PHOTOCHEMICAL OXIDATION OF WATER 69 



tions is not whether they are capable of complete photosynthesis, but 

 whether they too, retain some properties reminiscent of the part which 

 chlorophyll plays in photosynthesis. As shown in chapter 3, this part 

 is the utilization of light energy for hydrogen transfer against the gradient 

 of chemical potential. Chlorophyll may achieve this either by a purely 

 physical transfer of energy to a cellular oxidation-reduction system, or, 

 more probably, by direct chemical participation in such a system. 

 Consequently, what we ask is whether chlorophyll in vitro forms a 

 reversible oxidation-reduction system and, if it does, whether the oxi- 

 dizing capacity of its oxidized form, or the reducing capacity of its 

 reduced form (or both) are enhanced by the absorption of light. 



Indications that chlorophyll in vitro actually possesses the properties 

 of a light-activated oxidation-reduction catalyst, have been found by 

 Rabinowitch and Weiss (1937) in experiments which shall be discussed 

 in chapter 18. Some observations of Baur (1935), Baur and Fricker 

 (1937), and Baur, Gloor and Kiinzler (1928), which point in the same 

 direction, mil be described later in the present chapter (page 90). These 

 interesting, but as yet inconclusive results are the only indications 

 that chlorophyll outside the cell does retain certain of the properties 

 which make it "the most important single organic compound on earth" 

 as long as it is contained in living plant cells. 



B. The Photochemical Oxidation of Water * 



We now leave the living cell and the products obtained from it and 

 consider nonbiochemical systems whose behavior is of interest from the 

 point of view of artificial photosynthesis. 



The essence of photosynthesis is the reduction of the oxidant of an 

 oxidation-reduction system of a higher potential (carbon dioxide-carbo- 

 hydrate) by the reductant of a system of a much lower potential 

 (oxygen- water), with light supplying the necessary energy. The differ- 

 ence in total internal energy between the substrates and products of 

 photosynthesis is 112 kcal per gram atom of carbon; the difference in 

 free energy is a few calories larger {of. Table 3.V). Complete artificial 

 photosynthesis should bridge this whole gap at once. However, all 

 experiments which help to narrow it, may be considered as helpful 

 partial solutions. The bridging may begin at either end or in the middle. 

 It may include photochemical or nonphotochemical reactions likely to 

 bring the two reacting systems closer together. Nonphotochemical 

 reactions cannot contribute to the bridging of the energy gap; but they 

 can make the solution easier, by substituting catalytic reactions with 

 low activation barriers for reactions with the same net heat effect, but 

 with a larger energy of activation. 



* Bibliography, page 95. 



