Enantiomorphous pairs

The etching of an enantiomorphous pair of asn crystals in the presence of (/ )-aspartic acid and () )-A-methylasparagine is illustrated in Figure 12. Under these conditions, only the R crystals are etched (Figure 12a), whereas the S crystals dissolve smoothly (Figure 12b). The dramatic differences in the surfaces of the two enantiomorphous crystals after dissolution again make it possible to perform a manual Pasteur-type sorting of the R and S crystals with a quantitative enantiomeric yield. 

Crystal structures of two hexitols, galactitol and D-mannitol, have been published. Galactitol is meso, but the permissible intramolecular center of symmetry is not utilized in the crystal.28 The molecules crystallize as enantiomorphic pairs that, in conformation, are almost centrosymmetric molecules the difference therefrom is of the same order of magnitude as the thermal motion of the atoms. The carbon atoms and terminal oxygen atoms form an approximately planar chain. All of the oxygen atoms are both donors and acceptors for an intricate network of hydrogen bonding. 

Can you think of other examples of enantiomorphic pairs in our universe For example, your ears are enantiomorphs. (I like to imagine a race of aliens... 

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 ... 

Diastereoisomers. Whereas compounds with one chiral center exist as an enantiomorphic pair, molecules with two or more chiral centers also exist as diastereoisomers (diastereomers). These are pairs of isomers with an opposite configuration at one or more of the chiral centers, but which are not complete mirror images of each other. An example is L-threonine which has the 2S, 3R configuration. The diastereoisomer with the 2S, 3S configuration is known as i-a//o-threonine. L-isoleucine, whose side chain is -CH(CH3) CH2CH3, has the 2S, 3R configuration. It can be called 2(S)-amino-3(R)-methyl-valeric acid but the simpler name L-isoleucine implies the correct configuration at both chiral centers. 

Geometrical isomerism Geometrical isomerism is possible only in hexacoordinate complexes and in the case of 2 1 metal, e.g. chromium and cobalt, complexes arises from coordination of the ligand in a meridional (81) or a facial (82) mode in an octahedral complex. In the former case only an enantiomorphic pair of isomers is possible, but in the latter the possibility exists of four enantiomorphic pairs and a centrosymmetric isomer (Figure 1). 

Space groups (or enantiomorphous pairs) that are uniquely determined from the symmetry of the diffraction pattern and systematic absences are shown in boldface type. 

Lateral view of three kinds of carbon nanotubes with end caps (a) armchair (5,5) capped by one-half of (T.. (b) zigzag (9,0) capped by one-half of Cgo, and (c) an enantiomorphic pair of chiral SWNTs each capped by a hemisphere of icosahedral fullerene C140... 

To date, no chiral zeolite or molecular sieve has been obtained. However, Newsam et al. (48) have shown that zeolite beta is an intergrowth of two distinct structures polymorph A and B. Polymorph A forms an enantiomorphic pair. Thus, synthesis of one of the enantiomorphs of polymorph A would yield the first chiral zeolite and initiate the possibility of performing intrazeolitic asymmetric catalysis. Shape selective asymmetric catalysis would be the ultimate achievement in shape selective catalysis, and would certainly be a step closer toward truly mimicking enzyme catalysis. 

A special case of hypersymmetry is when the otherwise symmetrically independent molecules in the crystal are related by hypersymmetry operations making them enantiomorphous pairs. 

The two columns are here arranged so as to place an enantiomorphous pair on each line. In the following discussion we shall mention only members of the A series analogous energy relationships must of course exist among corresponding members of the A series. 

The significance of stereoselectivity in drug action in terms of drug-receptor interactions only becomes established when the more active isomers of groups of enantiomorphic pairs are shown to possess identical configurations. Stereochemical correlations within the methadone and isomethadone group are justified because the members of each possess chiral centers of similar nature (methadone type 29 isomethadone type 30) and subsequent... 

Stereochemical theory predicts the possibility of the existence of sixteen pentahydroxycyclohexanes. Of these, four have a plane of symmetry while the remaining twelve represent six enantiomorphous pairs. Since quercitol is optically active, only the twelve active isomers need be considered. Kiliani and Scheibler in 1889 showed that oxidation with nitric acid gave mucic acid—an observation which, as Karrer much later pointed out, further limits the possible configurations. Only four... 

A total of six stereoisomers might be expected when Ch is 8-quinolinolate (0 N), including two trans forms (III, IV) and two cis forms (V, VI) both cis forms would exist as an enantiomorphic pair. 

Pairs of screw axes, in which the sums of the subscripts equal to the order of the axis are called enantiomorphous pairs since one is the mirror image of another. The latter is reflected in the graphical symbols of the corresponding pairs of the enantiomorphous axes. These are 3i and 32l 4i and 43 61 and 65 62 and 64. Two enantiomorphous axes differ only by the direction of rotation or (which is the same) by the direction of translation. 

Figure 26.14 Supramolecular chirality provided by the tilt of the elements, (a) An enantiomorphic pair of polyhedrons,...

Figure 26.16 Enantiomorphic pairs of chiral clusters (a) and ladders (b) which are composed of primary ammonium carboxylates.

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