1520 



PHOTOCHEMISTRY OF CHLOROPHYLL 



CHAP. 35 



In ternary systems absorption bands of the sensitizers were not affected to more than 

 5-10% by 3 minutes illumination, while those of the oxidants were strongly reduced in 

 intensity. The results are summarized in table 35. IC. 



Table 35.IC 



Pekcent Reduction of Different Redox Systems by 3 Minittes of Illumination 



WITH Chlorophyll, Bacteriochlorophyll or Their Pheophytins as Sensitizers 



(After Krasnovsky and Voynovskaya 1952) 



* Complete bleaching of bacteriochlorophyll although the latter shows no reaction 

 with NazS in absence of riboflavin. 



A reduction of DPN could be observed with chlorophyll and ascorbic acid in pyri- 

 dine (as described before) but not with bacteriochlorophyll. (As in fig. 35.6, the reduc- 

 tion of DPN was derived from increase in the absorption of the illuminated solution 

 around 340 m^- ) 



mv 



300- 



400 



500- 



600 



Fig. 35. 7A. Changes of redox potential of chlorophyll and pheophytin 

 in pyridine upon repeated illumination and darkening: (A) 10 "^ mole/1. 

 Chi (a -f b), 0.8 X lO'^ mole/1, ascorbic acid; (B) 10~'» mole/1, pheophytin, 

 0.6 X 10~2 mole/1, ascorbic acid (after Evstigneev and Gavrilova 1953*). 



Although bacteriochlorophyll acts on the whole Hke chlorophyll, its reduced form 

 reacts back so much faster that no determination of its absorption spectrum could be 

 made. Krasnovsky also noted that while bacteriochlorophyll can be photochemically 

 reduced in vitro by sodium sulfide, it does not react with other reductants used by purple 

 bacteria, such as malic acid or propanol; he suggested that these hydrogen donors 

 must be acted upon by appropriate dehydrogenases before their hydrogen becomes avail- 

 able for photochemical transfer. 



