454 



CHLOROPHYLL 



CHAP. 16 



+ C0, 



Chapter 8 described several chemical mechanisms by which chlorophyll could 

 conceivably bind carbon dioxide in a nonionic reaction. One of them is carboxylation — 

 e. g., entrance of the — COO — group between the carbon atom in position 10 and the 



"labile" hydrogen attached to it (equa- 

 tion 16.5). However, no such carboxy- 

 lation has been observed so far. To 

 the contrary, many chlorophyll deriva- 

 tives have a tendency for decarboxy- 

 lation: for instance, pheophorbide a can 

 lose the — COOCHj group in position 

 10 by prolonged standing in 20% hy- 

 drochloric acid. It was mentioned on 

 pp. 91 et seq. that Baur, in one of his hy- 

 potheses concerning the chemical mechanism of photosynthesis, suggested that the 

 immediate substrate of photoreduction is a carboxyl group in chlorophyll, which is elimi- 

 nated after reduction and restored by means of a new molecule of carbon dioxide. How- 

 ever, no case of reversible decarboxylation is as yet known in chlorophyll chemistry. 



In addition to reversible carboxylation, a chemical binding of nonionized carbon 

 dioxide could conceivably be brought about by a reaction of the carbamination type. 

 Chlorophyll has no nitrogen-hydrogen groups; but it was pointed out on page 183 that, 

 in analogy to carbon-metal bonds, nitrogen-metal bonds may have a higher affinity 

 for carbon dioxide than have the nitrogen-hydrogen bonds. We may thus think of 

 equihbria of the following type: 



O 



N 



(16.6a) 



y 



R 



V 



N 



Mg + CO2 



O 



y 



N C 



R 



V 



o 



N— Mg 



(16.6b) 



R 



/■ 



N C 



\ 



O + CO2 ^ 



R 



Z' 



o 



II 



N— C— O 



N— Mg 



\ 



Mg 



N— C— O 

 O 



Reaction (16.6a, b) could explain the absorption of two molecules of carbon dioxide by 

 one molecule of chlorophyll (as extrapolated on page 452). 



In the case of pheophytin, the absorption of carbon dioxide could be attributed, 

 by analogy with (16.6), to a carbamination of the imino groups. The capacity of 

 pheophytin for carbon dioxide absorption is smaller than that of ethyl chlorophyllide, 

 but so is its capacity for hydration. The lower hygroscopicity of pheophj^in was 

 attributed by Hanson (page 451) to the influence of magnesium on the tendency of the 

 cyclopentanone ring for enohzation. One could ask whether a similar hypothesis could 

 be applied also to the absorption of carbon dioxide — thus locating this absorption in 

 the cyclopentanone ring. (The hypothesis of a carboxylation in position 10 would fit 

 into this picture.) However, it seems more probable that the two species, CO2 and H2O, 

 are attracted to different regions of the chlorophyll molecule. Rabinowitch has observed 

 that the capacity for binding carbon dioxide does not depend on whether the chloro- 

 phylhde molecules contain water or have been desiccated in high vacuum. The fact 

 that carbon dioxide is bound by colloidal chlorophyll under water also indicates that 



