of optical activity of a pure compound there are two and only two isomers, called enantiomers (sometimes enantiomorphs), which differ in structure only in the left-and right-handedness of their orientations (Fig. 4.1). Enantiomers have identical physical and chemical properties except in two important respects ... 

One fundamental property of an animal s immune system is that it does not normally react against its own body constituents, i.e. it exhibits tolerance. However, clinical and experimental evidence shows that certain diseases exist in which the patient apparently destroys his/her own cells. The reactions could involve Tc cells, B cells orNK cells, and the result of the reaction with antigen may result in a pathological condition arising (autoimmune disease). Autoimmunity is the mirror-image of tolerance and reflects the loss of tolerance to self... 

Enantiomers have structures of exactly the same kind and yet are different. Their structures correspond to mirror images. In their physical properties they differ only with respect to phenomena that are polar, i.e. that have some kind of a preferred direction. This especially includes polarized light, the polarization plane of which experiences a rotation when it passes through a solution of the substance. For this reason enantiomers have also been called optical isomers. In their chemical properties enantiomers differ only when they react with a compound that is an enantiomer itself. 

Many substances exhibit the property of isomerism they occur in two or more molecular forms that have the same composition but differ from each other in structure and in their properties. One type of isomerism, known as optical isomerism, is exhibited by molecules that have the same constituent atoms but are arranged in different spatial distributions, where one of the optical isomers is a mirror image of the other (see Textbox 63). 

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