PHOTODYNAMIC ACTION 697 



tion does not change, the product of the duration of irradiation and the 

 intensity of Hght gives a constant: 



I Xt = k. 



Attempts to determine whether this law appHes to photodynamic action 

 were made by Dognon (1928), using paramecia, and by Blum and Hyman 

 (1939), who studied photohemolysis, but in both cases the results sug- 

 gested some deviation from constancy for the product I X t. Recogniz- 

 ing that a biological reaction such as photohemolysis proceeds in several 

 stages, only one of which — the initial photochemical process — is subject 

 to the Bunsen-Roscoe law, Blum and Gilbert (1940a) devised experiments 

 to control such factors as the taking up of dye by the cells and the time 

 occupied by the final process of lysis of the cells. Their calculations 

 were based on the concentrations of dye taken up by the red cells, and 

 the lysis stage was eliminated by measuring the least time of irradiation 

 required to bring about hemolysis 24 hr after irradiation. Values for 

 / X t thus obt^^ined were remarkably constant throughout any one experi- 

 ment, indicating the relevance of the Bunsen-Roscoe law to this process. 



An additional characteristic of photodynamic hemolysis which is con- 

 sistent with the theory that it involves a simple photochemical reaction 

 is that the initiation of hemolysis requires the same total amount of 

 irradiation, whether this is applied continuously or intermittently. Blum 

 and Morgan (1939) quote times of 330-367 sec required to produce 50 per 

 cent hemolysis with intermittent hght and dark periods and 350-380 sec 

 with continuous illumination. Efimov (1923) had earlier obtained paral- 

 lel results in experiments on the photodynamic killing of paramecia. 

 Such results suggest not only that the process is of a simple photochemical 

 type, not involving chain reactions, but also that it is irreversible. 



Influence of Temperature. From their nature purely photochemical 

 mechanisms are independent of temperature, since the energy of acti- 

 vation of the reacting molecules is acquired by the absorption of quanta 

 of radiant energy and not through increase of kinetic energy of the mole- 

 cules by heating. Some photochemical reactions may, however, also be 

 thermal reactions, in which case they will show a temperature coefficient. 

 If an initial photochemical reaction is followed by a thermal reaction, the 

 over-all process may be similarly influenced by temperature. The photo- 

 chemical reaction may also depend upon collision between particles, and 

 the frequency of such collisions is influenced by change of temperature. 

 In practice, therefore, it is not uncommon for photochemical reactions to 

 show increases in velocity of about 10 per cent for a rise in temperature 

 of 10°C. 



If a temperature coefficient approaching 2 is found, it is most probable 

 that the over-all reaction contains one or more stages that are thermal 



