456 CHLOROPHYLL CHAP. 16 



chlorophyll in green leaves. (According to page 194, this absorption is 

 caused by buffer equilibria with phosphates and bicarbonates.) The 

 second mechanism (revealed by experiments with radioactive C*02, c/. 

 page 201, and "pickup" phenomena, page 206) is based on the presence 

 of a carbon dioxide absorber, whose concentration is of the same order 

 of magnitude as that of chlorophyll, but whose affinity for carbon dioxide 

 is many times stronger. In short, while chlorophyll is present in quanti- 

 ties roughly equal to those of the "strong" carbon dioxide acceptor, its 

 affinity for carbon dioxide is not larger than that of the "weak" absorber; 

 consequently, it cannot contribute markedly to the uptake of carbon 

 dioxide either under low or under high pressures. 



One could suggest that chlorophyll in vivo has a stronger affinity for 

 carbon dioxide than chlorophyll in vitro, e. g., because of its association 

 with a protein. However, the capacity of Chlorella for carbon dioxide 

 fixation in the dark was found not to be affected by variations in chloro- 

 phyll content; and radioactive indicator experiments showed that the 

 carbon dioxide-acceptor complex can be extracted by hot water, while 

 chlorophyll remains in the cells (page 204) . If one adds that some experi- 

 ments point to the location of the carbon dioxide acceptor in the cyto- 

 plasm outside the chloroplasts (page 204), one can scarcely avoid the 

 conclusion that chlorophyll probably bears no relation at all to the pri- 

 mary fixation of carbon dioxide in photosynthesis, and that the approxi- 

 mate equality in concentration between the acceptor and chlorophyll is 

 fortuitous. 



Thus, contrary to the conclusions of Willstatter and Stoll, the results 

 of the study of the interaction of chlorophyll with carbon dioxide in vitro 

 discourage the development of chemical theories based on the combination 

 of chlorophyll with carbon dioxide, whether by physical sorption, 

 carboxylation, or "chlorophyll bicarbonate" formation. 



This conclusion does not affect the hypothesis, expressed by equations (16.3), that 

 the elimination of magnesium from chlorophyll by carbonic acid includes, as a pre- 

 liminary step, the formation of a chlorophyll magnesium bicarbonate (it only denies 

 the importance of such a compound for photosynthesis). On page 493, we will find some 

 photochemical evidence in favor of this two-step mechanism of "pheophytinization." 



3. Oxidation and Reduction of Chlorophyll 



We shall discuss here, not the irreversible oxidative decomposition of 

 chlorophyll, or the equally irreversible catalytic hydrogenation of its 

 double bond system, but only milder changes, which could conceivably 

 bear some relation to the reversible transformation of chlorophyll in 

 photosynthesis. 



Despite a considerable number of investigations dealing with the 

 oxidation and reduction of chlorophyll, this problem has not yet been 



