PROTEINS 117 



tion. In fact, some natural forms are so exaggerated as to be of a 

 cigar-like shape. However, all these probably have their peptide chains 

 more or less looped back and forth instead of stretched into a single 

 filament. 



Actually, the bond angles of the atoms, the bond lengths, and the 

 sizes of the groups attached require that the peptide chain be a rather 

 complex affair. It has been proposed that this structure corresponds to 

 a helix with the side groups outside the spiral core of the peptide 

 chain. This configuration is believed to be stabilized by hydrogen 

 bonds between carbonyl and amide — NH — groups on different turns 

 of the spiral. How such a helix is folded to fit into a roughly globidar 

 pattern is still unknown. 



As dipolar ions, proteins react with both acids and bases, accepting 

 or giving up protons. Such reactions make them buffers resisting 

 changes in pH. Moreover, proteins exhibit characteristic isoelectric 

 points, depending upon their natures and their environments. They 

 undergo electrophoresis and may be separated by means of this 

 phenomenon when they migrate through the electrical field at differ- 

 ent rates. 



Proteins are enough alike chemically to undergo certain general 

 reactions. One of these is a test for the presence of two or more ad- 

 jacent peptide bonds and is called the biuret test. It is based on the 

 fact that a violet color appears when a protein or tripeptide is treated 

 with sodium hydroxide and dilute copper sidfate. The color arises 

 from the formation of a complex of cupric ion with two peptide bonds 

 or one peptide bond and one of several other kinds of structures. The 

 color is similar to that formed between copper and biuret, 

 HoNCONHCONHo, giving the test its name. 



Heating a protein with nitric acid produces a yellow color that turns 

 orange on addition of alkali. Aromatic amino acids, especially tyrosine 

 and tryptophan, are responsible for this test, known as the xantho- 

 proteic reaction. Many phenols yield red colors or precipitates when 

 treated with an acid solution of mercuric, nitrous, and nitrate ions. 

 This reaction is called Millon's test and is given by proteins by virtue 

 of the tyrosine normally present. 



Biological Properties 



Proteins serve in many biological roles. Most of these are critical to 

 life or at least to certain forms of life. Yet the diversity and com- 

 plexity of function are so great as to preclude extensive discussion 

 here. Some of these roles are outlined later, as noted below. 



Many proteins display catalytic activities, accelerating reactions to 



