52 



PLANT PHYSIOLOGY 



4. Thioamino acids: 



Z-Cystine, COOHCHNH2CH2SSCH2CHNH2COOH 



B. Aromatic series: 



1. Z-Phenylalanine, CeHs-CHrCHNHoCOOH 



2. Z-Tyrosine, C6H4OHCH2CHNH2COOH 



C. Heterocyclic compounds: 



1. Z-Proiine, (C4H7NH)COOH 



2. Z-Tryptophan, (C8H5NH)CH2CHNH2COOH 



3. Z-Histidine, (C3H2N2H)CH2CHNH2COOH 



A detailed study of proteins and their products of hydrolysis 

 would be out of place here. Further discussion of this subject 

 can be found in textbooks on organic or physiological chemistry. 

 It should be pointed out, however, that it is the aromatic nucleus 

 of tyrosine that gives the xanthoproteic and Millon's reactions. 

 Cystine is the typical compound in the form of which sulphur 

 enters into the composition of the protein molecule. From it is 

 obtained hydrogen sulphide, which is always formed during the 

 complete decomposition of proteins in the process of putrefaction. 

 Among the products of the hydrolysis of proteins, besides amino 

 acids, a small amount of ammonia and often carbohydrates also 

 are found. 



The percentage relationship of the amino acids varies in 

 different proteins, and it is exactly these differences that are the 

 basic cause of the enormous variety of protein substances. For 

 example, the reserve proteins from the seeds of some agricultural 

 plants contain the following quantities in percentage of the most 

 important amino acids and ammonia. 



Alanine 



Leucine 



Proline 



Phenylalanine 



Tyrosine 



Aspartic acid. 

 Glutamic acid 



Arginine 



Lysine 



Histidine 



Ammonia .... 



Legumin of 

 peas 



2.08 

 8.00 

 3.22 

 3.75 

 1 .55 

 5.30 

 16.97 

 11.71 

 4.98 

 1.69 

 2.05 



Gliadin of 

 wheat 



2.00 

 5.61 

 7.06 

 2.35 

 1.20 

 0.58 

 37.33 

 3.16 

 0.0 

 0.61 

 5.11 



Edestin of 

 hemp 



3.60 



20.90 



1.70 



2.40 



2.13 



4.50 



14.07 



14.17 



1.65 



2.19 



2.28 



