CHLOROPHYLL-SENSITIZED OXIDATION-REDUCTIONS 1513 



"absolute" maximum quantum yield of 0.92/2 = 0.46. If a value for k/ 

 is taken from earlier measurements of the life-time of the "long lived excited 

 state" (reversibly bleached state) of chlorophyll, we have: 



(35.24) k; = 2.5 X 10-^ sec.-' 



(35.25) A-i ^ 10^ (liters/mole) 



This high value for the constant of a bimolecular reaction indicates a reac- 

 tion with only a very small, or vanishing activation energy. 



In methyl red solutions, the life-time of the bleached state of chlorophyll 

 was estimated (c/. section 1) as '--'1 sec. This can be considered as the 

 life-time of the complex {tChl.MR"} (or, alternatively, as the life-time of 

 chlorophyll oxidized by methyl red, oChl). This leads to a value of the 

 order of 10^ for the constant kt, indicating that reaction (35.16) may re- 

 quire an activation energy. Consequently, the quantum yield of the over- 

 all reaction can be expected to show strong temperature dependence when 

 [PII2] is low, and (35.16) is therefore the "bottleneck" reaction. 



Watson (1952) in a subsequent paper (based on his work in Livingston's 

 laboratory) suggested a different interpretation of the mechanism of 

 chlorophyll-sensitized reduction of methyl red by phenylhydrazine. His 

 experiments dealt with the quenching of fluorescence by phenylhydrazine 

 in a polar solvent (methanol), and its activation, by the same agent, in 

 nonpolar solvents (benzene or heptane) (c/. page 768; Watson's experi- 

 mental results will be described in chapter 37C, section 4c). From these 

 experiments, and the strong effect of phenylhydrazine (PH2) on the ab- 

 sorption spectrum of chlorophyll h (cf. page 699 and chapter 37C, section 

 2), Watson concluded that phenylhydrazine forms two different complexes 

 with chlorophyll; its attachment to the chlorophyll molecule in one place 

 leaves the absorption spectrum of chlorophyll a unchanged and activates 

 fluorescence; its attachment in another place quenches the fluorescence 

 (independently of whether the first place is occupied by another phenyl- 

 hydrazine molecule or not). The calculated association constants were 

 1900 liters/mole for the fluorescence-promoting, and 16 liters/mole for the 

 fluorescence-quenching complexing (in benzene) ; the first constant was the 

 same, but the second was higher (58 liters/mole) in heptane, Watson 

 pointed out that if these constants are correct then, under the conditions 

 of Pariser's experiments, a large part of chlorophyll must have been as- 

 sociated with phenylhydrazine. The symmetry of equation (35.20) per- 

 mits substitution of the assumption of a primary reaction between PH2 and 

 excited Chi in a pre-formed complex for that of the primary formation of a 

 tChl-methyl red complex (e. g., 35.15e), and subsequent reaction of this 

 complex with phenylhydrazine (e. g., 35.16). 



This mechanism, involving primary interaction of chlorophyll with the 



