452 D. O. JORDAN 



0.10 A. in the C4— Cs bond distance, that in adenine being the longer. This 

 difference may be due to experimental error," but nevertheless is in agree- 

 ment with the different acid-base properties of adenine and guanine (see 

 p. 457). The Ce — bond in guanine, which has a length of 1.20 A. appears 

 to have predominantly double bond character, thus confirming the con- 

 ventional keto formula (VIII) ascribed to guanine. Broomhead" points out, 

 however, that an error of only —0.05 A. in this value (i.e., giving a bond 

 length of 1.25 A.) would make the bond of comparable length to that of 

 the C — bond in glycine and diketopiperazine, where the bond possesses 

 only 50% double bond character. 



The position of the hydrogen atoms has been considered by both Broom- 

 head" and Cochran,^* who conclude that in the adenine cation, hydrogen 

 atoms are at Ni , C2 , Cs , Ng , and two hydrogen atoms are at the amino 

 nitrogen Nio . The positive charge maybe located at Ni , N9 , or Nio and the 

 short length of the Ce — ^Nio bond (1.34 A. compared with the single C — N 

 bond length of 1.47 A.) may be attributed to the contribution of resonance 

 forms in which the Nio bears a positive charge and in which the Ce — Nio 

 bond is double. The location of the hydrogen atoms in the guanine cation 

 is not known so precisely. Four hydrogen atoms are located at Ni , Ng , Nio , 

 and Cs , and hydrogen bonds exist between N3 and Nio and between O and 

 N7 . There are thus four possible tautomeric forms, and a decision as to 

 which is the correct structure must await the results of a more precise 

 study. In aqueous solution, it will be difficult to distinguish between the 

 four "mesohydric tautomers" and indeed such distinction maybe meaning- 

 less. 



c. Nucleosides 



The structure of only one nucleoside, viz., cytidine (IX), has been de- 

 termined in detail although some preliminary studies have been made on 

 the other ribonucleosides. The deoxypentose nucleotides have yet to be ex- 

 amined. The structure of cytidine has been determined by Furberg^*''^ 

 and in addition to confirming the furanose structure of the D-ribose, the 

 point of attachment of the sugar radical as being atNs , and the /3-configura- 

 tion of the Na-glycosidic link, other information of fundamental impor- 

 tance to the structure of nucleic acids has emerged. The molecular projec- 

 tion is given in Fig. 4. The six atoms of the pyrimidine ring lie in the same 

 plane in agreement with the observations of Clews and Cochran''- and of 

 Pitf on some substituted pyrimidines. The C2 — distance (see IX) is 

 1.25 A., identical with the value found by Pitf* for the corresponding bond 

 in 2-hydroxy-4,6-dimethylpyrimidine, and the bond therefore possesses 

 some double bond character. The bond from the ring to the amino group, 



16 S. Furberg, Nature 164, 22 (1949). 

 '« S. Furberg, Ada Cryst. 3, 325 (1950) . 



