Apparent specific volumes and tastes of amino acids

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Abstract
The apparent specific volumes of 17 amino acids are determined and compared with their tastes and predicted ranges of taste reported in earlier work. Most amino acids elicit many basic tastes although one taste usually predominates. All D-amino acids tasted (except proline and hydroxy prohne) are sweet and seven of the L-amino acids are sweet. Ten of the 16 L-amino acids are bitter but two of these are both bitter and sweet in accordance with prediction from their apparent specific volumes. ‘Spatial barriers’ account for the exclusion of the larger L-amino acids from sweet receptors. However, D-proline, the only reportedly purely bitter D-amino acid, is cyclic and this explains its borderline position between the sweet and bitter regions, and the sweet-bitter taste of L-proline. Low mol. wt amino acids, irrespective of whether they are D- or L-enantiomers, are always sweet, providing that their apparent specific volumes are below 0.66 and that they do not generate suprathreshold concentrations of protons. The two dicarboxylic amino acids, glutamic and aspartic are sour. Apparent specific volumes represent the effective size of solutes in water due to their intrinsic molecular architecture. The values reported here are used to interpret steric exclusion of certain enantiomers from the taste receptors. These results support the idea that apparent specific volume is a determinant of taste quality.