302 P. Alexander, S. F. Cousens and K. A. Stacey 



free in the solution after reaction corresponded exactly to the 

 amount of acid produced. 



The absence of reaction with amino groups is shown still 

 more strikingly with solutions containing 0-12 per cent DNA 

 when 94 per cent of the mustard is used up in esterification 

 (Fig. 2) and only 6 per cent of it liberates acid. This 6 per 

 cent is entirely due to hydrolysis. 



In agreement with the prediction of Brown and Todd (1952) 

 we find that the triesters produced by alkylation of the phos- 

 phate groups of DNA are unstable and are hydrolysed com- 

 pletely by heating at 90° C for 20 minutes. Fig. 2 shows that 

 after this treatment the amount of acidity produced is equal 

 to the amount of esterification (i.e. after boiling, the total 

 amount of acid produced is equal to that which would have been 

 produced by hydrolysis in the absence of DNA). Within experi- 

 mental error ( ± 5 per cent) all the combined mustard residues 

 were released on hydrolysis and this shows that the triester 

 breaks down predominantly at the mustard linkage. Measure- 



1 I 



o o 



= P— OCH2CH2N<^0 = P— 0-+OHCH2CH2-N< 



I I 



o 



1 1 



ment of the molecular weight shows that breakdown also occurs, 



though very infrequently, at one of the sugar esters. When 



this happens a break in the main chain is produced (see p. 314). 



With epoxides, esterification of the mustard groups is also 



the most prominent reaction. In this case it results in the 



liberation of alkali which was followed with the automatic 



titrator (Fig. 3). 



I 

 CHo— CH— R + = P— 0-Na+ — > 



= P_0 . CHa • CH— R + NaOH. 



I I 



OH 



