PROTEINS 99 



In typical amino acids, the proton has migrated from the carboxyl to 

 the amino group, the actual structures thus becoming carboxylate and 

 ammonium ions. This method o£ writing the structure is preferred 

 and is adopted here to assure constant recognition of the presence of 

 the charged groups and their special contributions to the properties 

 of these systems. 



As typical amphoteric compounds, amino acids react with both 

 acids and bases as follows: 



RCHCOO- + HA ;^ RCHCOOH + A" 



I I 



NH3+ NH3+ 



RCHCOO- + BOH ±:; RCHCOO" + B+ + H-O 



I I 



NH3+ NHo 



where HA and BOH represent acid and base, and A- and B+ anion 

 and cation, respectively. Thus in acid solution the carboxylate ion 

 acquires a proton, and the amino acid becomes an ammonium salt 

 of the acid. Conversely, in alkaline solution the ammonium ion loses 

 a proton, and the amino acid becomes the anion of a salt. These 

 reactions are quite reversible when the pH of the solution is shifted 

 suitably. Therefore, amino acids serve as buffers in certain ranges of 

 pH and tend to prevent pH change when acid or base is added. This 

 class of compounds contributes to the buffer capacity of many biolog- 

 ical fluids and is used in certain artificial buffers, particularly in 

 regions of either high or low pH. 



Whenever a substance exists as a zwitterion with charges of equal 

 magnitude but opposite sign, it is said to be isoelectric. The pH at 

 which this state occurs is called the isoelectric point. Actually the 

 ionization constants of most of the polar groups involved are far 

 enough apart to allow isoelectric molecides over a range of pH, which 

 is nevertheless still referred to rather inaccurately at the isoelectric 

 "point." All dipolar ions large and small have characteristic isoelectric 

 points which depend upon the nature and number of the ionizable 

 groups present and to some extent upon the solvent and additional 

 solutes in the system. 



Other ionizable groups merely introduce another step into the 

 reaction process. Consider tyrosine as a typical example, starting with 

 it in strongly acid solution, where it exists as the ammonium ion, 

 shown on page 100. 



