468 D. O. JORDAN 



o 





Fig. 10. X-ray diagram of the paracrystalline form of sodium deoxj^pentose 

 nucleate (Franklin and Gosling^*). 



of X-ray diagram. The first (Fig. 9), which corresponds to a crystalhne 

 form (termed structure A by Frankhn and Goshng*^), is obtained in an at- 

 mosphere of about 75% relative humidity. This corresponds to a water 

 content of the sodium nucleate of 40-45% of the dry weight. At higher 

 humidities a different structure (B) showing a paracrystalline form with a 

 lower degree of order appears and persists over a wide range of humidity 

 (Fig. 10). The change from A to B is normally reversible.*'' In view of the 

 high water content of the form B it seems reasonable to suppose that in 

 this form the structural units of the sodium nucleate are relatively free 

 from the influence of neighboring molecules, each unit being shielded by a 

 sheath of water. Analysis of the X-ray diagrams of the B form of the sodium 

 deoxypentose nucleate from E. toli^^ and of the sodium deoxyribonucleate 

 of calf thymus** leads to a general confirmation of the helical structure 

 although the evidence is somewhat circumstantial. Furthermore it is 

 shown that the phosphate groups must lie on the outside of the structural 

 unit on a helix of diameter approximately 20 A. 



One of the important features of the Watson and Crick structure is that 

 it consists of two coaxial helical chains related by a dyad axis, the third 

 coaxial chain being absent in the nucleic acid but presumably occupied by 

 the protein in the nucleoprotein. Direct evidence of this two-chain helix 

 has been obtained by Franklin and Gosling,***^ who for structure A, which 



