166 CHEMISTRY OF PLANT LIFE 



AMINO-ACIDS AND PEPTID UNITS 



The products of hydrolysis of the common simple proteins are 

 all amino-acids. These are ordinary organic acids with one (or 

 more) of the hydrogen atoms of the alkyl group replaced by a 

 NH2 (or sometimes by a NH ) group. They may be 

 regarded as ammonia, NHa, with one of its hydrogen atoms 

 replaced by an acid radical ; or as the acid with one of its hydrogens 

 replaced by the NH2 group. For example, an amino-a^id derived 

 from acetic acid, CHa-COOH, is glycine, or amino-acetic acid, 

 CH 2 NH 2 -COOH; from propionic acid, CH 3 -CH 2 -COOH, there 

 may be obtained either a-amino-propionic acid, CHa-CHNH- 

 COOH, or /3-amino-propionic acid, CH 2 NH 2 CH 2 COOH, etc. 



All of the amino-acids which result from the hydrolysis of pro- 

 teins are a-amino-acids, that is to say, the NH 2 group is attached 

 to the a-carbon atom, i.e., the one nearest to the COOH group. 

 Hence, the general formula for all the amino-acids which are 

 found in plants is R-CHNH 2 -COOH. 



These amino-acids contain both the basic NH 2 group and the 

 acid COOH group. For this reason, they very easily unite 

 together, in the same way that all acids and bases unite, to form 

 larger molecules, the linkage taking place between the basic NH 2 

 group of one molecule and the acid COOH group of the other, as 

 indicated by the following equation: 



R R 



HOOC - C - N |H + HO|OC - C NH 2 



I I ! 



HH H 



R R 



= HOOC-C-N OC-C-NH 2 +H 2 O 

 H 



u 



It is obvious that the compound thus formed still contains a free 

 NH2 group and a free COOH group, and is, therefore, capable of 

 linking to another amino-acid molecule in exactly the same way; 

 and so on indefinitely. In actual laboratory experiments, as 



