166 J. D. BERNAL 



dimerSj but this may be simply because they camiot as yet be crystallized. Most 

 of those isolated by centrifugation are certainly very high polymers. Even here, 

 however, there seems to be some natural limit [14, 15]. The size of polymers is 

 of physico-chemical importance because it determines the nature of the forces, 

 not only between similar particles of polymer as described above, but also between 

 one particle and smaller molecules, offering, for instance, possibilities of 

 oriented adsorption. 



Nearly all linear polymers, and certainly those foimd in organisms, are not 

 rigid structures and can exist in a number of configurations depending on such 

 external circumstances as the temperature, pH and other properties of the 

 medium in which they are placed. This is also a fimction of the shape and 

 chemical activity of the monomers, particularly of the parts not immediately 

 involved in the main polymer chains, namely the residues or side chains. Where 

 these are neutral and small, as in the methyl residues of polyisoprene rubber, 

 different chain arrangements have all approximately the same energy and the 

 configuration is random. Where, on the other hand, the residues themselves 

 contain, as in the proteins, polar groups such as NH4+ or C00~ or, as in the 

 nucleic acids, the large aromatoid molecules of the pyrimidines and purines, 

 stable, low-energy configurations are preferred and these for steric reasons tend 

 to take on a helical form. Hence come the hydrogen-bonded Huggins-Pauling 

 helices of the protein or the double Watson-Crick helices of the deoxyribose- 

 nucleic acids, stabiHzed by the piling of their purine and pyrimidine residues. 



The major factor responsible for the configuration of the proteins is the 

 possibility for hydrogen-bond formation. It would appear that in all such con- 

 figurations every or nearly every available bond-forming hydrogen atom, that 

 is all in — OH, — NH or — NH2 groups, normally form hydrogen bonds to 

 suitable acceptors such as — O — OH or H2O, This naturally holds as much for 

 side chains as for main-chain groups. Configurations which do not permit the 

 maximum number of bonds will be of high energy and be unHkely to occur. 

 To judge from the degree of order observed in protein crystals, entropy effects 

 do not seem to be important. However, there will in general be a nimiber of 

 configurations of approximately the same maximal hydrogen-bond number 

 and consequently of approximately the same energy, and therefore the actual 

 configuration may depend on other factors — steric effects in the chain itself, 

 relations to other chains or with molecules in the mediimi. As hydrogen bonds 

 can be formed quite as easily between adjacent or distant portions of the same 

 chain or its side groups or between different chains, the possibility of alternative 

 quasi-stable structures is enormous. 



The simple a-helix form, internally hydrogen-bonded, though probably the 

 most stable for polypeptides with medium or large side chains is not so for small- 

 side-chain polypeptides, which favour the crossHnked j3-form of silk. Nor is it 

 that adopted by most of the natural crystalline or globular proteins. In the normal 

 pH range and in the absence of strong hydrogen-bond-breaking substances such 

 as urea, that is, in the imdenatured state, such proteins have molecules approxi- 

 mating more to the spherical than to the thread-like shape. This may be simply 

 a consequence of tlie large effective surface of the pattern. A quasi-spherical 



