698 RADIATION BIOLOGY 



reactions ; if the reaction rate is not influenced by change of temperature, 

 the reaction may be reasonably assumed to be free of thermal stages. 



Negligible influences of temperature have been recorded by Hannes 

 and Jodlbauer (1909) for the photosensitized inactivation of invertase 

 and by Blum et al. (1937) for photodynamic hemolysis. 



Effect of Concentration of Reactants. Determination of the order of a 

 reaction by following the effect of concentration of reactants on the rate 

 of formation of products is a common technique in studies of reaction 

 mechanisms. A photochemical reaction such as that postulated for 

 photodynamic action involves at least three steps — activation of the 

 sensitizer, colhsion of activated sensitizer with substrate, and collision 

 of substrate with oxygen — ^all of which may influence or determine the 

 over-all reaction rate, and none of which can be studied independently. 

 It is not to be expected, therefore, that reaction-order determinations will 

 yield much information on the mechanism of photodynamic action. 



Significance of Fluorescence. A property common to photodynamic 

 agents is fluorescence. Since fluorescence represents the ^mission from 

 activated molecules of energy previously absorbed during irradiation, 

 this property is consistent with the concept of photodynamic action 

 which has been outlined. 



ALTERNATIVE HYPOTHESES 



In developing the foregoing conception of the mechanism of photo- 

 dynamic action, Blum (1941a) made a critical examination of various 

 alternative hypotheses that have been proposed. Most of these involve 

 a fundamental photochemical mechanism but differ from Blum's concept 

 in method of utilization of the absorbed energy to effect the destructive 

 changes. One older hypothesis of a different type — that the effects of 

 photodynamic action are brought about by the light emitted as fluo- 

 rescence by the irradiated dye — is mentioned here only because the writer 

 has found it still commonly accepted among field workers concerned with 

 photosensitivity diseases. Apart from its inherent improbability, since 

 fluorescent light is not qualitatively different from radiation of the same 

 wave lengths emitted by other sources, it is sufficient to state that it was 

 tested and refuted by Raab (1900) himself. The chief alternative 

 hypotheses are discussed in the following paragraphs. 



Theories Involving Activation of Oxygen. A mechanism in which the 

 sensitizer transfers its energy directly to oxygen, that is, a transposition 

 of O2 and X in Scheme I, has been proposed (Kautsky et al., 1933). 

 These authors based their hypothesis largely on the effect of oxygen in 

 quenching fluorescence and supported it with an experiment in which the 

 sensitizer and the substance undergoing oxidation were kept apart by 

 absorption on separate particles of silica gel. When the two sets of 



