PHOTODYNAMIC ACTION 699 



particles were mixed and exposed to light, the extent of the photodynamic 

 oxidation was found to depend on the amount of oxygen admitted to the 

 system. This experiment has been criticized by Gaffron (1936) because 

 there was no certainty that exchange of sensitizer and substrate did not 

 occur when the silica-gel particles were mixed. 



The hypothesis of Kautsky et al. is, at all events, rendered untenable 

 as an exclusive mechanism by Gaffron's work on the energy requirement 

 of such a system. The energy of activation acquired by a molecule 

 through collision with another molecule previously activated by absorp- 

 tion of radiation cannot exceed the amount contained in a quantum, 

 which is in turn determined by the wave lengths of the radiation. It 

 follows, therefore, that a reaction cannot be initiated either directly or 

 indirectly by radiation unless the energy of activation of that reaction is 

 less than the energy contained in 1 quantum of the absorbed radiation. 

 According to calculations given by Gaffron (1935), the energy of acti- 

 vation required by Kautsky's hypothesis is 37 kcal, which is supplied 

 only by quanta at wave lengths below 7620 A. A photochemical reaction 

 involving activation of oxygen therefore cannot proceed if radiation below 

 7620 A is excluded. Gaffron (1935) found that photooxidation of thio- 

 sinamine by bacteriopheophytin occurred in filtered light from which 

 almost all radiation below 7600 A was absent, indicating that activation 

 of oxygen was not the mechanism involved in this reaction. 



Formation of Peroxides as an Intermediary Process. Hydrogen peroxide 

 is known to be formed when fluorescein dyes are irradiated in aqueous 

 solutions, and such previously irradiated fluorescein solutions are hemo- 

 lytic in the dark. However, it has been demonstrated that this effect is 

 still produced after the hydrogen peroxide has been destroyed and also 

 that the amount of hydrogen peroxide formed during photodynamic 

 action is insufficient to account for the degree of hemolysis (Blum, 1935). 



Formation of Active Compounds of the Sensitizer. INIenke (1935) 

 changed a dilute solution of sodium fluorescein into its photocompound 

 by exposure to sunlight for 300 hr and found that this compound pro- 

 duced hemolysis in the dark to the same extent as fluorescein did in light. 

 He also produced hemolysis in the dark with fluorescein solutions that 

 had been irradiated for only 2 hr and concluded that photodynamic 

 effects are due to the formation of minute quantities of photocompounds 

 of fluorescent substances. Blum (1941a) has raised objections to such a 

 mechanism on two grounds, namely, (1) that formation of such a com- 

 pound in effective concentrations is unlikely during the course of photo- 

 dynamic hemolysis, since other substances in a biological system would 

 compete with the dye for oxidation, and (2) that such a mechanism would 

 involve destruction of the dye after absorption of a single quantum, 

 whereas absorption of many quanta by each molecule has been observed 

 by Blum and Gilbert (1940b). 



