OXIDATION AND REDUCTION OF CHLOROPHYLL 457 



satisfactorily solved. Many organic dyestuffs — e. g., indigo, methylene 

 blue, thionine — are among the best-known examples of reversible oxi- 

 dation-reduction systems. The colored form, which usually possesses a 

 "quinonoid" structure, is an oxidant, while the reductant is colorless or 

 only weakly colored (e. g., indigo white, leuco methylene blue, etc.). 

 Seldom is the reductant the colored form and the oxidant the colorless 

 form (as in Michaelis' "viologens"). 



It has often been suggested that chlorophyll, too, may possess a 

 colorless or weakly colored reduced form, a "leuco chlorophyll," from 

 which it can be re-formed by oxidation. It has also been postulated 

 that chlorophyll is formed in the plants by oxidation (or photoxidation) 

 of a colorless "precursor," and many investigators have attempted to 

 isolate the latter from seeds or etiolated seedlings (cf. Chapter 15, 

 page 404). However, the only compound which has been isolated in 

 sufficient quantity to make its analysis possible — the "protochlorophyll" 

 from pumpkin seeds — has proved to be an oxidation product, rather than 

 a reduction product of chloroph3dl (page 445). This leaves us with 

 only the "etiolin," "chlorophyllogen," "leucophyll," and other hypo- 

 thetical chlorophjdl "precursors," which have never been isolated and 

 studied in the pure form, as alleged reduced forms of chlorophyll in nature. 



Attempts to prepare a "leuco chlorophyll" in vitro also have not 

 been completely successful. Many porphyrins can be reduced to 

 colorless "porphyrinogens" with the uptake of two, four, or six hydrogen 

 atoms by the conjugated porphin system, and restored by oxidation, 

 e. g., by atmospheric oxygen. A similar reversible reduction of chloro- 

 phyll and its derivatives has been described by Timiriazev (1903) and 

 Kuhn and Winterstein (1933). Timiriazev reduced chlorophyll in 

 pyridine solution by means of zinc and organic acids, and obtained a 

 colorless solution which slowly became green again upon exposure to air. 

 Kuhn and Winterstein found that the product obtained from chlorophyll 

 by reduction and reoxidation gave a positive "phase test" {cf. below), 

 and had the unchanged elementary composition and absorption spectrum 

 of chlorophyll. However, Albers, Knorr, and Rothemund (1935) found 

 that its fluorescence spectrum is different from that of the original 

 pigment, and Rothemund (1935) found differences in the absorption 

 spectra as well. The "cleavage test" (disruption of the cyclopentanone 

 ring by alkali) gave a product different from "chlorin e" (which is the 

 product of cleavage of intact chlorophyll a). The irreversibility of chloro- 

 phyll reduction by zinc and acids was confirmed by Godnev (1939). 



What changes were caused in these experiments by reduction and re- 

 oxidation is not known. Conant and Hyde (1931) found porphyrins 

 among products of reduction by hydrosulfite or Pd + H2 and oxidation 

 by air. It seems likely that hydrogenation of the vinyl group in posi- 



