Left-handed enantiomers

Since the metal complexes can assume a number of closely related conformational structures and form racemic mixtures containing both right- and left-handed enantiomers in... 

Another scenario for the origin of homochirality was suggested by Pearson [8,9] such that chance breaks the chiral symmetry. Though the mean number of right- and left-handed enantiomers are the same, there is a nonzero probability of deviation from the equal populations of both enantiomers. The probability of establishing homochirality in a macroscopic system is, of course, very small [10], but chance produces a slight majority of one type of enantiomer and asymmetric compounds when they have once arisen act as breeders, with a power of selecting their own kind of asymmetry form [8,9]. In this scenario, produced enantiomer acts as a chiral catalyst for the production of its own kind and hence this process should be autocatalytic. 

R)/ S) designations given to the right-handed and left-handed enantiomers in the traditional Cahn-Ingold-Prelog system... 

The term chiral is also used to describe a sample of a substance. When used in this sense, it is not necessary that every molecule in the sample has the same handedness see the definitions below for optical purity, enantiopurity, etc. A racemic sample is one containing (statistically) equal numbers of right-handed and left-handed enantiomers, and therefore showing zero optical activity at all wavelengths. A sample can also be chiral, nonracemic i.e., it contains an excess of one enantiomer. 

D-d(CGCGCG) and L-d(CGCGCG), pair only with complementary strands which have the same chirality. Racemic DNA molecules only interact at the end of the strand to form crystals in which the D-configured double helix s ends stick to two L-configured double helices and vice versa In the crystal, a pseudohelix is thus formed which contains half-a-turn of right-handed and half-a-turn of left-handed enantiomer structures (Figure 5.33). There is no formation of polyphosphate sheets. 

Of course, nature has been at it for millions of years. Since nature has chosen to work with only the left-handed enantiomer of amino acids,2 enzymes (made up of... 

Figure A2.5.30. Left-hand side Eight hypothetical phase diagrams (A through H) for ternary mixtures of d-and /-enantiomers with an optically inactive third component. Note the syimnetry about a line corresponding to a racemic mixture. Right-hand side Four T, x diagrams ((a) tlirough (d)) for pseudobinary mixtures of a racemic mixture of enantiomers with an optically inactive third component. Reproduced from [37] 1984 Phase Transitions and Critical Phenomena ed C Domb and J Lebowitz, vol 9, eh 2, Knobler C M and Scott R L Multicritical points in fluid mixtures. Experimental studies pp 213-14, (Copyright 1984) by pennission of the publisher Academic Press.

The difference m odor between (R) and (S) carvone results from their different behavior toward receptor sites m the nose It is believed that volatile molecules occupy only those odor receptors that have the proper shape to accommodate them Because the receptor sites are themselves chiral one enantiomer may fit one kind of receptor while the other enantiomer fits a different kind An analogy that can be drawn is to hands and gloves Your left hand and your right hand are enantiomers You can place your left hand into a left glove but not into a right one The receptor (the glove) can accommodate one enantiomer of a chiral object (your hand) but not the other... 

The enantiomers shown are related as a right hand and left hand screw respectively Chiral allenes are examples of a small group of molecules that are chiral but don t have a chirality center What they do have is a chirality axis, which m the case of 2 3 pen tadiene is a line passing through the three carbons of the allene unit (carbons 2 3 and 4)... 

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. 

The enantiomers shown are related as a right-hand and left-hand screw, respectively. 

Molecules that are not identical to their mirror images are kinds of stereoisomers called enantiomers (Greek encmtio, meaning "opposite"). Enantiomers are related to each other as a right hand is related to a left hand and result whenever a tetrahedral carbon is bonded to four different substituents (one need not be H). For example, lactic acid (2-hydroxypropanoic acid) exists as a pair of enantiomers because there are four different groups (—H, -OH, - CH3, -C02H) bonded to the central carbon atom. The enantiomers are called (-i-)-lactic acid and (-)-lactic acid. Both are found in sour milk, but only the (+) enantiomer occurs in muscle tissue. 

Figure 9.17 Imagine that a left hand interacts with a chiral object, much as a biological receptor interacts with a chiral molecule, (a) One enantiomer fits into the hand perfectly green thumb, red palm, and gray pinkie finger, with the blue substituent exposed. (b The other enantiomer, however, can t fit into the hand. When the green thumb and gray pinkie finger interact appropriately, the palm holds a blue substituent rather than a red one, with the red substituent exposed.

If two enantiomers are mixed together in equal amounts the result is a racemic mixture. We meet a number of enantiomeric items in daily life. The left hand, for example, is the mirror image of the right hand and they are not superimposable (see Figure 8.1). This becomes obvious if we try to put a right glove on a left hand. Similarly, a pair of shoes is an enantiomeric relationship while the stock in a shoe store constitutes a racemic mixture. 

Now, consider repeating these operations, but with the use of the observer s left hand to spin the nut in an anticlockwise direction as seen by the observer. It should be apparent that, in either orientation of the rod, the nut will now travel backward (toward the observer). This must suggest that an electron would be ejected in a reversed direction following excitation by a photon of opposite helicity, again mediated through the chiral interaction of the electron with the enantiomer s potential field. 

Figure 1.6 The left-hand panel shows a molecular model of the glycinate/Cu l 1 0 structure with both enantiomers present in the heterochiral (3 x 2) unit cell, superimposed on an STM image of this surface. (Adapted with permission from Ref. [12]. Copyright 2002,...

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