CHLOROPHYLL-SENSITIZED OXIDATION-REDUCTIONS 



1515 



(35.26) 2 D + 2 AII2 ;f 



light 



dark 



± 2 DH + 2 AH - 



± (DH2 + D) + (AII2 + A) 



In the presence of sensitizer, the latter was assumed to serve as intermediate 

 hydrogen acceptor. 



Figure 35.5 illustrates Krasnovsky's results. It indicates that rever- 

 sibility is more complete in pyridine than in methanol ; Krasnovsky suggests 

 as a possible reason the stabilization of the semiquinone DH by ionization 



DH ;^ 



± D- -1- H- 



encouraged by the proton affinity (basicity) of pyridine. The reaction with 

 pyruvic acid in methanol is at least partially reversible. 





CP 



o 



0.75 



Fig. 35.6. Chlorophyll-sensitized reduction of 

 coenzyme I (DPN) by ascorbic acid (after 

 Krasnovsky and Brin 1949). Chlorophyll (a + 

 b) in pyridine with 10 mg. ascorbic acid and 1 

 mg. DPN; (1) spectrum before reaction; (2) 

 spectrum after reaction (3 min. illumination in 

 red light) without DPN; (5) spectrum after 

 reaction (3 min. light) with DPN; (4) spectrum 

 of reaction product (DPNH2?) (obtained by 

 subtraction of 2 from 3). 



0.50 - 



0.25 



300 350 400 



Magnesium phthalocyanine also can be used as sensitizer, in the same 

 concentration (10"^ mole/liter) as chlorophyll. 



Krasnovsky pointed out that in the sensitized reduction of one mole 

 of safranin T by ascorbic acid under standard conditions as much as 16 



