PROTEINS 



483 



Amphoteric Properties. — Proteins and their structural units 

 the amino acids have the power to combine with both acids 

 and bases so as to form salts; that is to say, a protein such as 

 gelatin will react with a base such as sodium hydroxide or with an 

 acid such as hydrochloric to form a salt — in the former case, 

 sodium gelatinate; and in the latter, gelatin chloride. Because 

 they possess both acid and basic characters, proteins are said to 

 be amphoteric. All substances are amphoteric if they possess 

 groups that can take on or give up the hydrogen ion. Bredig 

 was a pioneer investigator of amphoteric electrolytes, and he 

 defined them as substances that play the part of an acid toward a 

 base or the part of a base toward an acid; in other words, they 

 are substances that spht off or combine with H+ or OH" ions. 

 Mann points out that, in this sense, water is an amphoteric 

 electrolyte because its hydrogen atom and its hydroxyl radical 

 may be converted into the chemically active H+ and 0H~ ions 

 whenever water comes into contact with certain salts (see also 

 page 373). 



The hydroxyl derivatives of many elements from the middle 

 of the periodic table, such as aluminum, chromium, zinc, lead, tin, 

 manganese, arsenic, and antimony, all behave like weak bases or 

 acids and are, therefore, examples of amphoteric electrolytes 

 among inorganic compounds. It is not the metals but their 

 hydroxides that are amphoteric; thus: 



A1(0H)3 + 3HCL = A1+++ + 301" + 3H2O 

 and 



A1(0H)3 + NaOH = AIO2- + Na+ + 2H2O 



If we represent the remainder of the protein molecule by R, 

 to which are attached the amino and carboxyl radicals, then we 

 can illustrate the amphoteric properties of proteins in the presence 

 of acids or bases as follows : 



/NH2 /NH3CI 



R< + HCl ^ R< 



^COOH \COOH 



R< 



/NH2 /NH2 



R/ + NaOH -^ R< 



^COOH \COONa 



.NH3 



^COOH 



ci- 



R< 



.NH2 

 ^COO 



Na+ 



