ISOLATION AND COMPOSITION OF DEOXYPENTOSE NUCLEIC ACIDS 



367 



(J3 O^^O (jJj— Q^^O (jJj O^^^O C3 



I Ca' — OH /\ C4 OH /\ 



1^ HO 0^1^ HO O 



C5. 



Cs' 



N. 



HO ^0, 



Cs- 



j^3*— %^0 C3— 0^^^ C3— 0^ ^ G3.- 



I Cv O^ \ Cv 0^ \ /\ — 



05' Cs' 05' C5' 



C^' 0. J) Cv 0. ,0 0»' 0. JD Gv- 



1 Cv 0H/'\ +C4— OHy^ + 



r HO OH 1^ ho oh 



HO Q 



p HO OH I 

 05' 05'— OH 05' — OH "05' 



(a) (6) (c) (d) 



Fig. 6. Cleavage of apurinic acid by alkali. (Taken from Tamm et a/."^) 



acids will yield entirely different results according to the particular nucleo- 

 tide arrangement. 



Up to this time, only the apurinic acid of calf thymus has been investi- 

 gated in some detail. While enzymic investigation methods were of little 

 avail — deoxyribonuclease apparently does not attack a chain that has been 

 deprived of its purines^*^' ^^^ — further degradation by alkali yielded some 

 results.^^' Following a mild treatment with alkali the apurinic acid is 

 cleaved in part with the formation of diffusible fragments and of a non- 

 diffusible residue. The nondialyzable fraction comprises about 85% of the 

 pyrimidine nucleotides, but only about 40 % of the nonglycosidic deoxyribo- 

 phosphate residues present in the starting material. If the sodium salt of 

 apurinic acid is assigned an average molecular weight of 15,000,-^* a "sta- 

 tistical polynucleotide" in accord with its composition and properties would 

 comprise 17 thymidylic acid residues, 12 cy tidy lie acid residues, and 29 

 deoxyribophosphate residues. About 10-* isomers are possible. As a plausible 

 explanation of the partial degradation of apurinic acid by alkali a mech- 

 anism similar to that proposed for the cleavage of ribonucleic acid-" (com- 

 pare, however, Lipkin et al}^^) was contemplated in which the formation, 

 and subsecjuent rupture, of a transitory cyclic triester of phosphoric acid 

 linking the 3'- and 4'-hydroxyls of the sugar phosphate residues occurring 

 in the aldehydo form was assumed (Fig. 6). A different explanation of this 

 reaction is given in Chapter 12. The experiments w^ere interpreted as per- 

 mitting the formulation of calf thymus deoxyribonucleic acid as 



[(Pui7Thy2Cy2) (Thyi5CyioPui2)]. , 



2" D. M. Brown and A. R. Todd, J. Chem. Soc. 1952, 52. 



"8 D. Lipkin, P. T. Talbert, and M. Cohn, J. Am. Chem. Soc. 76, 2871 (1954). 



