Mirror image relationship

The enzyme-catalyzed interconversion of acetaldehyde and ethanol serves to illustrate a second important feature of prochiral relationships, that ofprochiral faces. Addition of a fourth ligand, different from the three already present, to the carbonyl carbon of acetaldehyde will produce a chiral molecule. The original molecule presents to the approaching reagent two faces which bear a mirror-image relationship to one another and are therefore enantiotopic. The two faces may be classified as re (from rectus) or si (from sinister), according to the sequence rule. If the substituents viewed from a particular face appear clockwise in order of decreasing priority, then that face is re if coimter-clockwise, then si. The re and si faces of acetaldehyde are shown below. 

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. 

Enantiomers (Section 9.1) Slereoisomers of a chiral substance that have a mirror-image relationship. Enantiomers must have opposite configurations at all chirality centers. 

Optical isomers (Section 9.4) An alternative name for enantiomers. Optical isomers are isomers that have a mirror-image relationship. 

The precision of the data is not such as to allow non-dipole interactions to be definitively ruled out, and more detailed study of this topic by careful measurement of the full angular distribution, as opposed to detection at a single angle, will be required to provide a complete probe. In the meantime a clear observation that enantiomer PECD curves have a mirror-image relationship... 

The inactivity in the molecule is due to the fact that it is perfectly symmetric as shown by the dotted line, the upper half exactly coinciding with the lower half. Therefore, molecular asymmetry and not the presence of asymmetric carbon atoms is responsible for optical activity. Since the term asymmetric has been found to be inadequate, now the term chirality has been introduced. The word chiral (the Greek word cheir means hand pronounced kiral) signifies, the property of Handedness . An object that in not superimposable upon its mirror image is chiral and this mirror-image relationship is the same as left hand has with the right. If an object and its mirror image can be made to coincide in space, they are said to be achiral. 

The most noticeable features of the spectra, apart from the mirror-image relationship, are that ... 

In rigid molecules, where the geometries of the S0 and Si states are similar, there is a mirror-image relationship between the absorption spectrum and the fluorescence spectrum. This is due to the similarity of the energy spacing of the vibrational energy levels in the two states... 

Fig. 11.1 Mirror-image relationship ofthe amino acids. All amino acids except glycine) can exist in one of two mirror-symmetric forms, called enantiomers. Proteinogenic amino acids are almost exclusively the L-enantiomer. With some exceptions, biopolypeptides fold into right-handed a-helices. [1,2] In all cases,...

Fig. 3 Prolinamide (a) and 1-phenylethylamine (b) a-oxoamide salts. In the former, the anions are homochiral, while in the latter the anions have a near-mirror-image relationship...

If tetrahedral carbon has four different groups attached, it is found that they can be arranged in two different ways. These molecules are not snperimpos-able and they have a mirror image relationship to each other. This is most easily seen with models. 

Four different groups on tetrahedral carbon can be arranged in two ways - non-superimposable molecules with a mirror image relationship... 

Now the other three of the possible four stereoisomers are the (15,25), (l/f,2/f), and (15,2/f) versions. These are also shown, and mirror image relationships are emphasized. The (15,2/f) isomer is the mirror image of (—)-ephedrine, which has the (l/f,25) configuration. Therefore, it is the enantiomer of (—)-ephedrine, and can be designated (+)-ephedrine. Note that the enantiomeric form has the opposite configuration at both chiral centres. 

The other two isomers are the (15,25) and (IR,2R) isomers, and these two also share a mirror image relationship, have the opposite configuration at both chiral centres, and are, therefore, a pair of enantiomers. From a structure with two chiral centres, we thus have four stereoisomers that consist of two pairs of enantiomers. Stereoisomers that are not enantiomers we term diastereoisomers, or sometimes diastereomers. Thus, the (15,25) and (1R,2R) isomers are diastereoisomers of the (l/f,25) isomer. Other enantiomeric or diastere-omeric relationships between the various isomers are indicated in the figure. 

The luminescence spectra of rigid molecules like benzene, naphthalene, etc. show clear vibrational structures, especially in the gas phase. The first absorption band and the fluorescence band then show a mirror-image relationship in which the 0-0 vibrational transitions are nearly coincident (Figure 3.28). At room temperature only the lowest vibrational levels (v = 0)... 

Figure 3.28 The mirror-image relationship between the absorption and fluorescence spectra of a rigid molecule. In the gas phase the 0-0 bands would be theoretically coincident, and the spacing between the vibrational levels is not strictly identical in the ground and excited states...

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. 

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