Molecular Structure in Protoplasm 65 



possible in addition to direct ionic linkages and cystine bridges. 

 Thus all three types may be influential in the production of aggre- 

 gates of protein particles. The more complementary the two faces 

 are to one another, the greater their interaction should be (99) . 



Van der Waals' forces (65) are effective between all types of 

 atoms, while hydrogen bridges and ionic linkages, on the other hand, 

 are only possible between groups containing oxygen and nitrogen 

 atoms (19). Ionic linkages are possible only between oppositely 

 charged residues, such as ionized lysine and aspartic acid. In both 

 hydrogen bridges and ionic linkages, the atoms involved will 

 approach to a separation distance of approximately 2.8 A (19, 65) 

 in contrast to 3.5-4 A for van der Waals' distance (101) . A primary 

 valence bond, as in a cystine bridge, brings the bonded atoms to a 

 distance of approximately 1.5 A (19) . 



The relative stability of these linkages is of interest. Ionic link- 

 ages and gross electrical effects are markedly affected by changes 

 in hydrogen-ion concentration (102). In addition to this, cystine 

 bridges are influenced greatly by the concentration of electron- 

 donating systems within the cell. Van der Waals' forces, on the 

 other hand, are practically independent of these two factors. Tem- 

 perature, of course, affects all forms of cohesion. 



Recently evidence has been accumulating which indicates that 

 phospholipids and sterols may occur bound to the protein material 

 (103) . That the zwitterion portion of lecithin and cephalin, previ- 

 ously shown in Figure 7, may be effective as the means of attach- 

 ment is indicated by the similarity in the distance of 7 A between 

 the positive and negative charges on the lecithin, and the distance 

 of 7 A also between adjacent amino acid residues of the protein. 

 The lecithin might then become attached to two adjacent, oppositely 

 ionized residues of the protein through ionic attractions, or the 

 phosphoric acid group might bridge to a hydrogen-donating residue, 

 such as serine or tyrosine. If attached to the protein as shown in 

 Figure 13, it would project out about 25 A from the ends of the 

 amino residues (104, 105) . In certain instances the phospholipids 

 may act in blocking specific residue patterns where respiratory 

 prosthetic groups might otherwise become attached. 



Concerning the attachment of the smaller molecules, the respira- 

 tary prosthetic groups, to protein particles, it seems too early to make 

 much more than suggestive statements, but one thing at least may 

 be sensed from the work which has been done; that is, degrees of 

 specificity seem to exist, or rather, some prosthetic groups may be 



