AMINO ACIDS 93 



chemistry, chromatographic investigations of amino acids have greatly 

 outnumbered similar investigations of other classes of compounds. 

 Consequently, more workers are aware of the possible application of 

 chromatography to the study of amino acids than perhaps any other 

 group of compounds. Therefore, it is not surprising that some of the 

 earliest investigations into the application of chromatographic tech- 

 niques to systematics involved amino acid patterns. There is no 

 indication in these early studies that there was careful consideration 

 of the question of whether amino acids were, on apriori grounds, 

 likely to be of greater systematic value than other classes of substances. 

 As has already been noted amino acids are among the least useful 

 classes of substances if one concentrates upon the approximately 

 twenty amino acids of protein. Not only are these protein amino acids 

 nearly always present in tissues but, in addition their absolute and even 

 their relative concentrations are so closely dependent upon the 

 physiological state of the moment and so sensitive to metabolic dis- 

 turbances that their quantitative as well as qualitative relationships 

 are Kkely to be of little systematic value. This last point will be dis- 

 cussed further in a later paragraph. 



Before the advent of paper chromatography, the study of 

 amino acids contributed very little data of taxonomic importance. 

 Chromatographic techniques, however, not only provided new dimen- 

 sions of study of the common amino acids (for example, comparisons 

 of amino acids of individuals and accurate measurements of the con- 

 centrations of various free amino acids in a single root apex), but in- 

 advertantly disclosed the presence of a variety of "new" amino acids. 

 Fowden (1959) describes these latter compounds as "products of 

 the chromatographic revolution." It should not be inferred that all of 

 the non-protein amino acids owe their discovery to paper chromatog- 

 raphy. In lists compiled by Vickery (1941) and Dunn (1943), prior to 

 the development of chromatography, a number of suspected non- 

 protein amino acids were included (though they were reported simply 

 as not known to be constituents of protein). Each of these lists con- 

 tained approximately fifty compounds, about twenty of which were 

 the ubiquitous protein amino acids. 



Table 6-1 illustrates some of the non-protein amino acids of 

 plants and relates them structurally to protein amino acids when 

 possible. At least one acid of column two is found in protein 

 (a-aminoadipic acid is found in the protein of corn seeds) though it is 

 more typically associated with non-protein amino acids. 



New amino acids continue to be reported, and since 1958 

 some twenty or more additional amino acids have been characterized. 

 Several new amino acids have been discovered in the Mimosaceae, 



