ASSOCIATION OF CHLOROPHYLL WITH THE SUBSTRATE 545 



In discussing the sensitization phenomena in solution, we paid more 

 attention to mechanisms of type A — which can be called "sensitizations 

 by kinetic encounters" — than to mechanisms of type B — energy transfers 

 or oxidation-reduction reactions within a complex. We decided that a 

 complex formation of dissolved chlorophyll with oxygen was improbable 

 (page 492) ; its association with the organic substrates of the sensitized 

 reactions appeared more probable, but in no case was it proved by direct 

 evidence. 



The pigments in the living cell certainly are more or less rigidly 

 bound in a structure which includes proteins, lipides, and carotenoids 

 (cf. Chapter 14, part C). Franck and Herzfeld (1941) have postulated 

 that the carbon dioxide-acceptor complex, {CO2}, and its intermediate 

 reduction products ({HCO2}, etc.) also are associated with chlorophyll 

 (cf. Scheme 7.VA). However, the extraction of the carbon dioxide 

 acceptor from the cells by water and its possible location outside the 

 chloroplasts (cf. Chapter 8, page 204), make a stable association between 

 this component and chlorophyll improbable. On the other hand, 

 chlorophyll may well be associated with the intermediary catalysts, X 

 or Y, which first undergo a photochemical hydrogenation in photosyn- 

 thesis and later bring about the reduction of the complex, {CO2}, by 

 thermal encounters. The "substitute oxidants" (O2, HNO3) also are un- 

 likely to be directly associated with chlorophyll, but may replace carbon 

 dioxide (or the complex {CO2}) in kinetic encounters with the reduced 

 intermediate, HX. 



As to the primary reductant HZ, it, too, is likely to be associated 

 with chlorophyll (or may even be identical with it, cf. pages 551 et seq.). 



If we assume that both molecules which take part in the primary 

 photochemical reactions sensitized by chlorophyll (e. g., X and HZ), are 

 permanently associated with the pigment, the question of the "long-hved 

 activated state," which caused complications in the treatment of chloro- 

 phyll sensitization in vitro, appears in a new light. No " tautomeriza- 

 tion" of chlorophyll seems feasible under these conditions; while "re- 

 versible reaction with the solvent" (which was considered on pages 484 

 and 491 as an alternative mechanism of long-lived activation) is replaced 

 under these conditions by a "reversible reaction with the associated oxi- 

 dants and reductants." 



The weakness of the fluorescence of chlorophyll in vivo can be con- 

 sidered as a sign of the rapidity of the primary reaction, which leaves 

 only a small chance for the re-emission of light. Using a general formu- 

 lation which does not prejudice a possible identification of chlorophyll 

 with one of the primary reaction components, X or Z, we may write the 

 following two equations for the alternative: fluorescence or primary 

 photochemical reaction in a complex. 



