308 P. Alexander, S. F. Cousens and K. A. Stagey 



triesters which should be less stable than those from DNA 

 (Brown and Todd, 1952) it seems highly improbable that the 

 reduction in the amount of acid produced is due to esterifica- 

 tion of the primary phosphate groups. The stable esters which 

 are formed may be diesters derived from esterification of the 

 secondary phosphate groups at the end of the chain. These, 

 of course, would be very much more numerous in the relatively 

 low-molecular weight RNA than in DNA. However, this 

 whole reaction needs much more detailed study. 



Grosslinking of DNA 



When assessed by growth inhibition and the production of 

 chromosome breaks, the biological activity of alkylating 

 agents containing more than one reactive group per molecule 

 is in general much greater than that of monofunctional com- 

 pounds (Haddow, 1953). Although a direct comparison has 

 only been made in relatively few cases (Fahmy and Fahmy, 

 1956; Zamenhof et aL, 1956) this same difference seems to 

 persist also for mutagenicity. 



In principle, there are three reactions which distinguish a 

 bifunctional from a monofunctional reagent: the ability to 

 form crosslinks between different molecules (inter molecular 

 reaction); to join different groups in the same molecule (intra- 

 molecular reaction) and to form rings by reacting twice with 

 the same group (e.g. RNH2+X-M-X^RN>M+2HX) or on 

 two adjacent groups (Fig. 7). Since compounds which are 

 sterically prevented from forming a ring by double alkylation 

 such as CH3-S020-CH2-C=C-CH,-0-S02CH3 are highly 

 mutagenic, this last possibility need not be considered. 



With a relatively flexible macromolecule like DNA, intra- 

 molecular crosslinking will predominate in dilute solution, 

 while in more concentrated solutions reaction between mole- 

 cules will take place. Using synthetic polymers (Stacey et al., 

 1957) the change-over from one type of mechanism to the 

 other was followed in detail and was found to take place over 

 a remarkably small range of concentration. 



