210 



AMINO ACIDS, PEPTIDES AND PROTEINS 



pH 6. 4 was at first believed to be the cyclic 4-oxoazetidine-2-carboxylic acid (5a) but was 

 later shown to be fumaramic acid (5b). It should also be pointed out that the suggestion 



H 

 H C C-CO^H 



=C NH 



4-oxoazetidine-2-carboxylic acid 



H^N-C 



H ^ ^CO^H 



C 



II 

 C 

 / ^H 



fumaramic acid 



that asparagine exists in a hydrated cyclic imide form (6) had to be discarded when it was 

 shown that alpha-amino succinimide and asparagine were readily distinguishable in aqueous 

 solution (7). Due to the liability of the primary amide bonds neither glutamine nor aspara- 

 gine can be recovered from the acid or base catalyzed hydrolyses of proteins. 



The metabolic importance of these substances stems from the fact that they provide 

 links between carbohydrate and protein interactions, that they can act as precursors of 

 many other naturally occurring amino acids and that they appear to be involved in nitrogen 

 transport. 



The basic amino acids found in proteins and in the free state in higher plants are 

 histidine, lysine and arginine. Of these, arginine is by far the strongest base having an 

 isoelectric point of 10. 76. Although arginine is quite stable in acidic medium it is readily 

 hydrolyzed to citrulline, NH2C(=0)NH(CH2)3CH(NH2)C02H, or ornithine, 

 NH2(CH2)3CH(NH2)C02H, in basic solution. Both of these amino acids occur in higher 

 plants (8), (9). Lysine owes its basic character to the presence of two amino groups in 

 the molecule. Plant proteins are rather poor sources of this amino acid and since lysine 

 is an essential amino acid for man, a diet containing plant proteins as the sole source of 

 nitrogen will lead to a deficiency syndrome. The availability of synthetic lysine as a food 

 additive will go a long way in raising the nutritional standards of those countries in which 

 animal proteins are available only in insufficient amounts. 



Histidine is listed with the basic amino acids because it contains an imidazole group. 

 This group is also involved in the formation of colored derivatives when proteins are treated 

 with diazotized amines. 



SPECIAL AMINO ACIDS FOUND 

 IN HIGHER PLANTS 



Since the introduction of partition chromatography, the isolation techniques available 

 to chemists have gained in sophistication to the point where the number of new amino acids 

 being discovered is increasing at an aknost exponential rate. Therefore, no attempt will 

 be made to list all the new plant amino acids, but it is hoped that the compounds selected 

 for discussion will indicate the scope of the chemical variations being encountered. These 

 compounds are not constituent amino acids of proteins and are found as free amino acids 

 in a variety of higher plants. Comprehensive reviews of these non-protein amino acids 

 have been published (10 a and b). 



The discovery of amino acids which are widely distributed in higher plants in which 

 the amino group is not in the alpha position is of interest. Their structures are listed in 

 Table 2. Of these, gamma aminobutyric acid is the most ubiquitously distributed compound, 

 and it is a rare event when it is absent from a plant extract. 



