PHYSICAL PROPERTIES OF NUCLEIC ACIDS 



479 



TABLE VI 

 The Titratable Groups of the Sodium Salts of Some Desoxypentose Nucleic 



Acids 



Groups 



Calf Herring Wheat 



thymus sperm germ 



1. Amino (guanine) 



2. Amino (adenine) 



\ Equivalents/4P 



(pK'a 



\Equivalents/4P 



3. Amino (cytosine and 5- jpK'a 

 methylcytosine") \Equivalents/4P 



4. Secondary phosphoric acid ipK'a 

 dissociation \Equivalents/4P 



5. — NH — CO— dissociation of jpK'a 

 guanine and thymine \Equivalents/4P 



2.5 

 0.75 



3.7 

 1.02 



4.75 

 0.84 



6.5 

 0.33 



2.35 

 0.71 



3.7 

 1.0 



4.85 

 0.92 



6.5 

 0.18 



10.4,11.4 10.4,11.4 

 1.76 1.76 



2.9 

 0.66 



3.85 

 0.9 



4.5 



0.84 



5.5 

 0.42 



1.57 



" pK'o of 5-methylcytosine assumed the same as that of cytosine. 



sugar hydroxyl group to that bearing the phosphate group [cf . Brown and 

 Todd, Chapter 12.]. The presence of a small amount of secondary phosphoric 

 acid dissociation can only indicate that there is some chain-branching, 

 which will occur every ten or twenty nucleotides. This view is confirmed by 

 the dye-adsorption measurements of Cavalieri and Angelos,^"^ who, in order 

 to explain the experimental results, found it necessary to assume the exist- 

 ence of two binding sites. The ratio of these sites is lower than that found 

 from titration, being one in thirty. 



The initial (forward) titration curve of the high polymeric deoxypentose 

 nucleic acids is anomalous in that on the addition of acid or alkali to the 

 solution in water, no groups are titrated at first between pH 5.0 and 11.0, 

 but outside these limits there occurs a rapid ionization of the amino groups 

 and the — NH — CO — dissociations. This general behavior was first ob- 

 served by Gulland et al^^ for the acid from calf thymus and has been con- 

 firmed by electrometric titration of other acids and for different methods 

 of preparation by Signer and Schwander,''^ Cosgrove and Jordan, ^"^ and Lee 

 and Peacocke.^"^ It has also been confirmed by spectrophotometric titration 

 by Shack and Thompsett.^"^ The ionization of groups at pH 11.5 and in the 

 range pH 3.5-4.5 is accompanied by a marked fall in the viscosity'^" and 



io» L. F. Cavalieri and A. Angelos, /. A»i. Chem. Soc. 72, 4686 (1950). 



i«9 J. Shack and J. M. Thompsett, /. Biol. Chem. 197, 17 (1952). 



"0 J. M. Creeth, J. M. Gulland, and D. O. Jordan, J. Chem. Soc. 1947, 1141. 



