22O Biological Chemistry. 



The simplest of the amino-acids is known as glycine or 

 aminoacetic acid, the constitution of which is represented 

 by the formula CH 2 (NH 2 )-COOH. This substance is 

 therefore acetic acid in which a hydrogen atom is replaced 

 by an ammo group. It may be recalled that when acetic 

 acid is treated with chlorine, the hydrogens of the methyl 

 group may be replaced by chlorine, giving rise successively 

 to mono-, di-, and tri-chloracetic acid (see p. 90). When 

 monochloracetic acid is treated with ammonia, the chlorine 

 can be replaced by the amino group 



CH 2 C1- COOH + 2NH 3 = CH 2 (NH 2 ) - COOH + NH 4 C1 

 Now the group (NH 2 ) has basic properties by virtue of 



TTO1 

 which it forms salts such as CHg, COOH. Owing to 



the fact that this amino-radicle is contained in the mole- 

 cule, glycine only acts as a very weak acid, although it 

 contains a carboxyl group. Nevertheless, it can also form 

 salts of the type CH 2 (NH 2 ) COOM' by the replacement of 

 a hydrogen of the carboxyl group by metals. Substances 

 like glycine, containing both acid and basic groups, are 

 known as amphoteric electrolytes (or ampholytes). 



In addition to substances like glycine, which contain 

 one amino group and one carboxyl group, proteins yield 

 on hydrolysis amino-acids containing two amino groups 

 and one carboxyl group. In these the basic character pre- 

 dominates over the acid, and they were formerly known 

 as the " hexone bases." 



The proteins differ from one another chemically both 

 in the character and the quantity of the various amino- 

 acids which they yield on hydrolysis. The systematic 

 chemical examination and differentiation of the various 

 proteins was only possible after Emil Fischer and his 

 pupils had elaborated a method for separating in an 

 approximately quantitative way the amino-acids derived 



