MOLECULAR STRUCTURE IN PROTOPLASM 



175 



TABLE III 



Summary of Atomic Eadii and of Valence 

 Bonds and Angles 



hydration centers is less and will vary with 

 the proportion of the various amino acids 

 present in the chain. It may be seen in 

 Table IV that only a few residues contain 

 nitrogens and only a few contain oxygens, 

 aside from those in the back-bone. 



We turn again for a moment to dimen- 

 sions, since dimensions are important in 

 establishing the validity of the chain con- 

 ception and also in our study of the sizes 

 and forms of the particles in the proto- 

 plasm. I shall not enter into a description 



of the X-ray methods of determining these 

 dimensions but merely give some of the 

 results which are of direct use to us. It 

 was through the X-ray studies of silk pro- 

 tein in the form of silk fibers that the repe- 

 tition distance of the peptide group along 

 the back-bone, C N , was found to 



/ \ / \ 



C 

 be 3.5 A. This distance varies in different 

 proteins to as low^ as 2.8 A in gelatin, where 

 many proline linkages are involved. The 

 computed distance for the greatest exten- 

 sion of the chain is about ;3.(i5 A, but 

 neighboring atomic groups and proline 

 linkages reduce this in varying amounts. 

 A reasonable value where great exactness 

 is not demanded is 3.5 A. This means that 

 each amino acid has allotted to it 3.5 A of 

 the chain length, and that 300 residues 

 would thus form a chain about 1000 A in 

 length. 



The diffraction patterns obtained by X- 

 rays from the various proteins, with the 

 exception of protein crystals under certain 

 conditions, show that minute packets of 

 parallel chain-lengths occur in what ap- 

 pears otherwise to be amorphous masses .of 

 protein. These packets occur in random 

 arrangement similar to the minute crystals 

 in a crystal powder, differing, however, in 

 that they are attached to one another to 

 form a hard, solid mass when dry. 

 Stretching of this mass of protein in some 

 cases may orient these minute packets into 

 a less random arrangement (Astbury, 

 Dickinson and Bailey 1935; Astbury and 



TABLE IV 



IIydrophilic Groups on Various Protein Molecules 

 (Number based on a molecule of 288 residues.) 



