PHOTODYNAMIC ACTION 695 



or activated state; i.e., the absorbed energy is retained in a form in which 

 it can be transmitted to a second molecule with which the first may collide. 



2. In collision with another molecule of lower energy status, the acti- 

 vated molecule transfers its energy and reverts to its former energy level. 



3. The secondarily activated molecule (the acceptor) is then capable of 

 combining with a molecule of oxygen — a chemical reaction in which it 

 would not participate in its normal state. 



This mechanism can be expressed in the following scheme, where D 

 represents a molecule of the dye or other photodynamic agent, X is a 

 molecule of the acceptor or substrate, D' and X' are the activated or 

 reactive forms of these molecules, and A'ox is the final product. 



D + hv-^ D' 



D' + X^ D + X' (Scheme I) 

 X' + O2 ^ Xox 



The validity of this hypothesis can be best examined by considering 

 how it complies with the known characteristics of photodynamic actions 

 and with certain natural laws relating to photochemical reactions in 

 general. 



Relation of Effective Light to Absorption Spectrum of Sensitizer. A 

 fundamental law of photochemistry is the Grotthus-Draper law, which 

 states that only radiation absorbed by the reacting system is effective in 

 producing chemical change. In compliance with this law the radiation 

 producing a photodynamic effect (the action spectrum) should correspond 

 exactly to the absorption spectrum of the photodynamic agent as it occurs 

 in the system. In practice the comparison of action and absorption 

 spectra is often difficult because the absorption spectrum of a photo- 

 dynamic agent in a biological system ma}^ differ, through variation of 

 physical conditions, from its absorption spectrum as measured in vitro. 

 Screening or scattering of light by other substances (including the dye 

 itself if present in too high a concentration) may interfere with the meas- 

 urement. Methods of measurement also present difficulties, because usu- 

 ally the only practicable way to select portions of the spectrum is by use 

 of filters transmitting light unevenly over a comparatively wide band of 

 wave lengths and because measurement of the response in biological 

 systems is rarely precise. Nevertheless in a few systems, with careful 

 control of conditions, satisfactory agreement within the limitations of 

 methods and materials has been obtained between action spectra and 

 the absorption spectra of introduced photodynamic substances. These 

 include tropic bending of roots of wheat seedlings sensitized with eryth- 

 rosin (Blum and Scott, 1933), sensitization of human skin by hemato- 

 porphyrin (Blum and Pace, 1937), sensitization of the skin of sheep by 

 injected phylloerythrin (Clare, 1944), and sensitization of rabbits by 

 hypericin (Pace, 1942). 



