REVERSIBLE PHOTOCHEMICAL REACTIONS 493 



(a) the primary reaction in all cases (phase test, reaction with Fe+++ 

 and reversible bleaching) is tautomerization (e. g., enolization, as assumed 

 in the Fischer-Stoll theory) ; 



(6) the "colorless" (yellow or brown) phase is, however, not the enol 

 itself, but either a product of its dismutation (by reaction between the 

 tautomer and ordinary chlorophyll, as assumed by Livingston and 

 Franck) or the product of a reversible reaction of the tautomer with the 

 solvent; the transformation of the enolized phorbin into a chlorin (which 

 occurs under the influence of alcoholic alkali) removes at the same time 

 the decolorized product which is in equilibrium with the enol, and thus 

 causes the termination of the brown phase; 



(c) tautomerization (enolization) is possible also in the allomerized 

 state, but its velocit}^ in this case is so small (as compared with the 

 velocity of transformation of the enol into chlorin) that no "brown 

 phase" can be observed; in the reaction with ferric chloride (as well as 

 in the photochemical tautomerization), the decolorized stage is ob- 

 servable, even with allomerized material, because the conversion of the 

 enol into chlorin either does not occur at all, or is much slower than in 

 the alkaline medium of the phase test. 



This is merely a suggestion; it may well turn out that the similarity 

 between the phase test and the reversible decolorization of chlorophyll 

 in light is fortuitous; but this question is certainly worth closer study. 



It was stated on page 488 that the enhancing effect of formic acid 

 on the reversible bleaching of chlorophyll is different from the influence 

 of other acids, and must therefore be attributed to a specific reaction 

 (e. g., a reversible oxidation of chlorophyll by formic acid). However, a 

 much weaker reversible bleaching can be observed with other acids as 

 well. This phenomenon can be attributed to the existence of a reversible 

 and photosensitive first stage in the conversion of chlorophyll into 

 pheophytin. 



That "pheophytinization" can be accelerated by light was first noticed by Jorgensen 

 and Kidd (1916) when they observed the fading of chlorophyll solutions in an atmos- 

 phere of carbon dioxide. Closer examination (Rabinowitch, unpublished data) showed 

 that the rate of chlorophyll conversion into pheophytin is affected by light only if the 

 concentration of hydrogen ions is low. At pH 3, the rate of weakening of the red 

 absorption band (which can be used as a measure of this transformation, cf. page 467) 

 is markedly accelerated by illumination. At pH > 3, the transformation becomes 

 partly reversible, that is, the red band is restored to a certain extent in the dark. This 

 indicates that the reaction occurs in two stages {cf. page 467), and that the first, reversible 

 step is accelerated by light. 



dark tt tt 



and Y I 



light 1 dark 1 



(18.18) PhMg + 2 H+ ;f— ^ Ph— Mg+ + H+ > Ph— H + Mg++ 



dark 



Chlorophyll Pheophytin 



