Chiral hands

Many drugs are racemates, including 13-blockers, nonsteroidal anti-inflammatory agents, and anticholinergics (e.g benzetimide A). A racemate consists of a molecule and its corresponding mirror image which, like the left and right hand, cannot be superimposed. Such chiral ( handed ) pairs of molecules are referred to as enantiomers. Usually, chirality is due to a carbon atom (C) linked to four different substituents ( asymmetric center ). Enantiomerism is a special case of stereoisomerism. Non-chiral stereoisomers are called diaster-eomers (e.g., quinidine/quinine). 

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)-... 

Figure 7.9b shows aspartame. You can see here two isomers with a chiral carbon pointed out by the arrow. It turns out that one of these chiral isomers is sweet and is a component of the artificial sweetener Nutrasweet, and the other is bitter. As receptor proteins (which are chiral) mediate our sense of taste, our taste is inherently chiral. Thus, some, if not all, of our senses are chiral For example, lemons and oranges may contain the two different enantiomers of a molecule (called limonene), which gives these fruits their distinctive but different smells. So our sense of smell is chiral, and likewise our sense of touch could also be chiral since it utilizes our chiral hands. 

Find all the chiral (handed) carbon centers in a nucleotide of DNA and mark them hy asterisks on the drawing. You can use generic labels for the phosphate (as P) and the base (as B). 

It is important to understand that plane-polarized light can be produced both naturally (calcite and quartz crystals) and synthetically (with aligned polymers [Chapter 6]), and when such polarized light passes through chiral ( handed ) materials (or solutions of chiral materials), the plane of polarization is rotated. Historically, this property has been called optical rotation, and the chiral material was said to be optically active. Indeed, experimentally, the rotation of the plane of plane-polarized light demonstrates optical activity and thus, that the material is chiral. 

The R, S convention is a scheme which has largely superseded the D, i. system to denote configuration about a chiral centre in a molecule. The convention allows unequivocal designation of the absolute configuration in a description of the positions in space of ligands attached to a chiral centre, in relation to an agreed standard of chirality like a right-hand helix. 

Compounds in which a chirality center is part of a ring are handled in an analo gous fashion To determine for example whether the configuration of (+) 4 methyl cyclohexene is R or S treat the right and left hand paths around the nng as if they were independent substituents... 

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)... 

A particular point of interest included in these hehcal complexes concerns the chirality. The heUcates obtained from the achiral strands are a racemic mixture of left- and right-handed double heUces (Fig. 34) (202). This special mode of recognition where homochiral supramolecular entities, as a consequence of homochiral self-recognition, result from racemic components is known as optical self-resolution (203). It appears in certain cases from racemic solutions or melts (spontaneous resolution) and is often quoted as one of the possible sources of optical resolution in the biological world. On the other hand, the more commonly found process of heterochiral self-recognition gives rise to a racemic supramolecular assembly of enantio pairs (204). 

The four groups attached to the Cq atom are chemically different for all the amino acids except glycine, where two H atoms bind to Cq. All amino acids except glycine are thus chiral molecules that can exist in two different forms with different "hands," L- or D-form (Figure 1.3). 

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