HYDROGEN BONDING /O 



proteins, which are the subject of other papers in this symposium. In this case 

 the models must come first, and these models must be based on the precise 

 geometry as obtained from x-ray results on crystals of the simple substances. 

 As is well-known, that is the way the a and 7 helical structures for polypeptides 

 were discovered (Pauling and Corey, 1950) — they were not discovered by taking 

 X-ray photographs of hemoglobin or any other protein; they were discovered 

 by first making use of results such as those discussed above, and then looking 

 about in nature to see if one of them could be found somewhere. With regard 

 to a number of proteins of the fibrous variety, undoubtedly they do have the 

 a-helix structure, but in the case of a large number of other ones — and these 

 are the proteins which have had the most work done on them, insulin, hemo- 

 globin, ribonuclease — we must, I think, say that it is a Scottish verdict, of 

 neither innocent nor guilty, but "not proven," as to whether these have a basic 

 a-helix structure. 



There are two other polypeptide helices (Low and Baybutt, 1952; Donohue, 

 1953), discovered some time after the a-helix, which are not too improbable on 

 structural grounds, and which might have some significance in nature. These 

 are shown in Fig. 15. The a-helix lies between them with regard to diameter, 

 pitch, and number of residues per turn. It has not yet been shown whether either 

 one of these, or the 7-helix which was found at the same time as the a-helix, 

 exists in either native proteins or synthetic polypeptides. 



Quite recently, knowledge of hydrogen bonding was of great help in postu- 

 lating a structure for nucleic acid (Watson and Crick, 1953). The basic part of 

 the Watson-Crick structure for deoxyribonucleic acid is shown in Fig. 16. The 

 entire structure consists of two polynucleotide chains joined by hydrogen bonds 

 between their bases, which are adenine, thymine, guanine, and cytosine. Since 

 the bonds in the two chains from the base nitrogen to the carbon atom of each 

 deoxyribose residue are related by a two-fold axis, the sequence of the bases on 

 one chain is immaterial because the position of the base-nitrogen to sugar- 

 carbon bonds relative to the axis of the helix is the same for all four bases. How- 

 ever, for example, whenever adenine occurs on one chain, thymine must occur 

 opposite it on the other, so that the sequence on one chain fixes that on the 

 other. This gives the pairing which made the biologists so happy when this 

 structure was discovered. 



I think it is fruitful to attack this nucleic acid problem in much the same way 

 that the polypeptide problem was attacked, that is, to work with models made 

 according to interbond distances and angles from previous work on simpler 

 substances, and see what can be put together without any regard whatever as 

 to what nature is doing. Then, after some acceptable structures have been 

 found, one should look around and see if they exist in the natural substances. 



There is a different way of putting together these four bases (Fig. 17) so that 

 they pair in the same way as in the Watson-Crick DNA structure. One pleas- 

 ing feature of the Watson-Crick structure is that it predicts the observed ana- 



