Molecular Structure in Protoplasm 53 



an individual monolayer may become folded to form a three-dimen- 

 sional particle. Such particles are probably held together by cystine 

 bridges, ionic linkages, or hydrogen bridges. The specific configura- 

 tion within the globular particle might be quite irregular and 

 depend upon the distribution of cysteine and ionizable residues and 

 on the placement of proline and short residues which make folding 

 possible, although the general plan might be similar to this hypo- 

 thetical model. 



The features of the protein particle, size, shape, and configuration 

 of chain, just given, were taken from the results of work on proteins 

 in vitro. So far as the elementary particles in protoplasm are con- 

 cerned, reasoning by analogy, we may expect to find protein particles 

 of several shapes on this level where 1,000 A lengths or smaller are 

 under consideration; that is, as extended chains, as cylindrical or 

 globular structures, and possibly even as flattened discs. 



A clean-cut parallelism of the chains as indicated for the mono- 

 layer has not been demonstrated for the particles in protoplasm, 

 although a certain degree of parallel arrangement is indicated by 

 X-ray (33) and polarized hght investigations (53). For convenience 

 in discussion we are assuming a model which has a greater degree 

 of parallel placement of the chains than is justified by experiment, 

 feeling that the convenience outweighs the chances of serious 

 misinterpretation. Also for convenience we are making use of a 

 cubical shape for the elementary protein particle as one which may 

 be more easily treated. 



A chain of approximately 1,000 A in length, folded into a globu- 

 lar particle in the manner indicated above, may consist, for example, 

 of 20 folds, each about 50 A in length, arranged into three layers 

 of about seven in a row, as in Figure 5, making a packet 30 x 30 x 50 

 A of solid protein. The same chain length would make a block of 

 10 folds 100 A long but would be only about 25 A thick; that is, 

 about one-fourth its length. If, however, the particle consisted of 

 only four pieces of 250 A, its thickness would be about 15 A, or its 

 length would be nearly 20 times as great as its thickness. These are 

 comparable to the particle shapes mentioned as obtained experi- 

 mentally with proteins in vitro. 



Particles such as these in protoplasm would no doubt take on 

 water molecules through hydrogen bridge formation wherever 

 oxygen- and nitrogen-containing groups were spatially available, as 

 at the ends of polar residues and along the backbone (54) . In a 

 hydrated particle where 40 per cent water or more is included, there 



