458 CHLOROPHYLL CHAP. 16 



tion 2 is an important source of complications. This group is usually 

 the first to be attacked by reductants— e. g., the reduction of methyl or 

 ethyl chlorophyllide by palladium-hydrogen in dioxane can be carried 

 out so as to cause the uptake of only one molecule of hydrogen— and 

 this molecule goes to the vinyl group. The catalytic hydrogenation 

 product of chlorophyll a, whose fluorescence and absorption spectra were 

 studied by Knorr and Albers (1942), probably also is a meso derivative. 

 The reduction can be pushed further, until a colorless "leuco" compound 

 is obtained, but a subsequent reoxidation, while restoring the conjugated 

 double bond system (and thus the color) fails to restore the vinyl group. 

 The net result is the retention of two hydrogen atoms and the transfor- 

 mation of the original unsaturated compound into the corresponding 

 saturated meso derivative. This is probably what happened also in the 

 experiments of Timiriazev and Kuhn and Winterstein. 



The hydrogenation of the semi-isolated double bond in the nuclear 

 system, whose presence is shown by formulas 16. Ill, also could occur 

 before the attack on the conjugated double-bond system. (Stoll and 

 Wiedemann, 1932, had interpreted in this way the phenomena which 

 were later attributed to the hydrogenation of the vinyl group.) This 

 reaction would convert compounds of the chlorophyll class into the 

 corresponding compounds of the bacteriochlorophyll class; it could be 

 reversible, since bacteriochlorophyll derivatives are known to lose their 

 two excess hydrogen atoms easily. 



Besides its several reducible groups, chlorophyll and its derivatives 

 contain at least two potential centers of oxidation — the two "extra" 

 hydrogen atoms in positions 7 and 8, and the "lone" hydrogen atom in 

 position 10. In addition, the vinyl group may again cause compli- 

 cations, this time by its oxidation to a — CO— CH3 group (so-called 

 "0x0 reaction"). 



By an internal oxidation-reduction mechanism, the hydrogen atoms 

 m positions 5 and 6, can be lost even in reduction experiments. Fischer 

 and Bub (1937) found that, when pheophorbide a is reduced by palladium 

 and hydrogen in glacial acetic acid, the resulting leuco compound is 

 optically inactive, that is, it does not contain the hydrogen atoms which 

 made the carbon atoms 7 and 8 asymmetric. Upon reoxidation, these 

 hydrogen atoms do not re-appear in their original positions; since, at the 

 same time, two hydrogen atoms remain attached to the vinyl group 

 (as was described above) the net result of reduction and reoxidation is 

 an isomerization, the original chlorin or phorbin having been converted 

 into an isomeric mesoporphyrin. 



The treatment of chlorophyll derivatives with hydriodic acid usually 

 has the same isomerizing affect. One may imagine that, while hydrogen 

 iodide reduces the vinyl group, the liberated iodine oxidizes the hydrogen 



