PHYSICAL PROPERTIES OF NUCLEIC ACIDS 



463 



D C 



3.4 A 



Fig. 5. Structure of a pyrimidine nucleoside (Furberg"**). 



teins as helical polypeptide chains, Pauling and Corey'**'''' suggested a 

 structure for nucleic acid which they considered as compatible with the 

 main features of the X-ray diagram. The structure involved three inter- 

 twined helical polynucleotide chains, each chain having approximately 

 twenty -four nucleotides in seven turns of the helix. This structure, however, 

 has been severely criticized on several grounds. First, the Pauling and Corey 

 structure has the phosphate groups closely packed about the axis of the 

 molecule, with the pentose residues surrounding them and the purine and 

 pyrimidine groups projecting radially; the general behavior of the nucleate 

 ion in solution, however, would suggest the reverse configuration, i.e., with 

 the phosphate groups on the outside of the molecule and the purine and 

 pyrimidine rings on the inside. There is considerable experimental evidence 

 for this, the most important probably being that of Gulland et al.,^^ who 

 showed by titration that the phosphate groups are available for acid-base 

 equilibria in the normal way and that the amino and the — NH — CO — 

 groups on the purines and pyrimidines were inaccessible, unless the nucleic 

 acid was first subject to extremes of pH. Dye and protein absorption con- 

 firm that the phosphate groups must be accessible to large ions. Secondly, 



« L. Pauling and R. B. Corey, Nature 171, 346 (1953). 



« L. Pauling and R. B. Corey, Proc. Natl. Acad. Set. U. S. 39, 84 (1953). 



« J. M. Gulland, D. O. Jordan, and H. F. W. Taylor, J. Chem. Soc. 1947, 1131. 



