OXIDATION AND REDUCTION OF CHLOROPHYLL 465 



Rabinowitch and Weiss suggested that the reaction of chlorophyll 

 with ferric ions is a reversible oxidation: 



(16.11) Chi + Fe+++ . oChl + Fe++ 



where oChl ("oxychlorophyll") denotes a yellow, oxidized form of 

 chlorophyll. This explanation is supported by the fact that re++ ions 

 can be identified in the reaction products by means of a spot test with 

 l,l'-bipyridyl (which forms a red complex with divalent iron). Re- 

 versible bleaching of chlorophyll can also be caused by other strongly 

 oxidizing ions, e. g., eerie or thallic ions (c/. Fig. 51b). 



The "oxychlorophyll," assumed to be present in chlorophyll solutions 

 decolorized by ferric ions, is unstable. Illumination by blue or violet 

 light causes a rapid irreversible transformation: the solution becomes 

 straw yellow and cannot be restored again to the green form. "Oxy- 

 chlorophyll" is also irreversibly affected by water. 



Additional proof seems to be needed to establish the correctness of 

 interpretation (16.11). Some observations speak against it. For ex- 

 ample, the green color can be restored not only by ferrous chloride, but 

 also by other methanol-soluble salts (e. g., CaCl2 or NaCl), if used in a 

 comparatively large quantity and (although much more slowly) by 

 standing (in this case, the red band never reaches its original intensity). 

 Outwardly, the reaction bears a striking resemblance to the phase test, 

 and one is therefore tempted to explain it in a similar way, that is, by 

 an enolization in position 9, induced by the highly charged ions Fe+++ 

 or Ce++++. The return of the green color upon standing could then be 

 explained in the same way as in the phase test, that is, by a disruption 

 of the carbocychc ring and formation of a green chlorin; while the 

 restoration of apparently intact chlorophyll by immediate addition of 

 ferrous chloride or other salts can be considered as a "salt effect," which 

 shifts the enohzation equilibrium. Rabinowitch and Weiss decided for 

 the oxidation-reduction hypothesis, because of the above-mentioned 

 analytical proof of the formation of ferrous ions, because of the small 

 concentration of ferrous chloride sufficient for the restoration of green 

 color, and most of all because chlorophyll which was completely allomer- 

 ized by standing in methanol solution still proved capable of reacting 

 with ferric chloride. Since the hydrogen atom in position 10 has sup- 

 posedly already been oxidized by allomerization, the oxidation by ferric 

 chloride can plausibly be attributed to nucleus IV; the disappearance of 

 the red absorption band fits well into this hypothesis. On the other hand, 

 we have stated that the brown phase may not be visible in the reaction 

 of allomerized chlorophyll with alkah merely because of unfavorable 

 kinetic constants of its formation and decomposition. In the reaction 



