SWEETENERS

Carbohydrate MiniTopics Carbohydrates  Elmhurst College
Sweet Receptor Site Saccharin Sucralose  Chemistry Department
Aspartame Cyclamate Acesulfame-K  Virtual ChemBook


 Sweetness Receptor Site

Introduction:

A prevailing hypothesis for decades has been that the perception of sweetness is initiated by an interaction of a sweet molecule with a receptor site in the taste buds. The actual receptor site has not been isolated, therefore a schematic of the site is presented based upon the best available evidence. Once a molecule interacts with the receptor site, a cascade of other reactions is set off to ultimately send the taste sensation to the brain.

Various hypotheses have been proposed, but the most widely accepted one is the three-point attachment theory (AH-B-X).

The structure of Aspartame is shown on the left. It is a chiral molecule with two chiral carbons, and therefore could exist is four isomer forms. This aspect of the molecule is crucial for undeerstanding the sweet receptor site. One isomer is sweet, while another is is bitter.

Review Chiral Compounds.

Quiz: Which two carbons in aspartame are chiral? Check answer using pull down menu.





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Sweetness Receptor Site:

The current working hypothesis the is three-point attachment theory (AH-B-X). This theory suggests that there are three specifc areas of the receptor site.

Note: The model receptor site is this author's conception and is based only very loosely on theory and geometry of the molecules. It is only useful for students' initial conceptions of the fit of a molecule into a receptor site.

Area (AH+): This area contains functional groups such as an alcohol, amine, or some other goup that has hydrogens available to hydrogen bond to some partially negative atom such as an oxygen on the sweet molecule. (See the acid group (COO-) on aspartame on left.)

Area (B -): This area contains functional groups such as an alcohol, acid, or some other goup that has a partially negative oxygen available to hydrogen bond to some partially positive atom such as hydrogen on the sweet molecule. (See the amine group (NH3+) on aspartame on left.)

Area (X): This area is more or less perpendicular to the other two areas interacts through hydrophobic or non-polar properties. The receptor area is non-polar and the area on the sweet molecule is also non-polar. (See the benzene ring on aspartame on left.)

It seems that all of the sweet molecules most have at least two of these properties, but there may be others as well. This interaction with the receptor site may be somewhat hard to see in some of the sweet molecules.


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Comparison of Aspartame Isomers:

In support of AH-B-X sweetness receptor site theory, observe the interaction of the aspartame isomers with the receptor site. Aspartame must be the L,L isomer in order to work effectively.

If the isomer is the D, L isomer, then as shown on the left, the benzene ring is in the wrong position to interact with the receptor site. This isomer gives a bitter taste rather than a sweet taste.

The other two possible isomers for aspartame will not match the sites either. It is a good thing that the idea of the chiral isomers was known because this had to be incorporated into the synthesis of aspartame for it eventual commercial use.

Compare Aspartame Isomers - Chime in new window