PROTEINS 101 



formation of amides. Since amino acids contain both necessary func- 

 tional groups, two molecules should be able to combine with each 

 other. This process occurs, of course, and leads to the formation of 

 peptides and proteins. This biologically important chemical property 

 of amino acids is discussed more extensively under the peptide bond 

 (page 104). 



The ammonium ions may be replaced by hydroxyl groups by means 

 of various reactions, the most common employing nitrous acid: 



R— CHCOO- RGHCOO- 



I + HONO -^ I + H3O+ + N, 



NH3+ OH 



At a suitable pH this reaction is quantitative, and measurements of 

 the nitrogen gas indicate the number of amino groups or monosub- 

 stituted ammonium ions present. In fact, this determination can be 

 applied to peptides and proteins. Biochemical systems carry out the 

 same sort of replacement reaction in the metabolism of amino acids. 

 In cells the actual mechanism is different, but hydroxy acids and keto 

 acids are produced in large amounts. Moreover, the process is reversed 

 by cells using ammonia and keto acids to form amino acids (see page 

 464). 



Other chemical reactions occur and many are carried out in bio- 

 chemical operations of various kinds discussed in the sections on 

 metabolism (pages 463-494). Finally, as ionic compounds, amino 

 acids tend to be soluble in polar solvents like water and insoluble in 

 non-polar solvents. Such solubility behavior is required by the imiver- 

 sal biological roles of water. 



Optical Isomerism 



Most but not all of the naturally occurring amino acids possess 

 asymmetric carbon atoms. Hence there are possibilities for optical 

 isomerism in this group of compounds. In fact, those examples with 

 asymmetric carbons occur in given sources as particular isomers. It is 

 believed, therefore, that optical specificity of amino acids is preserved 

 with regard to each specific occurrence. In other words, a particular 

 peptide with o-alanine in it contains the d form only. The alanine of 

 proteins, on the other hand, is always of the l form. 



The conventional representation of the configuration of carbohy- 

 drates was explained on page 59. For amino acids the common 

 two-dimensional scheme is illustrated with alanine. 



