166 ''* G. B. Brown 



from a single sample of this ^^N-labelled nucleic acid. It 

 should be noted that these are mixtures of the isomeric 

 nucleotides demonstrated by Cohn and this fact must be 

 kept in mind when assaying the biological results obtained. 

 From the mode of production these nucleotides are labelled 

 in all of their nitrogens, including the substituent amino 

 groups. 



The group at Karolinska Institute has also utilized this 

 method of biosynthesis and has prepared the ribonucleosides 

 (Hammarsten, Reichard and Saluste, 1950), and more 

 recently they have extended this to the preparation of deoxy- 

 ribonucleosides from the DNA of Escherichia coli (Reichard 

 and Estborn, 1951). 



For certain purposes the homogeneously labelled derivatives 

 will be of limited usefulness, and we have also investigated 

 another biosynthetic approach which permits the production 

 of specifically labelled nucleosides or nucleotides. When 

 yeast (Torulopsis utilis) was grown in the presence of ^*C 

 labelled purines, either adenine or guanine was efficiently 

 utilized by the yeast and essentially all of the administered 

 radioactive purine could be recovered from the nucleic acids. 

 By the application of the ion-exchange method for the 

 separation of the nucleotides (Cohn, 1950), each of the 

 individual nucleotides may be recovered. The corresponding 

 nucleosides may be prepared from them by enzymatic hydro- 

 lysis and ion-exchange purification (Kerr, Seraidarian and 

 Brown, 1951). 



The specificity of the labelling of derivatives prepared by 

 this type of biosynthesis is dependent only upon the position 

 of the label in the purine furnished, and there is little waste of 

 isotope. Small samples of labelled adenosine, adenylic acid, 

 guanosine and guanylic acid, each labelled in the 8-position 

 of the purine ring, have been so prepared. 



The biosynthetic approaches are cumbersome for the 

 production of large samples, and offer no possibility of obtain- 

 ing the derivatives of the 2,6-diaminopurine in which we are 

 interested. We were therefore interested in total synthesis 



