6 The Structure of Protoplasin 



physical, cytological, and genetical. But the manner in which water, 

 salts, carbohydrates, fats, and proteins combine so as to fulfill the 

 biological requirements of a living system, only the chemist and 

 physicist can say. The direction which this cooperation between 

 physicist, chemist, and biologist is likely to take — in short, what the 

 future has in store for us in the matter of protoplasmic structure — 

 may be illustrated by a problem, a very fundamental one, namely, 

 the maintenance in protoplasm of two apparently incompatible prop- 

 erties, the capacity to flow, and the possession of structural qualities 

 necessary to satisfy elasticity and tensile strength. The biologist 

 presents the problem. The physical-chemist must answer, if answer 



Fig. 5 



there is to be. He suggests loosely attached cross-linkages, such as 

 those which are established by the hydrogen bond. 



The problein of the simultaneous presence in organic systems of 

 fluidity and structural continuity is one which confronts the chemist 

 as much as the biologist. Highly branched, cross-linked, three- 

 dimensional polymers always exhibit elasticity, though fluid. 



Side chains which make lateral connections between molecules 

 have long been recognized in stereochemistry. But the concept that 

 one end of the link is loosely attached and the other firmly so is 

 new, and of great biological importance. It has been suggested that 

 variations in the degree of hydration of the side chains constitute a 

 possible explanation of a weak attachment at one end. But it is 

 another interpretation of this feeble union which I have selected, 

 namely, the hydrogen bond. 



In a chemical cycle, as in muscle, during which accessory mole- 

 cules go in and out of combination with the side chains of the protein 

 main chains, the latter take up a cycle of configurations. In all such 

 dynamic systems, of which protoplasm is one, there is a constant 

 shifting of ties between the structural units. The loose union which 

 permits this is very probably a hydrogen bond. Linus Pauling*' 

 believes this bond to be of greater significance for physiology than 

 any other single structural feature. It serves our present purpose 

 well because it is not a strong bond. 



' The Nature of the Chemical Bond, Ithaca, N. Y., 1940. 



