CHAPTER IV 

 THE COMPOUNDS OF THE PROTEINS 



1. The Amphoteric Character of the Proteins. — The fact that 

 one and the same protein can combine either with a base or with 

 an acid appears to have been first clearly stated by Platner (33) 

 in 1866. The term '^ amphoteric" which is now used to desig- 

 nate substances possessing this power is due to Bredig (4). 



The source of this dual combining-capacity of the proteins 

 has been discussed in Chap. I. In 1886, Strecker (51) pointed 

 out that amino-acids can, like other substituted ammonias, bind 

 acids by their — NII2 groups, or, as we now know, through the trans- 

 formation of trivalent into pentavalent nitrogen; while they can 

 also bind bases in virtue of their possession of a — COOH group. 

 It is probable that in most cases a proportion of the amino- 

 acid, when dissolved in water, combines with water to form a 

 compound analogous to ammonium hydrate, which is in chemi- 

 cal equilibrium with the unhydrated form also present in the 

 solution. Thus amino-acetic acid, 



/NH2 

 ^COOH 



when dissolved in water, is believed to partially undergo the 



reaction 



/ NH2 • / NH3.OH 



CH2; +H.0H=CH2(^ 



^ COOH ^ COOH 



the trivalent nitrogen becoming pentavalent, and the molecule 

 splitting off both hydrogen and hydroxyl ions in definite pro- 

 portions. As I have stated, however, the transformation is not 

 complete, nor is it by any means an easy matter, as a rule, to 

 determine its extent (25) (39). 



Nevertheless, as I have pointed out in Chap. I and as will be 

 more clearly revealed in the chapters dealing with the electro- 

 chemistry of the proteins, it is very improbable that terminal 

 — NH3OH or COOH groups in the proteins molecule are involved 



67 



