548 PHOTOCHEMISTRY OF PIGMENTS IN VIVO CHAP. 19 



{HXChlZ}, which can be converted into {XChlHZ} by a monomolecular 

 back reaction, into the oxidized intermediate, {XChlZ}, which requires, 

 for the regeneration of the normal form, a bimolecular reaction with the 

 reductant A, and therefore has a comparatively long life. Scheme 19. Ill 

 could be simplified by assuming HZ = Chi (cf. pages 551 et seq.); but in 

 the present discussion we prefer to keep the question of the chemical 

 participation of chlorophyll in the primary photochemical process open. 



3. Problem of the Chemical Participation of Chlorophyll 

 in the Primary Process 



In chapter 18, we considered the energy transfer mechanisms (desig- 

 nated on page 514 as mechanisms of type a) as improbable (except for 

 the case of substrates whose absorption bands overlap with those of the 

 sensitizer — as in the case of the chlorophyll-sensitized reduction of azo 

 dyes, or of the carotenoid-sensitized fluorescence of chlorophyll), and 

 have analyzed mainly mechanisms of type jS, in which the sensitizer was 

 assumed to enter into reversible oxidation-reduction reactions. All 

 reductants and oxidants in table 19.1 are colorless, and therefore unlikely 

 to take over the excitation energy from chlorophyll in kinetic encounters. 



However, the improbability of energy transfer from chlorophyll to a 

 colorless substrate becomes less definite if we consider this substrate is a 

 part of the same complex. 



X 



It seems possible that in a complex, < Chi 



>, the excitation of 



HZj 



chlorophyll could cause a hydrogen (or electron) to be transferred from 

 HZ to X (as represented in scheme 19. Ill), leaving chlorophyll itself 

 unchanged. This process can be classified as an "energy transfer" as 

 far as the role of chlorophyll is concerned (even though the acceptor 

 uses this energy for a chemical transformation rather than for an elec- 

 tronic excitation). 



Although such a "physical" mechanism of sensitization by chlorophyll 

 in vivo cannot be entirely excluded, we are inclined to think that, in 

 the living cell, too, mechanisms of type /3, which involve a "chemical" 

 participation of chlorophyll in the primary reaction, are more probable. 

 However, we must admit that no direct proof of this hypothesis has as yet 

 been secured, even though the alternative, "physical sensitizer" or 

 "chemical photocatalyst," has been argued back and forth in the litera- 

 ture on photosynthesis, for, now, well over fifty years. 



The two opposing points of view were first formulated by Timiriazev 

 in 1885 and Reinke in 1886. Timiriazev (1885, 1904) suggested that 



