500 PHOTOCHEMISTRY OF PIGMENTS IN VITRO CHAP. 18 



Reaction (18.21), if it occurs with a large quantum yield, must be almost 

 completely reversible (to account for the low yield of bleaching) ; a small 

 "leak" may be caused by the elimination of a certain proportion of 

 HO2 radicals (e. g., by their dismutation to water and oxygen). A 

 residue of oxidized chlorophyll molecules will thus be accumulated after 

 prolonged illumination. 



However, it seems probable that the photoxidation of chlorophyll in 

 solution occurs not (or not only) in the way suggested by formula (18.21), 

 but is associated with reversible oxidation-reduction reactions with the 

 solvent, or impurities, such as were discussed in the preceding section. 

 Gaffron suggested that the photoxidation of chlorophyll may be an 

 indirect consequence of the sensitized oxidation of autoxidizable im- 

 purities ("acceptors") according to the following reaction scheme 

 (A = acceptor; asterisks denote excited states): 



(18.22a) Chi* + A > Chi + A* 



(18.22b) A* + O2 > AO2 



(18.22c) AO2 + Chi > oChl + A 



( 18.22) Chi* + O2 > oChl 



Reaction (18.22c) competes with the stabilization of the oxidized acceptor: 



(18.22d) AO2 > oA 



The discussion in section A (pages 484-486) suggests a twofold 

 modification of Gaffron's scheme. In the first place, the part of the 

 "acceptor," A, may be played by the solvent. In the second place, 

 the reaction between chlorophyll and the acceptor (or solvent) is unlikely 

 to be the simple energy transfer represented by (18.22a). The most 

 probable primary process is an oxidation-reduction reaction of chlorophyll 

 with the solvent (or with an impurity, or another chlorophyll molecule). 

 All reaction schemes discussed in section A, e. g., (18.11) or (18.15), 

 which assume the reversible formation of oxidized chlorophyll, oChl, may 

 serve to explain a small "residual" photoxidation, if one assumes that 

 complete reversibility is disturbed by side reactions, which deprive some 

 oxidized chlorophyll molecules of partners for the back reaction. For 

 example, if the primary reaction of excited chlorophyll molecules is 

 their oxidation by the solvent — as assumed in (18.15) — a partial re- 

 oxidation of the reduced solvent by oxygen would leave some chlorophyll 

 in the oxidized state: 



(18.23a) Chi* + S > oChl + rS 



(18.23b) rS + i O2 > S + ^ H2O 



(18.23) Chi* + § O2 > oChl + § H2O 



On page 491, we assumed that oxygen acts as a catalyst in the back reaction in 

 (18.23a), i. e., that in (18.23b) oxygen is reduced, not to water, but only to HO2, and 



