1490 PHOTOCHEMISTRY OP CHLOROPHYLL CHAP. 35 



was that the inhibiting effect of oxygen may be due to the catalytic acceler- 

 ation by oxygen of the "de-tautomerization" (or deactivation of a meta- 

 stable triplet state) of chlorophyll: 



(35.1) tChl ^'^ ) Chi 



a reversal of reaction (18.11a)— rather than to acceleration of an oxidation- 

 reduction reaction which follows tautoraerization, as was assumed, e. g., 

 in equations (18.12). The capacity of oxygen molecules to catalyze the 

 destruction of tChl can be made plausible, according to McBrady and 

 Livingston, by reference to the paramagnetism of the oxygen molecule, 

 and to the probability that the long-lived state of chlorophyll, tChl, is a— 

 mesomeric or tautomeric— triplet state and, as such, paramagnetic. 



McBrady and Livingston mentioned an alternative explanation of the 

 oxygen effect (suggested by Franck) involving the formation of a "mol- 

 oxide" of tautomeric chlorophyll: 



(35.2) tChl + O2 > tCh]-02 



capable of either reacting with hydrogen donors (such as an amine, AH2) : 



(35.3) tChl-02 + 2AH2 > H2O + 2 A + Chi 



or decomposing into ordinary chlorophyll and oxygen : 



(35.4) tChl-02 > Chi + O2 



Reaction (35.4) must be quite fast to be able to cause practically com- 

 plete suppression of reversible bleaching; but reaction (35.3) must be 

 even faster to account for the good quantum yield of sensitized photoxida- 

 tion of amines. These two requirements seem difficult to reconcile. 



Livingston and Ryan (1953) later adopted this scheme in the interpre- 

 tation of flashing light experiments, cj. reactions (g) to (i) in sequence 

 (35.12A), and calculated rate constants for reactions (35.2) and (35.4); 

 they did not discuss the rate constant of (35.3). 



McBrady and Livingston concluded that the earlier suggested mecha- 

 nism of oxygen inhibition [reaction (18.12) ] combined with a reaction of the 

 tautomer with impurities or with the solvent (as discussed on page 491, 

 Vol. 1) still affords the most plausible scheme of the oxygen effect: 



(35.5a) Chi* > tChl (equation 18.11a) 



(35.5b) tChl + S > rChl + oS (equation 18.14) 



(35.5c) rChl + O2 >Chl+H02\ , , ,. . ,. ,,„,,, 



(35.5d) HO2 + oS > S + O2 jfiompare back reaction m equation (18.14) 



(here, S can stand for impurity, or solvent). 



The following mechanism was suggested for the effect of iodine on re- 

 versible bleaching: 



