330 LIGHT AND LIFE 



quinone of the dye. This, then, finishes its reduction, either by com- 

 bination with a radical or by taking a j^roton cHrectly off the hydro- 

 gen donor itself, to give the colorless dyestuff and the dehydro- 

 ascorbic acid or other dehydro compound. This is a generalized 

 scheme which appears to apply for a whole variety of hydrogen 

 donors, hydrogen acceptors, and sensitizing dyes. The reaction will 

 work, for example, with acridine orange as the sensitizing dye, for 

 allylthiourea as the hydrogen donor, and for oxygen as the hydrogen 

 acceptor. 



In general, these reactions do not involve the storage of any energy, 

 that is, the reaction hydrogen donor -|- dyestuff — > reduced dye 

 -\- dihydro hydrogen donor (ascorbic acid, etc.) is thermodynamically 



Fig. 8. Chlorophyll-sensitized reduction of dye by ascorbic acid (Krasnovskii, 

 Evstigneev). Ch, chlorophyll; D, dye; A, hydrogen doner. 



positive; the energy is "downhill" in that direction. However, there 

 are a few cases in which the reaction seems to be reversible, that is, 

 when the light is turned off there follows the reappearance of oxidized 

 dyestuff. Whether this indeed represents a small degree of energy 

 storage (a few kcal) or whether it represents a trace of oxygen in the 

 reaction mixture which, in general, will oxidize most of these reduced 

 materials, remains to be demonstrated. In any case, there is no great 

 storage of free energy in this system; most frequently these reactions 

 are "downhill" thermodynamically and will not go backwards. When 

 they do go backwards, it seems as though these dihydro dyestuffs are 

 auto-oxidizable, and traces of oxygen in the reaction mixture may 

 carry them back. 



There are a nimiber of photochemical electron transfer reactions 

 involving dyestuffs unrelated to chlorophyll and which apparently do 



