56 The Structure of Protoplasm 



300 residues are capable of existing ionized at this concentration 

 of hydrogen ions. 



The manner in which the end groups of the various amino acid 

 residues become ionized is indicated for the two general types. 



A. The acidic groups lose a proton and thus bear a negative 

 charge. They include glutamic and aspartic acid residues. 



— COOH ^ H H COO 



B. The basic groups gain a proton and thus carry a positive 

 charge. Three types are of importance. 



1. The amino group of lysine. 



— NH, + H' ^ — NH.r 



2. The guanidine group of arginine (19). 



H H H + 



N N 



H II H + H II H 



— N— C-N +H — N-C-N 

 H H 



3. The imidazole ring of histidine (59) . 



-C = CH -C = CH 



I I + I I + 



HN N + H ^HN NH 



\ // \ // 



C C 



H H 



Studies (58) correlating" the range of hydrogen-ion concentra- 

 tion in which these groups ionize (7) with the resultant hydrogen- 

 ion concentration of cytoplasm (23, 60, 61, 62) , indicate that glutamic 

 and aspartic acid, as well as arginine and lysine, are probably com- 

 pletely ionized at the pH of cytoplasm. Histidine may be expected 

 to be only partially ionized, and tyrosine, serine, and the amide 

 groups of asparagine and glutamine probably not at all (7) . The 

 relative number of positively and negatively ionized residues, as 

 well as their spatial distribution (63) , may be expected to have a 

 great influence upon the electrical field surrounding the elementary 

 protein particles. Some of these ionized groups will be distributed 

 throughout the interior of the particle, perhaps forming ionic link- 

 ages which may help to maintain the specific configuration of the 

 chain (46 and 51); other ionized groups may exist on the surface 

 of the particle. A particle, such as in Figure 5, consisting of three 

 layers, will have six faces over which charged groups may be dis- 

 tributed, and these should have at least six to eight ionized groups 

 on each face; some plus, some minus. In the model at least four of 



I 



