152 PRIMARY PHOTOCHEMICAL PROCESS CHAP. 7 



in Chapter 4, page 77), despite the fact that the oxidation potential of 

 thionine is several tenths of a volt more positive than that of ferric iron. 

 This is the best-known photochemical reaction in vitro which is funda- 

 mentally similar to the postulated primary photochemical process in 

 photosynthesis — similar in that it, too, is an oxidation-reduction which, 

 with the help of light, proceeds against a considerable gradient of chemi- 

 cal potential. 



The unique characteristic of photosynthesis probably is not the 

 photochemical transfer of hydrogen from water to an oxidant much 

 weaker than oxygen, brought about by visible light — this may be a 

 common occurrence even in nonbiological systems — but the avoidance of 

 back reactions. The latter prevent a direct demonstration of primary 

 photochemical water oxidation in many simple inorganic systems, and 

 make even the photoxidation of ferrous ions by thionine a transitory 

 phenomenon. 



The secret of how back reactions are prevented in photosynthesis 

 must be sought in the heterogeneous structure of the photosynthetic appa- 

 ratus and the consequent topochemical mechanism of the whole process — 

 meaning by this term a chemical mechanism in which the participants 

 follow prescribed paths on the catalytic surfaces, without appearing as 

 free intermediates between the successive steps of their catalytic trans- 

 formations. The preservation of at least a part of this structure in 

 isolated chloroplasts may account for the success of Hill's experiments 

 on chloroplast-sensitized photoxidation of water by ferric oxalate. 



Theoretically, there is no reason why all electronic energy contained 

 in molecules excited by the absorption of light should not be available 

 for oxidation-reductions. A light-excited molecule is both an efficient 

 electron donor (that is, reductant) because it contains a "loose" electron; 

 and a potential electron acceptor (that is, oxidant) because (to use a 

 picture suggested by Weiss) it contains a "hole" in its usual complement 

 of electrons. 



All electronic excitation energy is "free energy" and thus available for chemical 

 reactions. Therefore, in a true thennodynamic equilibrium, hght-excited molecules can 

 be assigned oxidation-reduction potentials equal to those of the same molecules in the 

 normal state plus (or minus) their electronic excitation energy. Excitation by visible 

 light (X = 700-400 myu) should add (or subtract) from 1.7 to 3 volt to the oxidation- 

 reduction potential of the excited molecules, and thus make even the weakest oxidants 

 thermodynamically capable of oxidizing water, and even the weakest reductants able 

 to reducfe carbon dioxide. However, a photochemical reaction is practically never a 

 part of a true thermodynamic equilibrium (unless we consider systems at very high 

 temperatures, as, for example, the cosmic bodies). What is observed under ordinary 

 conditions is a progressive conversion of hght, partly into heat and partly into chemical 

 energy; the high theoretical oxidation or reduction potentials of the light-excited 

 molecules are of no practical avail if the conversion into heat occurs much more rapidly 



