We point out an ability of certain amino acids to be recognized at a biological receptor site as though their amino group bore, instead of an alpha relationship to a carboxylate group, a beta, gamma or delta relationship to the same or a second carboxylate group. For aspartate, the unbalanced position of its amino group between a pair of carboxylates allows its occasional biorecognition as a beta-rather than as an alpha-amino acid, whereas for proline and its homologs, their cyclic arrangement may allow the imino group, without its being replicated, to be sensed analogously as falling at either of two distances from the single carboxylate group. The greater separation might allow proline to be seen as biologically analogous to gamma-aminobutyric acid. This more remote positioning of the imino group would allow the D-form of both amino acids to present its amino group in the orientation characteristic of the natural L-form. The dual modes of recognition should accordingly be signalled by what appears to be low stereospecificity, actually due to a distinction in the enantiorecognition of the two isomers. Competing recognition for transport between their respective D- and L-forms, although it does not prove that phenomenon, has been shown for proline and, significantly, even more strongly for its lower homolog, 2-azetidine carboxylate. Such indications have so far revealed themselves rather inconspicuously for the central nervous system binding of proline, reviewed here as a possible feature of a role suspected for proline in neurotransmission.

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