DIFFERENTIAL DISTRIBUTION ANALYSIS 



135 



confirmed the occurrence of these diphosphates, prompted a dis- 

 cussion of the possible bearing of this finding on the problem of 

 nucleic acid structure (p. 366 of Ref. 7). It was clear that for 

 these fragments to serve as a tool in research on nucleotide 

 sequence the mechanism of their release had to be better under- 

 stood. It was also indispensable to be able to study them under 

 rigorously controlled conditions and on a quantitative basis. 



The kinetics of the liberation of the pyrimidine nucleoside 

 diphosphates were first studied, in collaboration with Shapiro, on 

 a series of models, namely, the several deoxyribo dinucleotides all 

 having cytidylic acid as one of their components'^. These inves- 

 tigations served as the basis of a well-controlled, differential 

 hydrolysis procedure which was usually performed in three 

 stages^i' '^. The diagnostically most valuable results are obtained 

 in stage I (0.1 M H2SO4, 35 min, 100°), when the release of 

 pyrimidine nucleoside diphosphates can be taken to reflect direct- 

 ly the abundance of these units as solitary pyrimidine nucleotides 

 in the polynucleotide chain. In the present survey I shall limit 

 myself to the findings based on this stage of the differential 

 hydrolysis procedure. The results yielded in stages II and III, 

 which involve longer periods of hydrolysis, are mainly indicative 

 of the secondary cleavage of bunched pyrimidine sequences; they 

 are of great value as further means of differentiation between 

 analytically indistinguishable nucleic acids of different cellular 

 origin. This will be exemplified in the following section. 



CHO 



CH, 



C^i- 



CHOH 

 .0 — CHo 



OH 



P 



ll\ 



o o- 



Py 



CH- 



I 



CH, 



I 



CH- 



I 



CH- 



I 



CHc 



-O OH 



\/ 



P 



CHO 



CHOH 



-O OH 



\/ 



P 



o o.. 



In its simplest form the cleavage of a polynucleotide chain and 

 the formation of the diphosphates of the pyrimidine nucleosides 

 (Py) may be regarded as a series of ^ elimination reactions^^. 

 After the liberation of the purines the first elimination pre- 



References p. 159 



