6 CHEMICAL SPECIFICITY OF NUCLEIC ACIDS 



obvious that the high and specific absorption in the ultraviolet 

 of the purines and pyrimidines could form the basis of a quanti- 

 tative ultra-micro method, if proper procedures for the hydrolysis 

 of the nucleic acids and for the sharp separation of the hydrolysis 

 products could be found. 



4. PREPARATION OF THE ANALYTICAL MATERIAL 



If preparations of deoxypentose nucleic acids are to be subjected 

 to a structural analysis, the extent of their contamination with 

 pentose nucleic acid must not exceed 2-3%. The reason will 

 later be made clearer; but I should like to mention here that all 

 deoxypentose nucleic acids of animal origin studied by us so far 

 were invariably found to contain much more adenine than 

 guanine. The reverse appears to be true for the animal pentose 

 nucleic acids: in them guanine preponderates. A mixture of ap- 

 proximately equal parts of both nucleic acids from the same tis- 

 sue, therefore, would yield analytical figures that would cor- 

 respond, at least as regards the purines, to roughly equimolar 

 proportions. Should the complete purification — sometimes an 

 extremely difficult task — prove impossible in certain cases, one 

 could think of subjecting preparations of both types of nucleic 

 acid from the same tissue specimen to analysis and of correcting 

 the respective results in this manner. This, however, is an un- 

 desirable device and was employed only in some of the prepa- 

 rations from liver which will be mentioned later. 



It is, furthermore, essential that the isolation of the nucleic 

 acids be conducted in such a manner as to exclude their 

 degradation by enzymes, acid or alkali. In order to inhibit the 

 deoxyribonucleases which require magnesium^'^, the preparation 

 of the deoxypentose nucleic acids was carried out in the presence 

 of citrate ions^^. It would take us here too far to describe in detail 

 the methods employed in our laboratory for the preparation of 

 the deoxypentose nucleic acids from animal tissues. They 

 represent in general a combination of many procedures, as de- 

 scribed recently for the isolation of yeast deoxyribonucleic acid^^. 



