544 PHOTOCHEMISTRY OF PIGMENTS IN VIVO CHAP. 19 



As discussed in chapter 7, the primary process in photosynthesis may 

 involve one of the ultimate reaction components, fCOa} or {H2O}, or 

 two intermediates, e. g., HZ and X, as in equation (7.10a). If the first 

 assumption is correct, the substitute reductants (or oxidants) may 

 interfere after the first photochemical reduction of {CO2} (or the first 

 photochemical oxidation of {H2O}), but before the conversion of the 

 products of these photochemical reactions into the final products, {CH2O} 

 or O2 (e. g., the first oxidation product, {OH}, may react with the sub- 

 stitute reductants — H2, H2S, etc. — before being converted into free 

 oxygen; cf. Gaffron 1944). The mechanism of action of substitute 

 reductants and oxidants appears even simpler if the primary process is 

 of the type (7.10a), that is, if it produces an oxidized intermediate 

 catalyst, Z, and a reduced intermediate catalyst, HX. In this case, we 

 have merely to assume that these intermediates can, under suitable 

 conditions, react further with O2 or HNO3 instead of CO2, and with H2, 

 H2R, or H2S, instead of H2O, respectively. 



CO2 

 HNO3 <- 



CeHaCHO 

 Scheme 19.1. — Chlorophyll-sensitized reactions in vivo. 



Arrows in scheme 19.1 represent hydrogen transfer. This scheme is 

 closely related to schemes 6. Ill and 9.V. 



In the latter, an intermediate, Y, was assumed between Z and X, and X was identi- 

 fied with the hydrogen acceptor in the hydrogenase system, Ah; this led to a closed 

 "hydrogen cycle" and thus permitted an explanation of the photochemical liberation 

 of hydrogen in adapted algae, and of the coupling of the oxyhydrogen reaction with 

 the reduction of carbon dioxide. 



2. Association of Chlorophyll with the Sensitization Substrates; 

 Fluorescence and Sensitization Yields in vitro and in vivo 



We recall the eight possibilities of sensitization, derived in chapter 18 

 from the three alternatives: free sensitizer, A, or sensitizer associated 

 with the substrate, B; physical energy transfer, a, or participation of the 

 sensitizer in the chemical reaction, /3; and primary interaction with the 

 oxidant, 1, or with the reductant, 2. We also recall the complications 

 arising from a possible preliminary transformation of the sensitizer into 

 a long-lived active form (by tautomerization, dismutation, or a reversible 

 reaction with the solvent). The same possibilities must also be taken 

 into account in the analysis of the primary process of sensitization by 

 chlorophyll in vivo. 



