MOLECULAR STRUCTURE IN PROTOPLASM 



177 



ular Aveight, with the tliickness dimension 

 taken from measurements of X-ray dif- 

 fraction spacings. These methods are too 

 involved to describe in detail, but several 

 sets of values are given in Table V for 

 comparison. The agreement for a given 

 protein is about as close as one could ex- 

 pect and makes mere coincidence seem im- 

 probable; especially when the cumulative 

 effect is attained by comparison with the 

 width and thickness values given in Table 

 VI, where these two dimensions are pre- 



undonbtedly essential, are ignored for the 

 time being. 



The structural nature of these protein 

 particles is very intimately associated with 

 the water molecules which occur in such 

 large numerical proportion in the cell ; in 

 fact, the very nature of the component 

 oxygen and nitrogen atoms of these two 

 substances, protein and water, demands 

 that an association stronger than mere co- 

 hesion of the van der Waals' type take 

 place between them. Both of these atoms, 



TABLE VI 



Occurrence of Back-bone and Side-Chain Spacings in Proteins 



sented for a great number of proteins. The 

 general likeness in dimensions of these 

 various proteins makes it seem almost a 

 certainty that the proteins which occur in 

 the living cell have chain dimensions of 

 similar thickness and width. It seems rea- 

 sonable also to make use of these values 

 for visualizing the structure of the par- 

 ticles which abound in such enormous 

 quantities in the submicroscopic region of 

 the cytoplasm as revealed by the ultrami- 

 croscope. These particles occur in the cell 

 apparently suspended in water, along with 

 a small percentage of other organic and in- 

 organic substances; the latter, although 



the oxygen and the nitrogen, when in place 

 in the molecule have strong residual fields 

 which are localized in the atom at tetra- 

 hedral points. Fig. 5 is a diagram to 

 illustrate these four coordination points. 

 It shows a pair marked oriented at 

 right angles to a pair marked 0, to which 

 four water molecules are coordinated in a 

 tetrahedral fashion. If a hydroxyl radical 

 were to replace the central water molecule, 

 only three places would then be left for the 

 coordination of three water molecules. The 

 number varies with the nature of the atomic 

 group in which the oxygen or nitrogen oc- 

 curs. Theory and experiment are in fairly 



