520 PHOTOCHEMISTRY OF PIGMENTS IN VITRO CHAP. 18 



To sum up, we are at present unable to select a single mechanism for 

 all autoxidations and oxidation-reductions sensitized by chlorophyll. 

 One thing is clear : the mechanism of sensitization by chlorophyll is more 

 complicated than that of the sensitization by reversibly reducible "vat 

 dyes," and the assumption of a long-lived activation state cannot be 

 avoided. A scheme of sensitized autoxidation based on long-lived ac- 

 tive state, and kinetic equations derived from it, will be found on pages 

 546-547. 



Not unconnected with the problem of long-lived activated states may be the experi- 

 ments on the " photodynamic activity" of pre-irradiated dyestuff solutions (e. g., 

 fluorescein). Moore (1928), Blum (1930, 1932), Menke (1935), and others found that 

 cytolytic and hemolytic effects can be obtained not only by illuminating the dyed tissue, 

 but also by first illuminating the dyes and then introducing them into the tissues. 

 Blum and Spealman (1934) found that these effects cannot be explained by the presence 

 of hydrogen peroxide in illuminated dyestuff solutions (c/. page 78), and attributed 

 them to some unknown decomposition products of the dye. 



Quantitative investigations with different solvents and acceptors and 

 with varying light intensities and wave lengths should bring clarity into 

 the problem of the chlorophyll-sensitized reactions in vitro; and this may 

 become an important step forward in the understanding of the role of 

 chlorophyll in photosynthesis. 



One prerequisite of such studies is that they be carried out with 

 fresh, pure chlorophyll preparations, and not with crude extracts, so-called 

 "pure" commercial products, or preparations which have been kept in 

 storage for a considerable length of time. The oxidation-reduction 

 properties of chlorophyll seem to be among the most sensitive character- 

 istics of this altogether very sensitive compound, and may undergo rapid 

 changes upon storage, not only in solution, but in the dry state as well. 



4. Sensitization within a Complex (Type-B Mechanisms) 



On pages 492 and 499, the possibility of a reversible association 

 of chlorophyll with oxygen was discussed in connection with the mecha- 

 nism of the bleaching of chlorophyll, and the conclusion reached was 

 that this association is improbable. Obviously, if it would occur, the 

 kinetics of chlorophyll-sensitized autoxidations would be largely changed 

 (mechanisms Bal or B^l taking the place of the mechanisms discussed 

 before, which were based on the encounters of the molecules tChl, rChl, 

 rS, or rA with free oxygen molecules). 



Another possibility is an association of chlorophyll with the acceptor A 

 (mechanisms Ba2 and Bl3'2). Such an association could explain the ab- 

 sence of a concentration-dependent effect of the acceptor on the fluores- 

 cence of the sensitizer, in the same way as this fact can be explained by a 

 reaction with the solvent or by tautomerization — namely, by making the 

 sensitization, like fluorescence, formally a monomolecular reaction. The 



