52 The Structure of ProtoplasTn 



Concerning the shape, a number of different experimental 

 methods show that many elementary protein molecules are globular 

 in outline rather than elongated as a completely extended chain, 

 although globular in this instance may mean anything from spherical 

 to considerable elongation. These methods include studies of vis- 

 cosity — concentration curves (40) , comparison of ultramicroscopic 

 observations (41) , determination of asymmetry by means of the 

 ultracentrifuge (9) , unit cell determinations of crystalline proteins 

 by means of X-ray analysis (36 and 39) , comparisons of double 

 refraction of flow (42) , and rotation in an electric field (43) . Certain 

 proteins, such as silk and gelatin apparently have their chains fully 

 extended for considerable lengths; others form particles having 

 various length-thickness ratios (44) ; and a few are practically iso- 

 diametric. A single chain of 1,000 A in length is about 100 times 

 as long as it is thick. Zein forms particles which are about 20 times 

 as long as their width; erythrocruorin, about 10 times; and insulin, 

 about 2 or 3 times; while for egg albumen the particles are nearly 

 isodiametric (44) . One of the most significant points, in addition to 

 the globular nature of many protein molecules, is that proteins 

 belonging to the same weight class may differ enormously in shape 

 (44). 



Regardless of shape or size the particles seem to consist of poly- 

 peptide chains folded in some manner to give the respective shape. 

 Very little is known concerning the exact configuration which the 

 chain takes, but it seems probable that it is a function of the nature 

 and distribution of its side chains (45) and of its environment at the 

 time of synthesis (46) . Whatever model is accepted, the general 

 configuration of the protein chain in the globular particle must be 

 consistent with the observations that reflections from parallel 

 packets of protein chains may be obtained fairly easily from the 

 protoplasmic material upon the removal of water (33, 47, 48) . A 

 working model has been proposed (46, 49, 50, 51) for the configura- 

 tion of the chain in the globular particle. The essential feature of 

 this hjrpothetical configuration is that the peptide chain occurs in 

 parallel folds forming a monolayer in which the residues extend out 

 approximately perpendicular to the surface, thus making the layer 

 about 10 A in thickness. Packets containing about 1,400 of such 

 monolayers, piled one upon another, have been prepared, and the 

 individual monolayer thickness of lOA has been found by direct 

 micrometer measurement (50 and 52) . These monolayers may 

 either attach to one another to form a layered globular particle, or 



