PHYSICAL PROPERTIES OF NUCLEIC ACIDS 



477 



I I I I I I 



2 3 4 5 6 7 8 



PH 



Fig. 15. The titration curve of sodium deoxypentose nucleate (Gulland, Jordan, 

 and Taylor«). 



I Forward titration curve 

 II Back-titration curve 



extremes of pH (Fig. 16). Whether this discrepancy is due to a difference 

 in drying procedure is not clear, but recent work by Garner et al}°^ using 

 undried nucleic acid has confirmed the results of Lee and Peacocke. 



As the acid-base characteristics of the deoxypentose nucleotides have not 

 yet been studied, interpretation of the back-titration curve has to be made 

 with reference to the p/v'a values of the ribonucleotides. This procedure is to 

 some extent justified by the similarity of the acid-base properties of 9- 

 methylxanthine and xanthosine, which suggests that the acid-base prop- 

 erties of the pyrimidines and purines are not dependent on the nature of the 

 substituent radical in the 9(or 3)-position so long as it is a nonresonating 

 system and that the glycosidic, C — N, link remains a single bond. The 

 titration curve (Figs. 15 and 16) shows that two main types of titratable 

 group exist in the molecule: that titrating in the range pH 2.0-6.0 and that 

 titrating in the alkahne range pH 8.0-12.0. From the discussion of the dis- 

 sociating groups in the nucleotides it is evident that these groups are the 



108 R. H. Garner, D. O. Jordan, and S. M. Matty, unpublished results. 



