OXIDATION AND REDUCTION OF CHLOROPHYLL 461 



phyll in air. They suggested that two atoms of hydrogen are lost in 

 this process (with the formation of one mole of hydrogen peroxide). 

 As mentioned above, allomerization was later attributed (by Stoll and 

 Fischer) to the oxidation of the "lone" hydrogen atom in position 10. 

 To explain how a molecule of oxygen can be consumed by the oxidation 

 of. one atom of hydrogen, Fischer suggested the formation of a "chloro- 

 phyll peroxide": 



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(16.8) — C(10)— H + O2 ^— C(10)— O— OH 



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He found that allomerization can be achieved also by means of quinone 

 (instead of air), and interpreted this as a result of the formation of a 

 hydroquinone ether (instead of peroxide) : 



(16.9a) — C(10)— H + Q=<^ ^=0 ^ — C(10)— O— /~\— OH 



and subsequent alcoholysis: 



(16.9b) — C— O^^^ ^— OH + CH3OH > 



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— C— O— CHs + HO^^ ^— OH 



The final products are hydroquinone and a compound with a CH3O — 

 group in position 10 (10-methoxychlorophyll). 



The chlorophyll peroxide assumed in (16.8), has not been isolated in 

 substance; but the autoxidation of the carbon atom in position 10 is 

 proved by the fact that hydriodic acid converts allomerized chlorophyll 

 into a porphyrin (so-called pheoporphyrin a?) which contains an hydroxyl 

 group in position 10. Since no allomerization occurs in the absence of 

 alcohol (see Table 16.11), while even 3% alcohol in pyridine is sufficient 

 to bring it about, it is possible that the chlorophyll peroxide postulated 

 in (16.8) is only an unstable intermediate (analogous to the hydroquinone 

 ether in 16.9a), and that reaction (16.8) is completed by the transfer of 

 oxygen to alcohol: 



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(16.10) — C— O— OH + CH3OH >— C— OH + CHj— O— OH 



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leading to the formation of methyl hydroperoxide, and of a 10-hydroxy- 

 chlorophyll. 



In addition to the solvent efifect, table 16.11 also shows the importance 

 of magnesium for the oxygen absorption by chlorophyll and its deriva- 

 tives. While elimination of phytol has no influence on allomerization, 

 the latter is prevented by the elimination of magnesium. 



