144 



FINE-STRUCTURE OF PROTOPLASM 



II 



about two, and for larger ones four to five (Fig. 95 a, b). This means 

 that the area of the flattened molecule is four to twenty times bigger 

 than the cross-section or projection of the spherical molecule before 

 spreading. The polypeptide chain may wind about in this area. If the 

 cross-section of such a chain measures 4.6 x 10 A as in the chain 

 lattice, its length L can be computed. 



Fig. 95. Surface film of a protein (from Frey-Wyssling, 1949a). a) Globular 



molecule of 100 A diameter; b) spread to a surface layer 7.5 A thick; c) denatured 



to a polypeptide chain 11,600 A long. 



The chain length L obtained for the globular particles is shown in 

 Table XV. For instance, a protein molecule of 24 Svedberg units 

 with a molecular diameter of 100 A harbours a chain of 11 600 A 

 = 1.16 ^ length (Fig. 95 b, c). An even greater length is obtained if 

 it is assumed that this molecule consists of 4800 amino acids, each of 

 which contributes 3.5 A to the chain length; this yields L = 1.68 fi. 



Since globular proteins denature so easily, we may ask what types 

 of force hold together the inner architecture of these macromolecules? 

 They must be rather weak, because they are broken by mere contact 

 of the globular molecules with a water surface. On the other hand, 

 the expanded molecules form a solid film, which has the character of 

 a fibrous protein. It must be supposed that the individual molecules 

 have been fused to a two-dimensional molecular aggregate. Here, 

 instead of intramolecular forces holding together the coiled, folded or 

 laminated internal structure of the globular molecule, inter molecular 

 forces unite neighbouring expanded molecules. The same thing occurs 

 when globular protein molecules are connected to form beaded chains. 



