Nucleic Acids and Nucleoproteins 63 



It had been found that the dose required (in solution), 

 expressed in r units, to bring about a given amount of change 

 increases directly with the concentration (Cox et ah, 1955). 

 This is, of course, a characteristic of indirect action and will 

 be true if a constant fraction of the radicals is effective in 

 bringing about the observed change. 



In dilute solutions about 100 ionizations are required per 

 molecule in the whole solution to reduce the viscosity by one 

 half, i.e. the process of degradation is comparatively inefficient. 



When irradiated in the solid state, much greater doses are 

 required to bring about a given change, about 5 X 10^ r is 

 required to reduce the intrinsic viscosity by one half, i.e. the 

 factor for solid/dilute solution (0-1 per cent) is of the order of 

 100 for viscosity (Fig. 1). However, the actual efficiency per 

 ionization in producing a viscosity change is indeed greater 

 in the solid than in the solution, since as pointed out by 

 Alexander and Stacey (1955) less than 3 ionizations per 

 molecule produced in the solid do enough damage to reduce 

 the molecular weight by one half, and Fluke, Drew and 

 Pollard (1952) found that about one ionization will inactivate 

 Pneumococcus-tT2i\\sioTYmi\g principle. The greater sensitivity 

 per ionization produced in the solid state might be antici- 

 pated, as in dilute solution many of the ionizations give rise to 

 radicals at considerable distances from the DNA particles, 

 and will recombine with each other before reaching them. 



This does not mean that the effect of radicals in vivo is 

 necessarily insignificant. The overall effect of irradiation is 

 greater in solution because ionizations occurring over a 

 considerable volume are effective by the radical mechanism. 

 When only a comparatively small amount of water is present, 

 the effect of ionizations in the water is relatively greater than 

 in dilute solutions and of the same order as that in an equal 

 volume of DNA; e.g. in 20 per cent DNA the sensitivity to a 

 given dose of radiation is 4 or 5 times that in solid DNA. 



The effect of X-rays is also similar to that of heat in causing 

 a denaturation of the DNA. This is primarily a breakage of 

 hydrogen bonding between the threads. The result is shown 



