270 RADIATION BIOLOGY 



to the specific change (loss of activity) investigated. It may rer|uire a 

 special orientation of the solute molecule and of the radical to achieve 

 inactivation, so that any collision of unsuitably orientated partners may 

 lead to elimination of the radical and some change of the solute molecule 

 but not to loss of its activity, or to no reaction at all. The shape of the 

 yield-concentration curves is, therefore, linked with the fate of the 

 radical. Any elimination of radicals through reaction in irrelevant 

 regions of the solute molecule will favor the attainment of constancy of 

 the ionic yield, down to low concentrations of solute. On the other 

 hand any collision of radicals with solute molecules which leaves the two 

 partners unchanged will favor recombination of radicals. This finds its 

 expression in a shift of the attainment of a constant ionic yield to ranges 

 of high concentrations of solute. It should be noted as a side line that 

 the orientation principle has its counterpart in another mechanism which 

 may occur in certain limited cases: energy transmitted from an active 

 radical to a solute molecule may migrate through the whole molecule, 

 leading to chemical change at a distance from the original point of impact 

 (Franck and Livingston, 1949). 



The foregoing considerations may be relevant to the difference in shape 

 of the yield-concentration curves, but they do not explain why the yield 

 of deamination reaches such high values. Here again we are not on very 

 firm ground. If every collision of an active radical with a solute molecule 

 led to a chemical change of the solute molecule which was registered by 

 the method of analysis, the number of changed solute molecules would 

 equal the number of radicals formed and this would be a simple means of 

 deciding how many radicals can be formed by any particular dose of 

 radiation. In fact the situation is more complicated since back reactions 

 and chain reactions can interfere. 



The number of radical pairs which can be formed for the absorption of 

 an energy corresponding to the production of one ion pair (approximately 

 32.5 ev) is not exactly known, though an estimate has been made which 

 puts the value at 3 (Dainton, 1948; Dainton and Miller, 1947; Miller, 

 1948). This includes radicals formed by excitation and charge neutral- 

 ization as well as by ionization and seems to be the limit energetically 

 possible. Thus an ionic yield of 3 can just be obtained if only one type 

 of radical enters into reaction and of 6 if two types enter ; but no radicals 

 must be lost by either recombination or back reactions. 



It is not yet clear whether H atoms as well as OH radicals are involved, 

 although there is one report to this effect (Stein and Weiss, 1949). 

 However, a yield of 4.2 for L-serine would be unlikely with a simple 

 radical mechanism and only one type of radical reacting. 



Yields of this magnitude can also be explained in terms of chain reac- 

 tions. Some support for this comes from other experiments, such as those 

 described in the section on protection effect where it was suggested that 

 energy could be handed on in a chain reaction. 



