458 D. O. JORDAN 



tures of the type considered by Taylor consistently make the bonds N9 — Cs 

 and N: — Cs single bonds and the bond N7 — Cs a double bond, whereas the 

 X-ray data indicate that these bonds in adenine all possess 20-50 % double 

 bond character. However, the correlation of bond lengths obtained for the 

 crystal with other properties obtained from measurements made on solu- 

 tions must be carried out with care as the different environments may 

 produce small changes in the molecule. In the present instance, the influence 

 of the chloride ion, which exists close to the adenine ion in the crystal lat- 

 tice, may produce significant effects. 



The group responsible for the acid dissociations of pK'a 6-9 which appear in the 

 oxypurines is not known with certainty. Existing evidence indicates clearly that it 

 is to be associated with the presence of the 6-oxy group in guanine, hypoxanthine, 

 and xanthine; the second oxy group in the 2-position of xanthine does not give rise to 

 a further dissociation, the case being somewhat analogous to that of uracil. Ogston^* 

 considers that the acidic properties of xanthine do not depend on the — NH — CO — 

 group in the 1- and 2-positions since substitution by a methyl group in the 1 -position 

 does not greatly alter the pi^'oi value of xanthine, nor that of the 3-, 7-, and 9-methyl- 

 xanthines. The actual form of the dissociating groups has been studied by Ogston^* 

 by electrometric titration in water and 90% ethyl alcohol. In xanthine, the group is 

 considered to be in the enolic form and the structure is — Ni^CeOH. In the 3-, 7-, 

 and 9-methylxanthines, however, the zwitterionic form is considered to predominate, 



+ 

 the acid dissociation being represented as that of an — NH^group in the imidazole 

 ring. If this interpretation is correct, the second dissociation in the imidazole ring 

 must be preceded, according to Taylor,*' by a tautomeric change, since Ogston''^ 

 has shown that the — NH — group in the 1 -position does not show acid properties in 

 these molecules. 



h. Nucleosides and Nucleotides 



The pK'a values of the ribonucleosides and ribonucleotides are given in 

 Table II. The values for the nucleosides correspond closely to those of the 

 parent purines and pyrimidines although there is a general tendency for the 

 pK'a values to be lowered, i.e., for the acid dissociations to be strengthened. 

 Except in the case of xanthosine, which has not been studied to a suffi- 

 ciently high pH value, an additional dissociation is observed at pK'a 12.3- 

 12.6, which is of the correct order for the first acid dissociation of a sugar. 

 No data are available for D-ribose, but glucose has a pK'a value of 12. 1.^--^^ 



The pK'a values of the ribonucleotides obtained from yeast nucleic acid 

 are given in Table II. These values will all refer to mixtures of the a and b 

 nucleotides, i.e., of the nucleoside-2'- and -3'-phosphates, in unknown 

 ratios. This isomerism, however, will have only a small effect on the dis- 

 sociation constants (see below). Comparison of the values for the nucleo- 



32 P. Hirsch and R. Schlags, Z. physik. Chem. 141, 387 (1929). 



33 F. Urban and P. A. Shaffer, J. Biol. Chem. 94, 697 (1931). 



3* F. Urban and R. D. Williams, J. Biol. Chem. 100, 237 (1933). 



