Novel amino acids from plants

Abstract

The availability of new chromatographic techniques in the late 1940s facilitated the study of complex mixtures of compounds present in extracts of plants, and paper chromatography was particularly useful for the separation, and subsequent characterization, of amino acid constituents. During the last 40 years, several hundred novel amino acids have been identified within the plant kingdom, and new characterizations continue as analytical techniques become more refined and sensitive. These ‘newer’ amino acids are not present normally in protein molecules of the ‘producer’ species, and for that reason often are called non-protein amino acids. Since they are now so numerous, it is not surprising that they display a very wide range of structural features. Some molecules contain groupings not encountered among the 20 protein amino acids, e.g. ethylenic and acetylenic linkages as part of aliphatic C chains, cyclopropane rings, novel heterocycles containing N, S or O atoms, etc. As a distinct group of plant secondary products, these metabolites pose many interesting problems in regard to biosynthesis and function. Some of the compounds are toxic, which sometimes can be explained by direct antagonism of intermediary metabolic processes concerned with amino acid biosynthesis, degradation, or incorporation into protein molecules. The poisonous nature of some of these substances suggests possible roles as protective agents produced by particular plants to confer a degree of resistance to attack by pests or pathogens. The remainder of this paper will elaborate the concepts expressed above through suitable examples.