METAL COMPLEXES AS REDUCTION INTERMEDIATES 239 



and "hydrogen chemosynthesis." In addition, however, they can also 

 use molecular hydrogen (or organic hydrogen donors) for photosynthesis 

 in the absence of oxygen. They must thus be capable of substituting 

 hydrogen for the intermediary reductant, H2Y, and oxygen for the inter- 

 mediary oxidant, Z, as well as using 



either hght energy or combustion h^y < ^'"' Q 2hz ^ ^ h^o 



energy for the reduction of car- ^ (^P y. ^^ °' 



bon dioxide. In other words, their ^\»(T) 



properties call for a combination (Hcoa) ^ (5^Jj^l(5^ ^Eojj^^. 



of schemes 6.III, 9.III and 9.IVB. {co^ ' x " e^O;, 



This synthesis is attempted in scheme I 



9.V. The intermediary reductant, €> 



H2X, at which the "energy dismuta- e„"" 



tion" was assumed to take place in g^^^^^ 9.V.-Metabolism of adapt- 



schemes 9. Ill and 9. IV, is identified ed algae. Arrows represent hydrogen 

 in scheme 9.V with the intermediary transfer from one oxidation-reduction 

 hydrogen acceptor HzAh of the hy- system to another, converting the oxi- 



droQ-pnasp svstpm in scheme 6 III ^'^^'^ ^°™ °^ *^® ^^**^'' ^«"o™) ^"^^ 



drogenase system n scheme b.iii ^^^ ^^^^^^^ ^^^^ ^^^p^_ 5,1,2, {4,3}- 



To avoid makmg scheme 9.V photosynthesis, 6,3,1,2, {4,3} -photore- 

 too complicated, we have adopted duction, 6 {4,8}— chemosynthesis. 

 the condensed method of presenta- 

 tion used before in scheme 6. Ill, that is, we have written out the oxida- 

 tion-redaction systems participating in the metabolism of adapted algae, 

 and indicated by numbered arrows the hydrogen transfers occurring be- 

 tween them. One is compelled to omit in this presentation all steps 

 (e. g., dismutations) which are not intermolecular oxidation-reductions. 



The three ways in which the carbon dioxide reduction can be brought 

 about by Scenedesmus are explained in the legend. It is essential that 

 these organisms can substitute, upon the activation of the hydrogenase 

 and oxidase by fermentation, the reduced hydrogenase, H2EH, for the 

 primar}^ photochemical reduction product, H2Y as a hydrogen donor, 

 and the oxidized oxidase, E'oOz, for the primary photochemical oxidation 

 product, Z, as a hydrogen acceptor, and that these two substitutions are 

 independent of each other. 



8. Metal Complexes as Reduction Intermediates 



It was stated on page 222 that iron complexes are unlikely to have 

 potentials positive enough to play the part assigned above to the radical, 

 HY (i. e., to reduce a carboxyl group). However, iron (or other metal) 

 complexes may conceivably play the part of the first, comparatively 

 stable reduction product, HX. Speculations in this direction are 

 encouraged by the conclusion of Hill (1939) and Hill and Scarisbrick 

 (1941) (c/. Chapter 4, page 63) that ferric salts of organic acids can 



