1514 



PHOTOCHEMISTRY OF CHLOROPHYLL 



CHAP. 35 



reductant, is in line with the observations of Krasnovsky and co-workers, 

 which we will now describe. 



According to the experiments of Krasnovsky which were described 

 above (section 3), chlorophyll can be reversibly reduced in light by ascorbic 

 acid. In the presence of riboflavin, or safranin T, the latter dyestuffs are 

 reduced instead of chlorophyll — presumably by reacting with the photo- 

 chemically reduced chlorophyll. Because of the relative positions of their 

 reduction potentials, Eo = 0.22 v. and 0.29 v., respectively (as compared 

 with —0.05 V. for ascorbic acid), riboflavin and safranin T are not reduced 

 by ascorbic acid in the dark. Reduction can be achieved by illumination 



a. 



E 

 o 



>- 



(D 



UJ 

 Q 



< 

 O 



I- 

 Q. 

 O 



0.20 



Uj 



0.10 



20 

 TIME, minutes 



30 



40 



Fig. 35.5. Chlorophyll-sensitized reduction of riboflavin by ascorbic or pyruvic 

 acid, and reoxidation in air (after Krasnovsky 1948). Curve i, ascorbic acid; 2, 

 pyruvic acid. Broken curves, in ethanol; soUd curves, in pyridine. (•) Light 

 off; (-(-02) air admitted. 



with light absorbed by the dyes themselves, or, in the presence of chloro- 

 phyll, by illumination with red light absorbed by chlorophyll. In addition 

 to ascorbic acid, pyruvic acid, too, can be oxidized in this manner. 



The reactions were carried out in ethanol or pyridine as solvent; the 

 concentrations were : 



[dye] = 10-5 mole/liter [reductant] = 6 X IQ-^ mole/liter 



The change in dyestuff concentration was determined photometrically. 

 The solutions were boiled in vacuum to remove oxygen, and illuminated 1-3 

 min. by focussed light from a 500-watt lamp. Admission of air accelerated 

 the back reaction. The following mechanism was assumed for the non- 

 sensitized reaction: 



