CHLOROPHYLL-SENSITIZED AUTOXIDATIONS 1525 



coloration. No nitrophenyl hydroxylamine was formed in controls without chlorophyll 

 or phenjihydrazine. Ascorbic acid could be used as hydrogen donor instead of phenyl- 

 hydrazine. (This reaction occurs spontaneously in alkaline medium, but not in neutral 

 solution. ) Similar results were obtained with eosin, erythrosin B and some other fluores- 

 cent dyes as sensitizers. 



Gurevich (1949) later found that oxidation of phenylhydrazine by o-dinitrobenzene 

 can be catalyzed in the dark by "chlorophjdl hemin." The latter was obtained from an 

 alcoholic leaf extract, by successive conversion to pheophytin and introduction of iron 

 (by means of heating with ferric acetate). The green crystalline product, added to a 

 saturated solution of o-dinitrobenzene in ethanol, also containing phenylhydrazine (free 

 base), catalyzed the reduction of dinitrobenzene to o-nitrophenyl hydroxylamine, which 

 can be recognized by its violet salts. The same result can be achieved with Fe^Oj as 

 catalyst, but the catalytic effect of the "chlorophyll hemin" does not depend on con- 

 tamination with inorganic iron salts. 



Pariser (1950) and Wcigl and Livingston (1952-) studied the chloro- 

 phyll-sensitized reduction of an azo dye (butter yellow) by ascorbic acid — 

 a reaction of the same type as was studied by Krasnovsky and co-workers. 

 According to Pariser, the maximum quantum yield of this reaction in meth- 

 anol is about 0.5. Weigl and Livingston used deuterated ascorbic acid to 

 see whether any deuterium could be found in chlorophyll after the reaction 

 with butter yellow is over. To avoid isotopic exchange with the solvent, 

 dioxane was used instead of methanol; this reduced the quantum yield to 

 a very low value. After about ten deuterium atoms were transferred to 

 the oxidant for each chlorophyll molecule present, analysis of chlorophyll for 

 deuterium content revealed <4% of the amount to be expected if one D 

 atom were to get stuck in chlorophyll in each reduction act. Experiments 

 on deuterium exchange between chlorophyll and heavy water (Weigl and 

 Livingston 1952^) showed that deuterium could not have been lost from 

 chlorophyll during the chromatographic purification; it thus seems that, 

 in this particular oxidation-reduction reaction, the primary photochemical 

 process is not the transfer of hydrogen from excited chlorophyll to the oxi- 

 dant (to be replaced later by hydrogen from the reductant). The primary 

 act can be the transfer of hydrogen from the reductant to chlorophyll (as 

 suggested by Krasnovsky's experiments) — at least, if one is permitted to 

 assume that the same hydrogen is later transferred, by a thermal reaction, 

 to the oxidant, without having been first pooled with other hydrogen atoms 

 in the sensitizer. 



6. Chlorophyll-Sensitized Autoxidations 

 (Second Addendum to Chapter 18, Part C) 



In Volume I (chapter 18) a number of chlorophyll-sensitized autoxida- 

 tions were described. Of these, only one was investigated quantitatively — 

 the ethyl chlorophyllide-sensitized autoxidation of allyl thiourea (Gaffron 



