174 



THE CELL AND PROTOPLASM 



carbon-nitrogen, for each amino acid resi- 

 due. This forms a back-bone to ^vhich the 

 remaining parts of the amino acid are 

 attached alternatel.y forming ribs or side 

 chains. The baclv-bone portion of the chain 



Fig. 2. Eeproduction of a protein chain model 

 showing about 15 amino acid residues of various 

 kinds. The model was made to scale. The white 

 horizontal lines serve to emphasize the zig-zag 

 back-bone. 



varies only slightly from one protein to 

 another; the side-chain portion, however, 

 varies to a considerably greater extent. 

 This may be seen by referring to Table II 

 and to the model in Fig. 3. 



The dimensions of the models made to 



scale were based on radii and valence 

 angles of the respective atoms which have 

 been determined from many measurements 

 obtained from X-ray crystal analyses and 

 from absorption spectra methods. These 

 atomic characteristics are summarized in 

 Table III (Pauling and Huggins 1934; 

 Pauling 1939). 



There are several points of especial in- 

 terest to us in Fig. 3. One important 

 feature of the protein molecule may be 

 recalled : that every ox^^gen and every 

 nitrogen atom present is potentially a 

 hydration center ; or more specifically, that 

 these atoms are capable of forming hydro- 

 gen bridges with water molecules (Huggins 

 1936; Lassettre 1937) and thus binding the 

 water molecules to the protein in much the 

 same manner tliat water of crystallization 

 is bound in a crj'stal. In a chain of 300 

 residues there will be approximately 300 

 oxygens and an equal number of nitrogens 

 in the back-bone alone, almost regardless 

 of the kind of protein ; while, in contrast to 

 this, ill tlie side chains the number of these 



V* 



V"^ 





Fig. 3. A model of a polypeptide chain made to scale, shuwing eight amino acid residues. The back- 

 bone and two residues are accentuated by the heavy lines. The black balls represent carbon atoms; 

 those with a central dot, oxygen, and those with a central cross, nitrogen. The smaller white balls 



represent hydrogen atoms. 



