Molecular Structure in Protoplasm 55 



patterns formed by these residues on the surface. The presence of 

 the water molecules within the particle may spread the chains lat- 

 erally from the 10.5 A distance when dry to perhaps as much as 

 15 to 20 A when water is plentiful as in an active cell. This would 

 enlarge the protein packet just mentioned to a cube of about 50 A 

 on an edge. 



Although the particles may appear regular in diagram, they will 

 no doubt be definitely irregular in outline due to the variable lengths 

 of the amino acid residues and perhaps to the uneven length of the 

 chain folds. Details of the structural features will depend to a 

 considerable extent upon the proportion and distribution of the 

 residues on the chain, as well as upon the number of chain folds 

 in the particle. 



We turn now to a more intimate consideration of the residues 

 and their influence upon the formation of these larger structures. 

 There are about twenty different amino acids obtainable from prac- 

 tically every protein, and one protein differs from another in the 

 proportion of the amino acid residues it contains. Thus while gelatin 

 contains 25.5 per cent glycine (55) , casein has only 0.5 per cent (56) 

 and so for the variations in quantity of all twenty. While the amino 

 acids themselves have a generic similarity, in that each has a group 

 consisting of a carboxyl and an amino group attached to the alpha 

 carbon, they differ specifically in the remainder of the molecule (57) . 

 In general, in the case of cytoplasmic proteins, at least one-third 

 of the total residues are polar; the remaining two-thirds have 

 residues which contain neither oxygen nor nitrogen atoms and thus 

 are comparable to fatty substances in their behavior toward water. 

 In contrast to this about one-third of the amino acid residues have 

 either OH or COOH as oxygen-containing groups, and NHj or some 

 other nitrogen-containing group such as the guanidine group or the 

 imidazole ring. Those having an affinity for water coordinate with 

 various, but rather specific, numbers of water molecules; for 

 example, OH and NH- may each form hydrogen bridges with three 

 water molecules (54) . 



In the formation of larger protein structures, ionization plays 

 an important part. From the amino acid analyses (8) of various 

 proteins whose residues are approximately one-third polar, it is 

 possible to ascertain that approximately 85 per cent of these polar 

 residues are ionizable at pH 7, the pH of cytoplasm; that is, about 

 one-fourth of the total residues on the chain or at least 60 out of 



