100 GENERAL BIOCHEMISTRY 



HO,.^^ NH;j+ 



I I 



Na+OH- 

 > 



HO^^^ NH3+ ^ .,,„_ HO,^.==^ NH 



Na+OH- 



-O/-^ NH2 



'^^GHsCHGOO- 



At 25°C. the first step is half complete at pH 2.20, still quite acid. 

 The second step is half complete at pH 9.11, and the first reaction was 

 complete much below this moderately alkaline value. Step three is 

 half complete at pH 10.07. In very alkaline solutions tyrosine thus 

 exists as a doubly charged anion. 



These additional ionizable groups include the phenolic hydroxyl of 

 tyrosine, secondary carboxyl groups as in aspartic and glutamic acids, 

 extra amino as in lysine, and the basic guanido and imidazolium 

 groups of arginine and histidine. (See Table 5-1 of amino acids.) 

 These structures are extra functional groups and are extremely im- 

 portant in the biochemistry of these compounds. 



When an aqueous solution of an amino acid like glycine is placed 

 in an electric field, the glycine does not move, since it has charges 

 of opposite sign and equal magnitude at its isoelectric point. In this 

 way glycine differs from simple ions which migrate rapidly toward 

 the electrode of opposite sign. This phenomenon of migration in an 

 electric field is called electrophoresis, or ionophoresis when the elec- 

 trolytes are small. Glycine will, of course, migrate in the field when 

 the pH is changed enough to remove either one of the ionic charges. 

 Other amino acids, like aspartic acid or lysine, already have an 

 unequal number of opposite charges. In pure water these compounds 

 move in the direction dictated by the preponderant charge type. 

 Taking advantage of the fact that different amino acids have difEerent 

 numbers and types of ionizable groups, mixtures of amino acids may 

 be separated into the individual compounds or into simpler mixtures 

 by means of ionophoresis. Special methods incorporating other effects 

 permit the complete separation of the amino acids in mixtures. 



Chemical Properties 



Only a very few of the known chemical properties of amino acids 

 can be mentioned here. One of the typical reactions of acids is the 



