8 The Structure of Protoplasm 



joined to it by primary valences, and two, farther away, by the 

 hydrogen bond. 



The situation existing in water is even more apphcable to pro- 

 teins where structural possibilities are infinitely greater; and proteins 

 are the building material of protoplasm. In amino acids the nitrogen 

 atom has one open coordinate position with which it may unite to a 

 hydrogen in the same or another molecule. This union is a hydrogen 

 bond; it may join the nitrogens of two amino acids. 



The essential feature of the hydrogen bond, insofar as our present 

 problem is involved, is its ease in shifting. This permits fluidity 

 v/hile maintaining continuity in structure. But protoplasm does not 

 flow with equal ease at all times; it is often of high viscosity, and it 

 may be firm. The lateral bonds are therefore more securely held at 

 one time than at another, and at still other times they are tightly 

 locked, wholly preventing flow. If the hydrogen bond is to satisfy 

 these conditions, it must show considerable variability in firmness, 

 from a feeble contact permitting ready readjustment, to a firm grip 

 tightly locking the fibrous units of the living three-dimensional lat- 

 tice. Pauling points out that firmness of the hydrogen bond varies 

 with the electronegativity of the atom. 



As one of the essential points of this discussion is the variability 

 in firmness of the weaker end of cross-chains, I want to dwell upon 

 it just a moment longer in order to emphasize an important differ- 

 ence between such a variability in nonliving and in living systems. 

 Usually, variability in the viscosity, elasticity, and tensile strength 

 of a solution of nonliving chain polymers is accomplished simply by 

 changing the concentration. At high concentration the chains will 

 join at more points. But in protoplasm the "concentration" remains 

 the same, yet the physical properties vary. This is accomplished by 

 change in the firmness of the cross-ties. How this change can come 

 about has been suggested. There is the further possibility that some 

 protons are added or subtracted with change in pH, forming or 

 breaking hydrogen bonds; change in salt concentration might also 

 alter their strengths. 



The hydrogen bond thus provides a mechanism by means of 

 which continuity in structure, elasticity, and rapid changes in viscos- 

 ity in protoplasm may be explained. 



Astbury' calls attention to another attribute of the hydrogen 



' The hydrogen bond in protein structure, Trans. Faraday Soc. 34: 871-880, 

 1940. 



