134 PROTEINS 



on X-ray data, have been advanced regarding the arrangement of the 

 chains in the protein molecule. Since the carbon and nitrogen atoms 

 in the backbone of the chain are at angles to one another, the chain has 

 a zig-zag appearance with hydrogens, oxygens, and "tails" (side chains 

 of the amino acid residues) sticking out at various angles with the back- 

 bone carbons and nitrogens. (See hydrogen bonding, p. 135.) 



If such a chain is folded back on itself, or if two chains are arranged 

 in the right order, the result is a puckered two-dimensional structure 

 or layer. Such a layer may be compared to a piece of lace crocheted 

 from a single thread. Several such layers may be superimposed on 

 one another to build a three-dimensional structure, as in a layer cake. 

 Layers may also be folded back and forth in some such fashion as a 

 road map is folded. When a protein is spread on a water surface, the 

 layers unfold and form a film only one layer thick. In such cases the 

 polar groups {e.g., amide, carboxyl, hydroxyl, phenol, etc.) are drawn 

 into the water surface while the nonpolar groups {e.g., hydrogen, paraffin, 

 benzene, etc.) are repelled and extend upward from the water surface. 



A somewhat different arrangement of the chains has been proposed by 

 Pauling for the structure of fibrous proteins, for example, a-keratin of 

 wool. According to this view, the chains are arranged in the form of 

 a helix (a spiral spring is an example of a helix) to give a hollow cylinder- 

 like structure. For collagen, Pauling concludes that the molecule con- 

 sists of three chains twisted about one another to give a rope-like effect. 



Theories on the structure of proteins are constantly changing as more 

 X-ray, infra-red, and other physical measurements are made. To quote 

 Bernal, ''The problem of the protein structure is now a definite and 

 not unattainable goal." 



One of the difficult problems in protein structure is to find a satisfac- 

 tory explanation for the manner in which the folds in a molecular chain, 

 or the layers in a molecule, are held together. Several types of forces 

 have been postulated, three of which will be mentioned here. 



Salt Linkage. Basic groups of one side chain (the second amino group 

 of lysine, the imadazole group of histidine, and the guanidino group of 

 arginine) may be united to acidic groups (the second carboxyl group of 

 aspartic and glutamic acids) of another chain to form an electrostatic 

 bond, that is, a salt. 



Chain 1 NH-CH-CO NH-CHCO--- 



I I 



(CHO4 (CH,)4 



NH, »- NH; 



+ . 



COOH *■ COO- 



I I 



CH2 CH2 



Chain2 OC-CH-NH--- ••OC-CHNH- 



Salt linkage 



