Shallenberger, R.S., Acree, T.E. and Lee, C.Y. 1969.
Sweet taste of D- and L-sugars and amino acids and the steric nature of their chemoreceptor site.
Nature 221(5180): 555-6.
The tastes of 10% solns. of various enantiomorphic pairs of D and L sugars previously allowed to come to mutarotational equil. were compared by a taste panel. The solns. were all rated as rather sweet (scores 4-6) but the intensity of sweetness of a given D sugar and its enantiomorph were not distinguishable. A theoretical stereochem. structure for the taste bud receptor site is presented. Since any individual sugar OH group can act as an acidic or basic unit at the receptor site, the stereochem. nature of the OH groups with respect to each other does not affect their suitability for reaction with the receptor site. Thus, there is little difference in the tastes of the enantiomorphic sugars. If amino acids are considered, only the protonated amino group can serve as an acidic unit, and only the carboxylate group can serve as a basic unit. Since glycine and D- and L-alanine taste sweet, the size of the R group (in RCNH2CO2H) in nonsweet L amino acids must be greater than C2H5. A spatial barrier probably restricts the ability of nonsweet L amino acids to gain access to the receptor site. Regardless of the barrier, the OH groups of both D and L sugars can form an intermol. H bond at the receptor site. A series of 10% aqueous solutions of D- and L-sugars was submitted to a taste panel using a paired comparison technique, to compare the sweet taste intensity of each enantiomorphic pair. The sweetness scored for D- and L-isomers respectively were: arabinose, 5.2, 5.6; xylose, 4.6, 4.4; glucose, 5.4, 6.0; rhamnose, 4.6, 6.5; mannose, 4.9, 5.0; galactose, 5.6, 6.0; glucoheptulose, 5.0, 6.6; fructose, very sweet. The panel could not distinguish significantly between the sweetness of a D- and L-sugar. Some stereochemical implications of these data concerning the structure of taste bud receptor sites are discussed.