Biology Question #4024

Kevin Swope, a 25 year old male from San Diego/California asks on November 28, 2007,

Why does Aspartame taste so sweet? What is the chemistry involved in something that tastes sweet?

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The answer

Barry Shell answered on November 29, 2007

While great progress has been made in recent years, the exact chemistry of taste (and smell) is not completely understood. It remains one of the outstanding questions of science. It appears that taste receptors in the tongue are able to sense molecular shape. However a competing theory (or it may be a complementary theory) proposes that the receptors can sense molecular vibrations and that it is the vibrational state of a molecule rather than its shape that determines taste and smell. Perhaps it is a combination of both. (Image of aspartame molecule, below.)

Experiments have shown that  taste (and smell) is sensed by a particular complex of proteins formed of two protein subunits that cross the outer membrane of sensory cell walls in the nose and tongue. We know what these proteins look like and how they go together, but we don't know precisely how they sense taste and smell. The ends of these protein molecules that stick outside the sensory cells can take on thousands of different configurations based on an efficient genetic coding system for smell and taste in the genes. The many thousands of configurations match possibly millions of different molecular shapes when they are all working together. The shape theory assumes that certain parts of a molecule bind to certain shape receptors on the ends of the taste cell proteins. For instance on sugar there is always a double bonded oxygen in a ring of carbons and hydrogens and a similar structure also appears in aspartame.

There are various problems with this theory however.  Why, for instance, does aspartame taste about 250 times sweeter than sucrose (table sugar)? Some sweet-tasting proteins can be thousands of times sweeter than sugar. Also, some sweet tasting proteins are positively monstrous in size compared to sugar, but they taste the same. If you replace all the hydrogens of a sweet smelling molecule with deuterium (the nucleus of the hydrogen atom has a neutron as well as a proton, while ordinary hydrogen has only a proton and no neutron) most molecules will smell or taste differently. Since you have not changed the shape but you have changed the vibration of the molecule by the addition of deuterium atoms instead of hydrogen atoms, it does make you wonder about the shape theory and that there must be something else going on. That is why the chemistry and in particular the chemical mechanism of taste and smell is still open for exploration.


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