Enantiomeric molecules

Although Pasteur was unable to provide a structural explanation—that had to wait for van t Hoff and Le Bel a quarter of a century later—he correctly deduced that the enantiomeric quality of the crystals was the result of enantiomeric molecules The rare form of tartanc acid was optically inactive because it contained equal amounts of (+) tartaric acid and (—) tartaric acid It had earlier been called racemic acid (from Latin racemus meaning a bunch of grapes ) a name that subsequently gave rise to our pres ent term for an equal mixture of enantiomers... 

FIGURE 4.12 Enantiomeric molecules based on a chiral carbon atom. Enantiomers are nonsuperimposable mirror images of each other. 

The recognition of differences in the pharmacological activity of enantiomeric molecules has created the need to administer them - and therefore to obtain them -as isolated enantiomers. However, nowadays this problem affects not only the pharmaceutical industry, but also the agrochemical industry and food additive producers, both of which are increasingly concerned by this subject. 

Strictly speaking, the term racemic mixture applies only when the mixture of molecules is present as separate solid phases, but in this book we shall use this expression to refer to any equimolar mixture of enantiomeric molecules, liquid, solid, gaseous, or in solution. 

Fig. 9.6 Mirror image behaviour of enantiomeric molecules. The left-handed L-a-amino acid is converted to the right-handed D-a-amino acid by reflection... 

Compound (I)—The protons are enantiotopic because respective replacement of enantiotopic atoms by another ligand affords enantiomeric molecules. 

Mason, S. F. and Tranter, G. E. (1983). The parity violating energy difference between enantiomeric molecules. Chem. Phys. Lett., 94, 34. 

The parity violating energy difference between enantiomeric molecules. Mol Phys., 53, 1091-111. 

Optical isomerism is the result of a dissymmetry in molecular suhstitution. The basic aspects of optical isomerism are discussed in various textbooks of organic chemistry. Optical isomers (enantiomers) may have different physiological activities from each other provided that their interaction with a receptor or some other effector structure involves the asymmetric carbon atom of the enantiomeric molecule and that the three different substituents on this carbon atom interact with the receptor. The Easson-Stedman hypothesis assumes that a three-point interaction ensures stereospecificity, since only one of the enantiomers will fit the other one is capable of a two-point attachment only, as shown in figure 1.13 for the reaction with a hypothetical planar receptor. However, it is reasonable to assume that receptor stereospecificity can also undergo a change when the receptor conformation is altered by a receptor-drug interaction. 

Self-resolution of Enantiomeric Molecules 3.2.1 Enantiomeric self-resolution on crystallisation... 

Self-resolution of Enantiomeric Crystals Containing Both Enantiomeric Molecules... 

Of course, the probability is small that at any instant, the enantiomeric mixture at equilibrium is exactly equimolar the absence of observable chirality phenomena, such as optical activity, is the result of rapid cancelations of random statistical fluctuations of activity in the time domain of observation. In other words, although, at any instant, the mixture (with a high degree of probability) has an excess of one enantiomer or the other, under measurement conditions, it effectively contains an equal number of enantiomeric molecules. When 10,000,000 dissymmetric [i.e., chiral] molecules are produced under conditions which favor neither enantiomorph, there is an even chance that the product will contain an excess of more than 0.021 % of one enantiomorph or the other. It is practically impossible for the product to be absolutely optically inactive [12],... 

Similar considerations apply to enantiomeric molecules that are represented by static models. Although the time-averaged structures of two such enantiomers (say, d- and L-alanine) will have approximate mirror-image relationships, it must be remembered that we are not dealing with mathematical objects Exact mirror-image relationships belong exclusively to Kelvin s ideally realized ... 

When do enantiomers have different properties Again, it is helpful to draw analogies with everyday objects that are chiral. When do your hands have different properties They are different when you put on a glove they are different when you write they are different when you shake hands. What do these objects or activities have in common A glove, writing, and shaking hands are all chiral Hands are different when they interact with one enantiomer of a chiral object or activity. Likewise, enantiomeric molecules are different when they are in a chiral environment. Most commonly, the chiral environment is the presence of one enantiomer of another chiral compound. Otherwise, their properties are identical. For example, the naturally occurring ketones (R)- and (5)-... 

This paper thus clearly defined the possibility for a three-point attachment to provide for the differentiation of enantiomeric molecules. In support of the theory, they cited results on the action of epinephrine and related compounds. Cushny had reported that (— )-epinephrine is 12-15 times as active as (+ )-epinephrine. Easson and Stedman deduced that of the four groups around the chiral center, the hydrogen played no part in the drug-receptor attachment. Thus for (—)-epinephrine the complex could be represented as 7. With the (+ )-enantiomer the hydroxyl group cannot be positioned on the hydroxyl receptor. It was suggested that a less perfect combination was possible if the hydrogen were located on the normal hydroxyl... 

The relationships existing within molecules possessing some asymmetric characteristics have been treated in detail by several authors. Protons HA and HB in a molecule such as I-III (with P, Q, and R different) are said to be diastereotopic, whereas if P = Q, they would be enantiotopic. In the latter case, as in the case of two enantiomeric molecules, the NMR spectrum normally does not distinguish between the two, and the chemical shifts of HA and HB are the same. However, in a chiral solvent the interactions between the solvent and the two protons are not necessarily equivalent, and chemical shift differences (usually small) may be found. 

Many of the CSPs have been developed by systematically applying the principle of reciprocity of (enantioselective) molecular recognition. This states that if a single enantiomeric molecule of a chiral SO has different affinities for the enantiomers of a pair of selectands, then a single enantiomer of the latter will have different affinities for the enantiomers of the initial selector molecule, as well as for structurally related solutes. 

OMS140 77AC1071). An interesting effect has been observed in the efficiency of formation of attachment ions [2M + H] of optical isomers of dialkyl tartrates by chemical ionization. The abundance of attachment ions consisting of two enantiomeric molecules ( meso ions ) is lower than that of the [2M + H]" ions in which both molecules have the same configuration. This was the first report of detection of chirality by mass spectrometry (77JA2339). 

Enantiomers are nonsuperimposable mirror image isomers. Enantiomeric molecules are able to rotate the plane of polarization of plane-polarized light. 

Because each member of a pair of enantiomeric molecules differs from the other only in the spatial arrangement of the ligands attached to the chiral center, their physical properties, that is, melting point, boiling point, refractive index, solubility, etc., are identical. The major difference between the isomers of an enantiomeric pair was first observed by Biot in 1815 when he noted that one form of tartaric acid rotated plane-polarized light, whereas another form did not (1). 

This convention can be used to rapidly and unambiguously specify the configuration of a chiral center. As would be expected, for an enantiomeric molecule the direction of the sequence for one enantiomer is reversed for the other enantiomer. If one enantiomer has an R designation, its mirror image has the S configuration. 

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