CHLOROPHYLL-SENSITIZED OXIDATION-REDUCTIONS 1511 



If this scheme is compared with the various mechanisms discussed in 

 chapler 18 (Vol. 1), it appears as a variation of mechanism A/31 (page 515), 

 exemplified in equations (18.33a-18.33d) for the case of molecular oxygen 

 as oxidant. The alteration consists in the assumption that tChl associates 

 with the oxidant in a complex, and in this form catalyzes the transfer of 

 hydrogen from reductant to oxidant; while in its original form, scheme 

 A/31 assumed that this catalysis occurs in two steps, tChl first transferring 

 a hydrogen atom to the oxidant, and then recovering it from the reductant. 

 The first mechanism is similar to an often postulated mechanism of enzymic 

 catalysis, based on complex formation of enzyme and substrate ; the second 

 one is more in line with the known mechanisms of nonenzymic oxidation- 

 reduction catalysis. 



By permitting competition between the catalytic reaction (35.16) and 

 the decomposition of the complex (35.15e), Livingston's scheme gives a 

 possibility of explaining the dependence of the quantum yield on the con- 

 centration of the reductant, [PHo]. To achieve a similar result in the original 

 reaction scheme A/Sl, we would have to admit the possibility of a back reac- 

 tion there, too. In scheme (18.33), e. g., a reaction oChl + HO2 -^ Chi + 

 O2 would have to be postulated, reversing reaction (18.33b). 



Livingston and Pariser derived, from the above reaction scheme, an ex- 

 pression showing the dependence of the average quantum yield, 7, on the 

 concentration of phenylhydrazine (which remains practically constant 

 during a run) and the "logarithmic mean" of the (strongly changing) con- 

 centration of the reacting dyestuff : 



r<^^ iQ^ Tmt^i - [MR-Jo - [MR-] 



•^■^^•^^^ ^^^^^ ' ~ In [MR"]o - In [MR"] 



[MR"]o is the initial, and [MR"] the final concentration of the reactive 

 form of the dye. 



The equation obtained for the mean quantum yield has the form 



^Qron^ - - ^' V (WA^O [PH .] {k,/k; )[MR"] 



[6b.Z()) 7 - 2(k, + kj)^ 1 + {k,/k[) [PH2] ^ 1 + {kjh') [MR"] 



With the below-determined constants, and applied to momentary concen- 

 trations of all components, this equation becomes 



i-ir. 91 , - n dR V 1 X 10-^ [PH2] ^ 5 X 10* [MR"] 



(35.21) 7 = 0.46 X 1 I 1 w in2 rPTj 1 X 



1 + 1 X 102 [PH2] 'M + 5 X 10' [MR"] 



The factor i/2 comes, as mentioned before, from the dismutation of the 

 half-reduced dye; the factor kt/{kt -\- k/) from competition between fluores- 

 cence and tautomerization ; the factor ki/kf from competition between 

 complex formation (35.15d), and the detautomerization of chlorophyll, 

 (35.15c). As mentioned above, oxidation and reduction of tChl could be 



