MUTAGENESIS 



101 



sulfate or dimethyl sulfate reacts mainly with the 7-position of 

 guanine (see Fig. 20) . This automatically induces a quaternary nitrogen 

 at this place that is positively charged. But this molecule is in reso- 

 nance with another state, in which the charge is not on this nitrogen 

 but on the 9-position nitrogen on which the ribose phosphate is 

 attached. 



It is clear that the positive charge makes this molecule rather un- 

 stable and, subsequent to the attachment of the ethyl group, either 

 the ethyl group itself can separate off again or the sugar-base bond can 

 break. 



To test this idea we first dialyzed DNA in order to remove all 

 the nucleotides and bases which contaminated it, then added diethyl 

 sulfate to the dialysis bag, incubated it at 37° for two days, and then 

 examined the dialysate. Indeed, we found that the dialysate contained 

 7-eth3d guanine. The amount of ethyl guanine liberated is about 0.5 per 

 cent of the guanine present in the DNA, in this particular treatment. 



in 



Figure 20. Proposed mechanism of depurination by ethylation. I and 

 II: mesomeric states of 7-ethyldeoxyguanosine. IV: removal of R- 

 caused by state II. R- = deoxyribosyl-. 



