696 RADIATION BIOLOGY 



Energy Requirements of Photodynamic Systems. The second law gov- 

 erning photochemical reactions is that of Einstein, according to which 

 one molecule is activated for each quantum absorbed. This does not 

 imply that every molecule that has absorbed a quantum of energy will 

 react, since not all molecules will make collisions with other suitable 

 molecules during their activated lifetimes. A quantum yield (i.e., num- 

 ber of quanta recjuired per molecule transformed) of 1 is possible in the 

 ideal case, and yields approaching 1 have been obtained in some reactions 

 (Gaffron, 1933). In photochemical mechanisms involving chain reac- 

 tions, quantum yields of less than 1 may be obtained, but, in general, 

 yields are greater than 1 because a number of the activated molecules 

 lose their energy before making an effective collision. A further compli- 

 cating factor is the fact that sensitizer molecules may undergo recurrent 

 activation, returning to a nonactivated state each time they have made 

 an effective collision or have otherwise lost their energy. It therefore 

 follows that ciuantum yields in a scheme such as that suggested for photo- 

 dynamic action will be dependent on the concentrations of oxygen and of 

 the substance undergoing transformation but are virtually independent 

 of the concentration of the photodynamic agent, since the latter concen- 

 tration influences not only the number of molecules capable of being acti- 

 vated but also the number of quanta that may be absorbed. 



Using the hemolysis of red blood cells sensitized by dyes of the fluo- 

 rescein group as their experimental system, Blum and Gilbert (1940b) 

 estimated the number of quanta necessary to hemolyze a single red cell. 

 In such a system the effective dye concentration is the amount of dye 

 actually taken up by the cells, and the primary photochemical reaction 

 involved is assumed to be the alteration or destruction of a substance 

 within the cell membrane which affects the permeability or fragility of 

 the latter. Evidence of such a process is the existence of an induction 

 period of irradiation which must precede the onset of hemolysis. Within 

 this system the number of quanta required per cell hemolyzed was found 

 to be constant, of the order of 10^", for both rose bengale and erythrosin 

 over a wide range of concentrations of the dyes taken up on the cell. 

 A further observation in this experiment was that the number of quanta 

 absorbed per dye molecule during the course of the reaction ranged from 

 tens to thousands of quanta and was inversely proportional to the con- 

 centration of the dye. This is consistent with the concept of a photo- 

 chemical process in which each activated dye molecule reverts, after 

 either effective collision or dissipation of the acquired energy, to its initial 

 state without destruction and is then capable of reactivation by further 

 irradiation. 



Another law that follows from the equivalence relation of Einstein is 

 the Bunsen-Roscoe law, which states that, provided the degree of absorp- 



