494 PHOTOCHEMISTRY OF PIGMENTS IN VITRO CHAP. 18 



At the high hydrogen-ion concentrations, the photochemical contribution to reaction 

 (18.18) is neghgible; but at pH ^ 3, this contribution— which is independent of pH— 

 becomes commensurate with that of the thermal reaction (which is proportional to 

 [H+]). At very low hydrogen-ion concentrations, the first step of (18.18) is practically 

 purely photochemical. The velocity with which the intermediate product, HPhMg+, 

 is transformed back into chlorophyll is independent of [H+], whereas the velocity with 

 which this intermediate is converted (irreversibly) into pheophytin, is proportional to 

 [H+]. Thus, at very low hydrogen-ion concentrations, the two [H+]-independent 

 reactions— photochemical bleaching and monomolecular restoration of chlorophyll — 

 must predominate over the two [H+]-proportional, bimolecular thermal reactions, and a 

 reversible photochemical bleaching supplants the irreversible thermal pheophytinization. 



Experiments described in this section provide a first glimpse into the 

 mechanism of the primary photochemical process in chlorophyll solutions. 

 They show that "hidden" reversible changes— tautomerizations, oxida- 

 tion-reductions, perhaps also dismutations— occur in illuminated (even 

 if outwardly photostable) chlorophyll solutions. The last-discussed 

 reaction, the elimination of magnesium by acids, shows how, under 

 appropriate conditions (in the presence of "acceptors" which react with 

 the products of the primary reversible process), the reversible primary 

 reaction is replaced by an irreversible secondary transformation. Many 

 photochemical reactions of chlorophyll (and of other organic dyestuffs as 

 well), whether they affect the dyestuff itself or are merely sensitized by 

 it, are likely to originate in a similar way, that is, through irreversible 

 secondary transformations of the products of reversible primary processes. 



B. The Irreversible Photochemical Transformations 



OF Chlorophyll* 



1. Bleaching of Chlorophyll 



In all probability, the rate of any of the well-known chemical reactions 

 of chlorophyll could be accelerated by light under appropriate conditions 

 (we encountered an example above in the conversion of chlorophyll into 

 pheophytin). However, the only effect of light on chlorophyll which 

 has been repeatedly investigated was bleaching, a change which is prob- 

 ably caused by a complex series of transformations (often involving the 

 solvent, or impurities), rather than by a single, well-defined, chemical 

 reaction. 



Everyday observation teaches us that chlorophyll in the plants is 

 stable to air and light; but we also learn from experience that, when 

 illumination becomes too strong, or when photosynthesis is inhibited 

 (by drought, poisons, or carbon dioxide starvation), the plants become 

 yellow or colorless, that is, their pigments undergo a photochemical 

 decomposition. A similar effect can be produced much more rapidly 



* BibUography, page 523. 



