CHLOROPHYLL AND CARBON DIOXIDE 451 



of pH). Other arguments against magnesium as the hj^dration center 

 are the hygroscopicity of magnesium-free derivatives of chlorophyll and 

 the position of magnesium in the monolayer (marked by a cross in Fig. 

 49a) which should preclude its contact with water. Considering probable 

 negative hydration centers, Hanson decided in favor of the cyclopenta- 

 none ring V, with its enolizable carbonyl group. (The pH-dependence of 

 the surface area of allomerized chlorophyll — which, according to page 459, 

 has no capacity for enolization — is quite different from that of the 

 nonallomerized compound.) The hygroscopicity is somewhat larger in 

 chlorophyll b than in chlorophyll a, and may be ascribed to the effect 

 of a second carbonyl group. Hanson interpreted the disappearance of 

 the pH effects in magnesium-free compounds as an indirect influence of 

 magnesium on the properties of the cyclopentanone ring, an influence 

 which has been noticed on several occasions (see pages 454 and 462). 

 Hanson treated the whole theory of photosynthesis from the point of 

 view of the primary formation of a chlorophyll-water complex, stressing 

 that the pH of the chloroplasts is probably alkahne (pH = 7.0 to 7.5, 

 according to Menke), and that this is the region in which chlorophyll 

 hydration reaches its maximum. 



In chapter 7 we have considered, with van Niel (and others) the 

 decomposition or oxidation of water as a possible primary photochemical 

 process in photosynthesis; Hanson's concept of a chlorophyll-water 

 complex obviously fits into this theory. However, hygroscopicity is 

 such a common property of organic compounds that the hygroscopicity 

 of chlorophyll can scarcely be considered as an important argument in 

 favor of this specific chemical theory of photosynthesis. Furthermore, 

 if the hygroscopicity of chlorophyll in the cell is not larger than that of 

 sohd chlorophyll in vitro, less than one-half of all chlorophyll molecules 

 in the chloroplast are hydrated at room temperature — and if this is so, 

 how can light quanta absorbed by all chlorophyll molecules, be utilized 

 for photosynthesis? (This remark is not meant as an argument against 

 the primary water decomposition theory of photosynthesis, but merely as 

 an indication that observations of the hygroscopicit}^ of chlorophyll in 

 vitro cannot be quoted as arguments for this theory.) 



2. Chlorophyll and Carbon Dioxide 



In chapter 7 we have also considered, as an alternative to the hy- 

 pothesis of a primary photochemical oxidation of water, the hypothesis 

 of a primary photochemical reduction of carbon dioxide. The first 

 detailed theory of this type was suggested by Willstatter and Stoll 

 (1918), and was described on page 287. These authors thought that the 

 proof of a chemical association between chlorophyll and carbon dioxide 

 would be an important argument in favor of their theory. They found 



