342 



ERWIN CHARGAFF 



100 



Hours 

 Fig. 2. Liberation of free purines from the sodium deoxyribonucleate of calf thy- 

 mus at 37° and pH 1.6. The duration of the reaction is plotted as the abscissa against 

 the concentration of adenine (A) and guanine (G) in the dialysate (left ordinate) and 

 against the quantities of liberated purines as per cent of total adenine and guanine 

 contained in the starting material (right ordinates) . The upper part of the graph indi- 

 cates the molar ratio of adenine to guanine in the dialysate. (Taken from Tamm 

 eia/.i") 



added gradually, when a pH of 1.6 was reached. During the addition a heavy precipi- 

 tate formed in the viscous solution. The mixture was immediately transferred to a 

 cellophane bag and dialyzed at 37° against 440 cc. of dilute HCl of pH 1.6 for 26 

 hours. "2 No change in pH occurred during this time. The clear inside fluid was di- 

 alyzed against 750 cc. of 0.2 M borate buflfer (pH 7.3) for 22 hours at 4°, against run- 

 ning tap water (about 12°) for the same period, against frequent changes of distilled 

 water at 4° for 24 hours, and evaporated in the frozen state in a vacuum. The apurinic 

 acid formed a pure white fluff, weighing 76.7 mg. and containing 9.7% of moisture. It 

 was very hj^groscopic when completely dehydrated and was easily soluble in water 

 to give a solution of about pH 6.5. The yield corresponded to 94% of the starting ma- 

 terial, when allowance was made for the loss of purines amounting to about 20% of 

 the initial weight. 



{2) Properties. Information about two typical preparations of apurinic 

 acid is provided in Table III. It will be seen that the recovery of the 

 pyrimidines and, even more significantly, of the phosphorus is almost 

 quantitative. The comparison with the data given below in Section VII will 

 show that no distortion of the inter pyrimidine ratios characteristic of the 

 parent nucleic acid has taken place. All this is not true of thymic acid 

 preparations. In apurinic acid the nucleotide sequence of the parent sub- 

 stance presumably is preserved, except that the positions of the purine 



"'^ When larger quantities of apurinic acid are to be prepared, it is not necessary to 

 carry out the acid treatment under conditions of dialysis. The subsequent neutrali- 

 zation and dialysis are, of course, required. 



