PHYSICAL PROPERTIES OF NUCLEIC ACIDS 461 



a and b isomers of cy tidy lie acid. From electrometric titration (glass elec- 

 trode) the following pK'a values were obtained: 



a pK'aj 4.36 (4.3) pK'aj 6.17 (6.2) 



6 pK'a2 4.28 (4.3) pK'a^ 6.0 (6.0) 



The values in parenthesis were those obtained by Loring et al.,^^ and Cava- 

 lieri considers that since the amino and phosphate groups will be closer in 

 the 2'- than in the 3'-phosphate the a isomer is probably the 2'-phosphate. 

 A similar conclusion was reached by Loring et al.^"^ on the grounds that the 

 isomer with the smaller distance between the charged groups would show 

 the greater tendency towards zwitterion formation and hence have the 

 lower solubility, acidity, and ultraviolet absorption. 



II. Nucleic Acids 

 1. Structure Determinations by X-Ray Diffraction 



Although the general chemical picture of the nucleotides and of the nu- 

 cleic acids is one of some complexity, the basic macromolecular configura- 

 tion which emerges from X-ray diffraction studies is one of comparative 

 simplicity. The studies, in the main, have been concerned with the sodium 

 salt of deoxypentose nucleic acid since this substance can be comparatively 

 readily isolated in a state of purity and of high molecular weight. Ribo- 

 nucleic acids have only been examined when a high-molecular-weight 

 sample, such as that obtained from tobacco mosaic virus, has been available. 



The main features of the molecular structure of deoxypentose nucleic 

 acid have been known since 1938, but it is only recently that the accumula- 

 tion of knowledge has permitted the suggestion of a structure concerning 

 which there appears to be a large measure of agreement. Astbury and 

 BelP^ showed first that the X-ray fiber photograph gave a prominent re- 

 flection which corresponded to a spacing of 3.34 A. along the fiber axis. 

 This was interpreted as due to a succession of fiat nucleotides standing out 

 perpendicularly to the fiber axis to form a relatively rigid structure. This 

 rather small distance between the nucleotides is in agreement with the 

 density of the dried nucleic acid, Avhich is 1.62-1.63 g./cc. In addition to 

 this spacing of the nucleotides, Astbury and Bell and later Astbury^^ con- 

 cluded that the structure pattern along the axis of the molecule repeated 

 at a distance of about 27 A. Since the Astbury structure assumed a pile of 

 nucleotides, one on top of the other, this distance corresponds to approxi- 



■" H. S. Loring, H. W. Bortner, L. W. Levy, and M. L. Hammell, /. Biol. Chem. 196, 



807 (1952). 

 « H. S. Loring, M. L. Hammell, L. W. Levy, and H. W. Bortner, J. Biol. Chem. 196, 



821 (1952). 

 « W. T. Astbury and F. O. Bell, Nature 141, 747 (1938). 



