518 PHOTOCHEMISTRY OF PIGMENTS IN VITRO CHAP. 18 



in the absence of oxygen, in neutral (instead of acid) solution, Fe+++ ions 

 form an insoluble hydroxide and the dyestuff is progressively bleached. 



Can this mechanism also account for the sensitizing action of chloro- 

 phyll? Two observations argue against such an hypothesis: chlorophyll 

 solutions are not bleached by reducing ions, e. g., Fe++ or I~, either 

 reversibly or irreversibly (Rabinowitch and Weiss, unpublished); and 

 the fluorescence of chlorophyll is not quenched by these ions. 



However, the first result could possibly be explained by assuming 

 that, in the case of chlorophyll, the back reaction in (18.39) : 



light 



(18.39) Chi* + A V rChl + oA 



dark 



is so rapid that even the strongest illumination cannot appreciably 

 disturb the equilibrium, and the concentration of the reaction products, 

 [oA], never becomes large enough to allow their removal (as by precipi- 

 tation of Fe+++, in the case of A = Fe++). 



The second result, the nonquenching of chlorophyll fluorescence by 

 the substrates of photoxidation, also does not entirely preclude a mech- 

 anism of the Weiss-Fischgold type, since it can be attributed to a pre- 

 liminary tautomerization (or a reversible reaction with the solvent), as 

 described on pages 483^84, for example : 



(18.40a) Chi* . tChl or (18.41a) Chi* + S v oChl + rS 



(18.40b) tChl + A )■ oA + rChl (18.41b) oChl + A > oA + Chi 



(18.40c) rChl + J Oj > Chi (18.41c) rS + i O2 > S 



(18.40) A + i02 >oA (18.41) A + i O2 > oA 



(As mentioned on page 484, contrary to the suggestion of Weiss and 

 Weil-Malherbe, 1944, strong self-quenching cannot explain the absence 

 of quenching by sensitization substrates, when the quantum yield of the 

 sensitized reaction is close to 1.) 



(e) Comparison of Mechanisms of Type A 



In (18.33), (18.34), (18.40), and (18.41) we have formulated four 

 alternative "type-A " mechanisms of chlorophyll-sensitized autoxidation, 

 each of which could explain why the oxidation substrates do not quench 

 chlorophyll fluorescence and why the photoxidation occurs with a high 

 quantum yield even at low oxygen pressures. The only experimental 

 results which can be used to test these formulas are Gaffron's data on 

 the quantum yield of sensitized oxidation of allyl thiourea (Eq. 18.32). 

 Apart from the [Chi] term in the denominator, equation (18.32) is of 

 the familiar " Stern-Volmer " type, indicating that activated chlorophyll 

 has the alternative of either being deactivated by a monomolecular 

 process, or reacting with acceptor A by a bimolecular process. Of the 

 four mechanisms mentioned above, mechanism (18.40), in which tau- 



