GENERAL CHEMISTRY 



Proteins 

 The organic basis of protoplasm is generally considered to be protein. 

 In terms of elementary chemical composition, proteins contain carbon, 

 hydrogen, oxygen, nitrogen, and sulfur. They are complex molecules with 

 molecular weights varying from 10"' to 10*' and yield a mixture of amino 

 acids when hydrolyzed by alkali, acid, or enzymes. Amino acids are chain 

 carbon compounds containing carboxyl ( — COOH) and amino ( — NHo) 

 groupings. The structure of a typical amino acid is shown in Formula 

 (2-1), where "R" represents a specific group which determines the par- 

 ticular kind of amino acid. 



a-carbon (2-1) 



In the protein molecule, the amino acids are linked together by bonds 

 between the carboxyl group of one amino acid and the amino group of 

 another (Formula (2-2)). The — CO — NH — linkage between the two 



H H 



R— C— CO— NH— C— Ri ( 2-2 ) 



II 

 NH. COOH 



amino acids is called a peptide bond. When only two amino acids are 

 involved, the resulting protein is called a dipeptide. The combination of 

 an amino group and/or carboxyl group in the dipeptide with other amino 

 acids makes possible a large variety of combinations which are spoken of 

 as polypeptides. Because the component amino acids of a protein con- 

 tain potential acidic (carboxyl) and basic (amino) groups, proteins 

 may act either as acids or bases depending on the pH of the en- 

 vironment. Proteins, on this basis, are considered to be amphoteric 

 substances. Using glycine as an example, the formula of an amino acid 

 (protein) in its fully dissociated form may be illustrated in Formula 

 (2-3). In this form, the amino acid carries both positive and negative 



/NH3+ 

 Ch/ (2-3) 



\coo- 



14 / CHAPTER 2 



