Chirality in Nature and Chiral Environments

Although the different enantiomers of a chiral molecule have the same physical properties, they usually have different biological properties. For example, the (+) enantiomer of limonene has the odor of oranges, but the (-) enantiomer has the odor of pine trees. 

More dramatic examples of how a change in chirality can affect the biological properties of a molecule are found in many drugs, such as fluoxetine, a heavily prescribed medication sold under the trade name Prozac. Racemic fluoxetine is an extraordinarily effective antidepressant but has no activity against 

The hand-in-glove fit of a chiral substrate into a chiral receptor is relatively straightforward, but it s less obvious how a prochiral substrate can undergo a selective reaction. Take the reaction of ethanol with NAD+ catalyzed by yeast alcohol dehydrogenase. As we saw at the end of Section 9.13, the reaction occurs with exclusive removal of the pro-R hydrogen from ethanol and with addition only to the Re face of the NAD+ carbon. 

The hundreds of different pharmaceutical agents approved for use by the U.S. Food and Drug Administration come from many sources (see the Chapter 5 Focus On). Many drugs are isolated directly from plants or bacteria, and others are made by chemical modification of naturally occurring compounds, but an 

Those drugs that come from natural sources, either directly or after chemical modification, are usually chiral and are generally found only as a single enantiomer rather than as a racemic mixture. Penicillin V, for example, an antibiotic isolated from the Pemcillimti mold, has the 2S,SR,6R configuration. Its enantiomer, which does not occur naturally but can be made in the laboratory, has no antibiotic activity. 

The S enantiomer of ibupro-fen soothes the aches and pains of athletic injuries much more effectively than the R enantiomer 

The hand-in-glove fit of a chiral substrate into a chiral receptor is relatively straightforward, hut it s less obvious how a prochiral substrate can 

FIGURE 5.16 (a)When a prochiral molecule is held in a chiral environment, the two seemingly identical substituents (red) are distinguishable, (b) Similarly, when an achiral coffee mug is held in the chiral environment of your hand, it s much easierto drink from one side than the other because the two sides of the mug are now distinguishable. 

In this chapter, we ve looked at some of the causes and consequences of molecular handedness—a topic crucial to understanding biological chemistry. The subject can he a hit complex, hut is so important that it s worthwhile spending the time needed to become familiar with it. 

Enantiomers are identical in all physical properties except for their optical activity, or direction in which they rotate plane-polarized light. 

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PHTP is a chiral host which can be resolved into enantiomers DCA and ACA are (or derive from) naturally occurring optically active compounds. Using these hosts inclusion polymerization can be performed in a chiral environment and can be used for the synthesis of optically active polymers. This line of research has been very fruitful, both on the synthetic and on the theoretical plane. It has been ascertained that asymmetric inclusion polymerization occurs by a "through space" and not by a "through bond" induction only steric host-guest interactions (physical in nature) and not conventional chemical bonds are responsible for the transmission of chirality (W). 

Many xenobiotics and pollutants are chiral in nature and the two enantiomers of these pollutants may have different toxicities [13]. Additionally, the degradation of some chiral pollutants is stereospecific in the environment, and the degradation of some achiral pollutants may result in chiral toxic metabolites. Moreover, it has also been reported that enantiomers may react at different rates with achiral molecules in the presence of a chiral catalyst [13]. It is also obvious that most of the identities and the structures in nature are chiral and, therefore, that there is a greater chance that the environmental pollutants will react at different rates. Therefore,... 

The economic growth of any country is directly related to health, a disease-free atmosphere and an eco-friendly environment. Therefore, to avoid any outbreaks of disease and other tragedies due to the adverse effects of chirality, it is essential to monitor chiral environmental pollutants. As discussed above, chiral agrochemicals are carcinogenic in nature and hence may create serious problems. Accordingly, it can be seen that chirality indirectly affects the world s economic growth. 

This example illustrates a subtle control of a chemical reaction by a delicate manipulation of tire stereochemical environment around a metal centre dictated by tire selection of tire ligands. This example hints at tire subtlety of nature s catalysts, tire enzymes, which are also typically stereochemically selective. Chiral catalysis is important in biology and in tire manufacture of chemicals to regulate biological functions, i.e., phannaceuticals. 

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