CYTOPLASM 



155 



:h, ch. 





NH-, 



SH 



OH 



CH3 



cules. Established N-numbers are 96 = z% 144 = z*-f, 288 = z*-^^, 

 ^^^ ^ 2«-32 etc. (ScHEiBE, 1948). These complicated proportions 

 have been derived from crystalline proteins. 

 In this respect there exist two different types : 



a) Globular proteins consisting of isodiametric macromolecules 

 which preferably crystallize in the system of cubic, hexagonal or 

 •orthorhombic closed packing (Fig. 90a). 



b) Fibrillar proteins formed by expanded polypeptide chains aggre- 

 gated to a chain lattice (Fig. 90b). In the chain lattice they may assume 

 a spiral configuration (spiral chains, Perutz, 195 i). 



In the second type the crystallization depends on the regularity of 

 the side chains R. If these side radicals are simple as in silk fibroin 

 (Fig. 170), where they consist mainly of H- and CHg-groups, the 

 chains combine as easily as polysaccharides to form a crystal lattice. As 

 will be obvious from Fig. 89, however, this is not possible if the side 

 chains happen to be of 

 quite different lengths and 

 confio-urations. These con- 

 ditions can be compared 

 with the arrangement of 

 bean- or peastalks. Where- 

 as there is no difficulty in 

 uniting a great number of 

 smooth bean stalks into a 

 btmdle, it is not so easy 



to obtain a parallel order in pea stalks with their numerous twigs 

 pointing sidewise; and if, moreover, the lengths of these twigs alter- 

 nate in an irregular manner, the resulting structure becomes so spaci- 

 ous that it is almost impossible to bundle them together. This is the 

 case with complicated polypeptide chains. 



In general these unwieldy chains are folded up in some complex 

 manner to form globular molecules. Open spaces inside these macro- 

 molecules are occupied by bound hydration water. The protein part- 

 icles crystallize in a molecular lattice of close packing. As their size 

 is considerable, some space accessible to additional water or even dye- 

 stuff molecules is left between the spheres (Fig. 90a). Such crystals 

 therefore swell or shrink and can be stained in aqueous solutions. 



Rigorous dehydration removes not only the water between the 



CH. 



COOH 



NH2 



NH 



Fig. 89. Unequallengths of polypeptide side chains R. 



