EFFECT OF PROTECTIVE SUBSTANCES ON BLEACHING 501 



that the latter radical reduces oChl to Chi; however, if the HO2 radicals dismute into 

 O2 and H2O, instead of reacting with oChl, the catalytic system "springs a leak," and 

 the net result is that represented in (18.23). 



Experiments of Knorr and Albers (1935) and Albers and Knorr (1935) on the 

 fading of chlorophyll fluorescence show that photodecomposition may also occur in an 

 atmosphere of pure nitrogen or carbon dioxide, sometimes even more rapidly than in 

 oxygen. As described on page 492, this phenomenon may be attributed to a primary 

 reversible oxidation-reduction reaction of activated chlorophyll with the solvent, which 

 is converted into irreversible decomposition by the absorption of violet or ultraviolet 

 light by oxidized chlorophyll (oChl). Whether this second irreversible step also is an 

 oxidation we do not know. It will be noted that Wurmser (1921), also working with 

 chlorophyll in acetone, found no bleaching in the absence of oxygen. Perhaps this 

 discrepancy may be attributed to the high intensity of short-wave radiations in the 

 experiments of Knorr and Albers (four 150-watt Pyrex mercury lamps in the axis of a 

 cyUndrical vessel containing the chlorophyll solution). 



3. Effect of Solvents and Protective Substances on Bleaching 



According to the preceding two sections, one of the ways in which 

 the solvent can affect the photoxidation of chlorophyll, is by direct 

 participation in the bleaching process, either in the reaction by which 

 the bleaching is produced or in the reaction by which the original color 

 is restored. In the presence of oxygen, this reaction cycle may leave, 

 as a net result, a sensitized oxidation of the solvent. Thus, the solvent 

 may "protect" the pigment from oxidation in light by a diversion of the 

 oxidative action: a sensitized photoxidation of the solvent can be substi- 

 tuted for the direct photoxidation of the pigment. A similar diversion 

 may be caused by dissolved "antioxygens," which are themselves 

 oxidized in light, but prevent the photoxidation of the sensitizer. How- 

 ever, not all "antioxygens" act in this way; others exercise a truly cata- 

 lytic influence, by accelerating the return of the oxidized pigment into 

 its normal state, as assumed on pages 490 et seq. in the interpretation of in- 

 hibitory effects of the systems O2-HO2 and Fe+++-Fe++ on the reversible 

 bleaching of chlorophyll. It is not always clear whether the protective 

 action of a given "antioxygen" is of a "diversionary" or "catalytic" 

 character, although effects of the second type should be distinguishable 

 by a greater permanency. 



Examples of substances which protect chlorophyll from photoxidation 

 by diverting the reaction to themselves are benzidine and the carotenoids 

 (Noack 1925, 1926; Aronoff and Mackinney 1943); also, hydroquinone, 

 p-benzohydroquinone, phenol, resorcinol, pyrogallol, diphenylamine, and 

 aniline (Weber 1936). Their action may be based either on a primary 

 interaction of excited chlorophyll with the protecting "acceptor," A: 



(18.24) Chi* + A > oA + rChl (compare 18.14) 



(18.25) rChl + O2 > Chi (compare 18.16) 



(18.26) A + O2 >oA 



