Our Senses of Taste and Smell

Diastereomers interact with highly specific sensory receptors. For example, D-marmose, a carbohydrate, exists in two diastereomeric forms that differ in the configuration of a hydroxyl group at one center. The two isomers are designated (X and p. The a form tastes sweet, but the P form tastes bitter. 

Sensory receptors also readily distinguish enantiomers. The specificity of response is similar to the relationship between our hands and how they fit into gloves. Because sensory receptors are chiral, they interact stereospecifically with only one of a pair of enantiomers. The two enantiomeric forms of carvone have very different odors. (+)-Carvone is present in spearmint oil, imparting its odor. In contrast, its enantiomer, (-)-carvone, is present in caraway seed. It has the famihar odor associated with rye bread. 

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Using information about electron-dot formulas, we can predict the three-dimensional shapes of many molecules. The shape is important in our understanding of how molecules interact with enzymes or certain antibiotics or produce our sense of taste and smell. 

For example, our ability to taste and smell is regulated by chiral molecules in our mouths and noses that act as receptors to sense foreign substances. We can anticipate, then, that enantiomers may interact differently with the receptor molecules and induce different sensations. This appears to be the case. The two enantiomers of the amino acid, leucine, for example, have different tastes—one is bitter, whereas the other is sweet. Enantiomers also can smell different, as is known from the odors of the two carvones. One has the odor of caraway and the other of spearmint. 

The inability to taste food is a common complaint when nasal congestion reduces the sense of smell. Thus, smell greatly augments our sense of taste (also known as gustation), and taste is, in many ways, the sister sense to olfaction. Nevertheless, the two senses differ from each other in several important ways. First, we are able to sense several classes of compounds by taste that we are unable to detect by smell salt and sugar have very little odor, yet they are primary stimuli of the gustatory system. Second, whereas we are able to discriminate thousands of odorants, discrimination by taste is much more modest. Five primary tastes are perceived bitter, sweet, sour, salty, and umami (the taste of glutamate from the Japanese word for "deliciousness"). These five tastes serve to classify compounds into potentially... 

The inability to taste food is a common complaint when nasal congestion reduces the sense of smell, Fhus, smell greatly augments our sense of taste (also known as gustation), and taste is, in many ways, the sister sense to... 

In some cases relevant to food, our sense of taste may serve as the detector. Conversely, the patient may not be conscious of detection except by exhibiting the response. Early in our evolutionary history, this reflex probably served a very useful function since food which previously caused problems could be eliminated before absorption occurred. Since taste and smell are our body s most sensitive detectors, this reflex prevented unwanted intake of poisons. Our olfactory systems can detect chemicals at concentrations far below those necessary to induce a biological response. Today the mind is inappropriately associating certain distinctive organic smells with harm. 

I he sensations of taste and smell greatly affect our daily experience. For example, memories are often triggered by an odor that matches one that occurred when an event was originally stored in our memory banks. Likewise, the sense of taste has a powerful effect on our lives. For example, many people crave the intense sensation produced by the compounds found in chili peppers. 

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. 

Flavorings—These are the substances that stimulate the senses of taste and/or smell. With the exception of the four primary sensations—sweet, bitter, salty, and sour— flavor characteristics are the result of our perception of odor the difference between flavor and fragrance is in large part only a semantic distinction. Thus, a substance that provides an odor in perfumes may also be used to add flavoring to a food. 

As part of his investigation of the Psilocybe genus, Guzman noted a common trait among the hallucinogenic species, in addition to the bluing reaction a flour-like smell or taste. Apart from the inherent subjective nature of our sense of smell and taste, a common odor is a trait that definitely does not apply to the European species (also see Chapter 3.2). 

Molecular bonding and structure play the central role in determining the course of chemical reactions, many of which are vital to our survival. Most reactions in biological systems are very sensitive to the structures of the participating molecules in fact, very subtle differences in shape sometimes serve to channel the chemical reaction one way rather than another. Molecules that act as drugs must have exactly the right structure to perform their functions correctly. Structure also plays a central role in our senses of smell and taste. Substances have a particular odor because they fit into the specially shaped receptors in our nasal passages, and taste is also dependent on molecular shape. 

With the exception of the four primary taste sensations— sweet, bitter, salty, and sour—food flavors are the result of our sense of smell. Today, chemists can make chemicals in the laboratory which alone or in various combinations can imitate many of the natural food flavors. These are synthetic flavors. In many cases the synthetic flavors are superior to natural flavors in terms of (1) withstanding processing, (2) cost, (3) availability, and (4) consistent quality. Synthetic flavors may be substances that are prepared in the laboratory but chemically identical to those found in nature, or substances prepared in the laboratory which as yet have not been found to occur in nature but which produce familiar aromas. 

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