552 PHOTOCHEMISTRY OF PIGMENTS IN VIVO CHAP. 19 



Theories belonging to all these groups have been suggested by different 

 authors. 



(a) Photochemical Oxidation, Nonphotochemical Reduction of Chlorophyll 



Weigert (1923) postulated that the primary photochemical process in photo- 

 synthesis is an electron transfer from chlorophyll to water, followed by the oxidation of 

 water by oxidized chlorophyll and reduction of carbon dioxide by reduced water: 



(19.10a) Chi* + H2O > Chl+ (= oChl) + H2O- 



(19.10b) Chl+ + OH- > Chi + OH ( > O2) 



(19.10c) H2O- + CO2 > H2O + CO2- ( > carbohydrates) 



(19.10) OH- + CO2 > CO2- + OH ( > carbohydrates + oxygen) 



Our four quanta schemes (7.7) and (7.10), and the eight quanta 

 schemes (7.14) and (9.10), can be classified as "primary chlorophyll 

 oxidation schemes" if one identifies the reductant, HZ (or HX), with 

 chlorophyll, and thus writes, for example, instead of (7.10a): 



(19.11) Chi + X "-^ oChl + HX or {ChlX} "—^ (oChlBX} 



where oChl may stand for a "monodehydrochlorophyll." 



One is free to speculate about the possible role in this one-step oxidation of the 

 "lone" hydrogen atom in position 10 (c/. Stoll 1932, 1936), or of the "extra" hydrogen 

 atoms in positions 7 and 8. 



We can use the elementary photochemical process (19.11) to rewrite 

 the various mechanisms of photosynthesis and photoxidation suggested 

 in chapters 7, 9, and 18 with chlorophyll as the primary photoreductant. 

 For example, the steps a, h, d and g in equation system (9.10), which 

 represented photosynthesis according to the theory of "energy dismu- 

 tation," become: 



(19.12a) 8 {ChlY} ^ {8 oChlHY} 



(19.12b) 8 {oChlHY} > 4 H^Y + 8 oChl 



(19.12d) 8 H2X + 4 oChl > 4 Chi + 4 HX 



(19.12g) 4 oChl + 4 H2O > 4 Chi + O2 + 2 H2O 



The mechanism of photoxidation in vivo, represented by scheme 9. Ill, 

 becomes, with HZ = Chi: 



(19.13a) {ChlX) "-^ {oChlHX} 



(19.13b) {oChlHX} + i O2 > (oChlX} + | H2O 



(19.13c) {oChlX}+A ^oA-hlChlX) 



(19.13) A + i O2 > oA + ^ H2O 



