466 CHLOROPHYLL CHAP. 16 



with ferric chloride, these conditions may be more favorable, allowing 

 one not only to observe the formation of the brown phase, but also to 

 reverse the reaction before the beginning of its second, irreversible stage. 



An additional argument in favor of the oxidation-reduction hypothesis 

 (16.11) is provided by the effect of light on the reaction between chloro- 

 phyll and ferric chloride, to be described on page 488. An oxidation 

 reaction of chlorophyll is likely to be accelerated by Hght absorption, 

 while an effect of illumination on the allomerization equilibrium (16.7) 

 is much less probable. Because of the importance a definite proof of 

 reversible oxidation of chlorophyll could have for the explanation of the 

 role of this pigment in photosynthesis, the reaction of chlorophyll with 

 ferric chloride certainly deserves further study. It must be proved, for 

 example, that this reaction leaves the magnesium in the chlorophyll 

 molecule unaffected, and does not cause a replacement of this metal by 

 iron or hydrogen. 



Altogether, the chlorophyll molecule appears to be a very delicate 

 system, with several groupings (vinyl group, CH — group in position 10, 

 the two extra hydrogen atoms in positions 7 and 8) easily susceptible to 

 oxidation and reduction. Interesting, and far from understood, internal 

 relations exist between these spacially separated groups; and a close 

 interaction is apparent also between them and the complexly bound 

 magnesium. 



For none of these groups has a capacity for reversible oxidation or 

 reduction been definitely proved by experiments in vitro. Such a proof 

 would be very valuable, since it would indicate that chlorophyll can serve 

 as an oxidation-reduction catalyst and would thus provide a firm basis 

 for speculations as to the part which it plays in photosynthesis 

 {cf. Chapter 19). A first confirmation that chlorophyll can act as such 

 a catalyst (even in the dark) can be seen in the observation of Rabino- 

 witch and Weiss (1937) that, when ethyl chlorophyllide is left standing 

 with ferric chloride in methanol, ferric chloride is reduced slowly in 

 quantities far in excess of that of the chlorophyll present in solution. 

 Probably chlorophyll catalyzes the oxidation of methanol by ferric chlo- 

 ride; but this explanation has yet to be confirmed by analysis. 



We have considered so far the oxidation-reduction systems formed by phorbin- 

 dihydrophorbin, dihydrophorbin-tetrahydrophorbin and vinyl phorbin-ethyl phorbin, 

 as well as the transformation of chlorophyll into the radical (monodehydrochlorophyll) 

 by the loss of the hydrogen atom in position 10. Another oxidation-reduction system 

 is represented by the pair chlorophyll a-chlorophyll b (since according to Fischer these 

 two pigments differ only by the replacement of the methyl side chain in modification a 

 by a methoxyl chain in modification b). However, the interconversion of these two 

 forms is difficult, if at all possible (cf. Stoll and Wiedemann 1932'). 



