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



coil up the molecule (see Table IV) and only after very exten- 

 sive reaction is there a fall in molecular weight (this will be 

 discussed on p. 315). With a monofunctional reagent such 



REAGENT ESTERIFICATION 



5* 



49% 



2200 



1950 



Phenylalanine ^ 



mustard 20 /o 



Bisepoxypropyl ^ 



ether 42 /o 



920 



875 



300 



■ ^ 6/ 

 sin 72 



Fig. 8. Light scattering plots (the reciprocal of the scattering 



at each angle is plotted as a function of that angle), for DNA 



treated with various alkylating agents. 



pg,w, the radius of gyration which is obtained from the slope 



of the corresponding line, is a measurement of the extension 



in space of the DNA molecule after reaction. The molecular 



weight is inversely proportional to the intercept. 



as propylene oxide no change in shape or molecular weight is 

 observed, proving that esterification of the phosphate groups 

 alone is not sufficient to coil up the molecule. 



Electron microscope photographs (Fig. 9) also reveal the 



