PHOTOCHEMISTRY OF CHLOROPLAST PREPARATIONS 1529 



by Friedel, and later described by Molisch and by Inman ; and the enhance- 

 ment and stabihzation of this phenomenon which Hill (1939, 1940) had 

 achieved by providing an oxidant, such as ferric oxalate. 



As anticipated in chapter 4, the study of the "Hill reaction" has proved 

 one of the most promising approaches to the analysis of the mechanism 

 of photosynthesis. 



Little doubt now remahis that the "Hill reaction" represents a part of 

 photosynthesis which can be reproduced with nonliving material — al- 

 though as yet only with complex colloidal systems obtained by mechanical 

 disintegration of cells. 



This part consists of photochemical oxidation of water leading to the 

 liberation of oxygen, undoubtedly with the participation of an enzymatic 

 system closely linked to, and surviving with, the photochemical apparatus. 

 The part lost in the preparation of the chloroplasts is that concerned with 

 the use of carbon dioxide as acceptor for the hydrogen taken away from 

 water. Only relatively strong oxidants, such as ferric salts or quinones, 

 can be used as hydrogen acceptors in the Hill reaction with a good quantum 

 yield. True, it has been found possible to couple this reaction, through 

 the intermediary of pyridine nucleotides, to enzymatic systems permitting 

 the reduction of pyruvate to lactate, or its reductive carboxylation to mal- 

 ate, or (with the help of ATP), the reduction of phosphogly eerie acid to 

 phosphoglyceraldehyde. 



However, so far, this nearest approximation to photosynthesis outside 

 the living cell could be achieved only with a very low quantum yield ; there- 

 fore it remains an open question whether this reaction in vitro represents 

 a significant approach to the reconstruction of the actual mechanism of 

 photosynthesis in vivo. 



The interpretation of the Hill reaction as oxidation of water is supported 

 by oxygen isotope tracer studies. 



In chapter 3 (Vol. I, page 54) we described tracer experiments with 

 heavy oxygen which directly demonstrated that all oxygen evolved in 

 normal photosynthesis originated in H2O (and none in the oxidant, CO2). 

 In the case of the Hill reaction, many oxidants — such as Fe[(CN)6]"'"' — 

 contain no oxygen at all; furthermore, the ratio of the amount of oxygen 

 liberated to the amount of oxidant reduced agrees with the assumption that 

 oxygen comes from the oxidation of water. An exception is chromate, 

 where oxygen could conceivably come from the anion, and where the 

 amount of liberated oxygen, as found in Holt and French's experiments, 

 was much smaller than stoichiometrically expected. Holt and French 

 (1948^) made a mass spectroscopic analysis of the oxygen evolved in the 

 Hill reaction from normal water and from water enriched in 0(18), using 



