MOLECULAR STRUCTURE IN PROTOPLASM 



185 



and also that packets of small sizes nor- 

 mally occur. Studies of mono-films have 

 demonstrated that layers of the chains may 

 be formed. It seems to me then that in 

 view of this cumulative infornmtion it is 

 not an enormous jump to assume that the 

 particles which may be seen as diffraction 

 spots or visible light in the ultra-microscope 

 are composed actuall.y of these protein 

 chains in various arrangements. 



I want to think of the chain lengths and 

 of the particles they form as being of no 

 standardized size, for a standard size seems 

 to me to be inconsistent with protoi^lasm, 

 as a biologist knows it. Instead, I should 

 expect to find in the cytoplasm particles of 

 many sizes which are held intact by means 

 of i^rimary valence bonds only. These, for 

 the sake of simplicity, may be considered as 

 structural units. I do not feel easy in 

 speaking of them in that way, despite the 

 fact that they are to serve as a first ap- 

 proach and therefore must be devoid of 

 detail to a great extent. This much we may 

 say concerning them : that for some of them 

 the molecular weight may be as low as 9000 

 or even less, while for others it may be soTue- 

 what larger; and further, that the com- 

 ponent protein chains must have arrange- 

 ments which are specific for specific pur- 

 poses. These small particles or structural 

 units may be considered as super-molecules. 



Aggregation of these super-molecules 

 may be accomplished in two relatively dis- 

 tinct and easily conceived ways, which form 

 separate categories of units on a higher 

 structural level. .The difference between 

 these two constructions lies in the closeness 

 or compactness of the component super- 

 molecules. This gives us now a series of 

 structures of more or less continuous levels, 

 in which the first is the super-molecules ; a 

 second is a compact aggregate of super- 

 molecules ; and a third, a loose aggregate of 

 super-molecules together with the compact 

 forms, both acting as components of this 

 loose sponge-like third aggregate. 



It will simplify- matters if we do not con- 

 sider intermediate forms of these three 

 structural units, that is, between super- 

 molecules, compact aggregates, and loose 



aggregates. But a word or two about the 

 second and third may clarify the concep- 

 tions. In order to make the picture more 

 tangible the compact aggregates may be 

 thought of as having perhaps 40 per cent to 

 50 per cent water and a specific gravity of 

 about 1.15; and the water as being bound, 

 to a ver}' great extent, to the hydrophilic 

 groups of the protein b.y means of hydro- 

 gen bridges. In shape they may be some- 

 what cubical or elongated, but rectangular. 

 It would be surprising if they were found 

 to be as regular in outline as these terms 

 imply; but for a first imj^ression the com- 

 pact aggregates described in such general 

 terms are probably more useful than more 

 specific attempts. The angular form seems 

 to be a consequence of being built fi-om an- 

 gular super-molecules. 



I think we should digress for a moment 

 here to refer to a possible structure of the 

 super-molecule. As a small packet con- 

 structed of polypeptide chains it wnll pos- 

 sess a certain degree of internal parallelism. 

 If we accept an extreme as a basis for fur- 

 ther modification and suppose that it con- 

 sists of several layers of parallel chains of 

 indefinite lengths, built up to form a cube- 

 like packet, the surfaces of the cube sides 

 will be molecularly different in pairs. One 

 pair will have only the ends of the side 

 chains exposed; another pair will have the 

 back-bones exposed, while the third pair will 

 have the ends of the polypeptide chains ex- 

 posed. Thus, at least three distinct sorts 

 of surfaces are available in which the 

 smaller atomic groups or residues may form 

 mosaic patterns of various molecular con- 

 tent and of various force fields. The pair 

 of faces which present only the side-chain 

 ends to the water channel will probably 

 produce footholds for prosthetic groups. 

 With this very brief mention of the sur- 

 faces of the super-molecules, we return to 

 the discussion of the influence of these sur- 

 faces on the compact aggregates when ag- 

 gregation takes place. If like surfaces are 

 most effective in producing cohesion of one 

 super-molecule to another, then the larger 

 compact aggregate becomes an enlarged 

 replica of the super-molecule, with similar 



