50 



The Structure of Protoplasm 



the dimensional properties are of chief interest to us. It has become 

 well known that although many different kinds of proteins are 

 recognized they are all built on practically the same structural 

 scheme. Since the amino acids are all of the alpha form, they link 

 together to form a long peptide chain in which the adjacent amino 

 acid residues alternate from one side of the molecule to another. 

 The model in Figure 4 represents only a small portion of such a 



chain but nevertheless shows 

 its characteristic features; 

 that is, the central zig-zag 

 backbone portion and the 

 various side chains, or amino 

 acid residues, which extend 

 out at right angles to it. The 

 backbone feature is the fea- 

 ture which is common to all 

 proteins, while the kind and 

 arrangement of the amino 

 acid residues give the protein 

 its specific qualities. 



The dimensions of this 

 basic, structural chain are 

 nearly the same for all of the proteins (32, 33) . Along the backbone 

 each amino acid residue is allotted about 3.5 A or slightly less. As a 

 result of this, the length of the chain depends directly upon the total 

 number of residues. Thus, a chain of 300 residues is likely to be about 

 1,000 A long. The width varies somewhat depending upon the size 

 and the proportional content of the various residues. The shortest 

 residue, glycine, extends scarcely an Angstrom from the backbone; 

 while the longest, arginine, or perhaps tryptophane, may extend 6 

 to 8 A. The width of the chain as a whole, then, depends upon the 

 weighted average for the 300 or so constituent residues of various 

 lengths, and while it is slightly different from protein to protein, 

 depending upon the percentages of the various short and long 

 residues (32) , the width is, despite this, fairly consistently close to 

 10 A, rarely more than 12 A, and seldom less than 9 A. The thick- 

 ness of the chain through the backbone and normal to the 10 A 

 width is consistently 4.5 A for all of the many proteins which have 

 been measured (32) . A few are 4.4 A, and an occasional one is 

 4.6 A. From these values a generalized conception is obtained of 

 the chain dimensions, 4.5 A x 10 A x (3.5) 7i, where n is the number 



H-1.1A 



Fig. 3. Diagram of hydrogen bridge be- 

 tween two water molecules. Note attrac- 

 tion of residual negative charge on one, 

 for hydrogen nucleus, or proton, on the 

 other, and the separation distance of 2.8A. 



