Mutagenic Alkylating Agents 309 



When the reaction occurs within the same molecule it causes 

 a reduction in viscosity (Alexander and Fox, 1952) because 

 the molecule is coiled up so that it occupies less volume*. On 

 intermolecular reaction the viscosity of the solution rises 

 steadily until quite suddenly it sets to a gel, when the number 

 of crosslinks formed exceeds, on average, one per molecule. 



( 



(6) (c) 



Fig. 7. Different types of reactions produced by bifunctional reagents 

 with a flexible macromolecule like DNA 



(a) Intermolecular crosslinking. 



(b) Reaction with neighbouring groups. 



(c) Intramolecular crosslinking. 



Internal crosslinking can be recognized very clearly from 

 light scattering measurements. By plotting the light scattered 

 at different angles, as shown in Fig. 8, the volume occupied 

 by the molecule in solution (expressed as the radius of gyra- 

 tion) is given by the slope and the molecular weight by the 

 intercept (the smaller the value for the intercept the higher 

 the molecular weight). Using this technique it is clear that 

 the first change produced by a polyfunctional reagent is to 



* The viscosity of a solution of a macromolecule depends both on the size 

 and shape of the molecule dissolved, and a reduction will be observed both 

 after a reaction which degrades the molecule into smaller fragments and after 

 internal crosslinking which causes a reduction in size. Following the observa- 

 tions of Chanutin and Gjessing (1946) and Sparrow and Rosenfeld (1946) that 

 the viscosity of DNA solution was reduced by treatment with nitrogen mustard 

 and X-rays respectively, Butler (1950) assumed that both agents degraded the 

 molecule. We now know (see p. 310) that initially the two reactions are quite 

 different. While radiation degrades, the mustards reduce the viscosity by 

 internal crosslinking. 



