Molecular Structure in Protoplasm 57 



the six surfaces would contain ionized residues available for the 

 attachment of small organic molecules. 



The interaction of one charged particle with another in its imme- 

 diate environment depends largely upon the pattern of the electrical 

 field surrounding it. The magnitude of this field diminishes with 

 distance. It is possible (58) , by arbitrarily placing a single positive 

 charge at various short distances out from the charged groups on 

 the surfaces, and by making use of Coulomb's law (65) in which 

 the force of attraction and of repulsion varies inversely as the 

 square of the distance, to gain some notion of the variation of the 

 resultant field around a particle. In general the resultant effect 

 is such as to give strong local areas of positive and of negative 

 fields close to the surface of the particle, while weaker resultant 

 fields occur farther away (58). The net charge may be effective 

 only at distances of 50-100 A and may be either positive or negative. 

 It may be thought of as the algebraic sum of the positively and 

 negatively ionized residues. 



An additional feature of the protein molecule which may prove 

 to be significant when considering attachment of other molecules 

 is that concerned with residue arrangement. The frequency of 

 distribution along the chain of polar residues, such as lysine with 

 an amino group and glutamic acid with its carboxyl group, has a 

 strong bearing on the location of these groups on the surface of the 

 particle. It has long been realized (66) that, given twenty different 

 kinds of amino acid residues with no restrictions as to the relative 

 number of each, it is possible to obtain millions of different proteins 

 on the basis of composition alone without considering the number 

 of additional types possible when their arrangement is also varied. 

 Recent experimental studies (67, 68) of quite homogeneous pro- 

 teins, using carefully tested methods of amino acid analysis, seem 

 to indicate that the residues do not occur in random amounts but 

 instead are present to the extent of 1/2, 1/3, 1/4, 1/6, 1/8, 1/9, 1/12, 

 1/16, 1/18, 1/36 . . . etc., of the total residue number. Although 

 this generalization may be based on insufficient experimental evi- 

 dence, it furnishes an indication that some numerical rule may be 

 involved in the synthesis of the protein molecule in the living cell. 

 This observation has led to the hypothesis (67, 68) that the various 

 amino acid residues may occur at constant intervals along the 

 protein chain. Thus if there are 24 glutamic acid residues in a chain 

 of 288 residues a uniform spacing along the chain will place one at 

 every twelfth residue position; if there are 32 lysine residues, 



