68 



The Structure of Protoplasm 







points obviously to the necessity for attachment of certain small 

 molecules to specific larger ones in order to bring about particular 

 activities, and where the only means of attachment is through a 

 few sorts of residues restricted to limited localities on the larger 



molecule to specific larger 

 ones in order to bring about 

 particular activities, and 

 where the only means of at- 

 tachment is through a few 

 sorts of residues restricted 

 to limited localities on the 

 larger molecule, the concep- 

 tion of specificity is given a 

 spatial as well as a structur- 

 al connotation. 



There is still one more 

 class of components which 

 is involved in the construc- 

 tion of particle aggregates in 

 cytoplasm; this comprises the 

 inorganic ions. In general 

 the interactions of these 

 small ions and the protein 



» 



"^ 







r% 



Fig. 15. Nucleic acid, a tetranucleotide, 

 plus a protein showing dimensional rela- 

 tionship between four phosphoric acid 

 groups and four adjacent amino acid resi- 

 dues. 



molecules are of two sorts; 

 the ions may attach to the protein, and the protein may have a dis- 

 placing effect on the neighboring ions in the surrounding medium. 

 The kind and number of ions adsorbed will depend upon the nature 

 and distribution, both quantitative and spatial, of the charged groups 

 on the protein particle. The monovalent ions, Na+ and K+, will have 

 some tendency to associate with ionized carboxyl groups, but the 

 divalent ions, Ca"l + and Mg++, will have a more pronounced tend- 

 ency to become absorbed (112, 7) . The negative ions, NOs", SO4 , 



and PO4 , have been shown to associate with basic proteins (113) 



probably with the residues of lysine, histidine, and arginine. Studies 

 of the displacement of the isoelectric point of various proteins with 

 changes of ion concentration of the solution (7, 114) give evidence 

 of ion association with the protein. 



These examples of aggregation in which only one protein molecule 

 is involved with another protein, or with a fatty molecule, or with 

 another small organic molecule, or an inorganic ion, are instances 

 of simple interactions involving only two molecules at a time. It 



