450 CHLOROPHYLL CHAP. 16 



B. Chemical Properties* 



We know that chlorophyll sensitizes the reduction of carbon dioxide 

 by water. It is probable that, in carrying out this function, it enters 

 into reversible reactions with the substrates of photosynthesis (c/. 

 Chapter 19). Two types of such reactions can be envisaged — complex 

 formation, and oxidation-reduction. In the first case, chlorophyll would 

 serve as an "acceptor" for carbon dioxide {cf. Chapter 8) or water (c/. 

 Chapter 11), or both. In the second case, chlorophyll would play the 

 part of an oxidation-reduction catalyst {i. e., an acceptor for hydrogen 

 atoms or electrons). The hypothetical complex formation of chlorophyll 

 with carbon dioxide or water should be a "dark" reaction, while the 

 reversible oxidation-reduction of chlorophyll should be a photochemical 

 reaction, with light activating either the "forward" or the "back" 

 step, or both. 



In considering the chemical properties of chlorophyll in relation to 

 the role of this pigment in photosynthesis, we are thus interested, first 

 in the interaction of chlorophyll with water and carbon dioxide and 

 second in the capacity of chlorophyll for reversible oxidation-reduction. 



1. Chlorophyll and Water 



Willstatter and Stoll (1913) and Fischer and Stern have assumed 

 that both chlorophyll and the alkyl chlorophyllides contain one-half mole 

 of water per mole. Rabinowitch (1938) has noted, however, that ethyl 

 chlorophyllide (and, to a smaller extent, chlorophyll as well) has "zeolitic" 

 properties {i. e., it reversibly absorbs different gases, including water 

 vapor, the absorbed quality being a smooth function of temperature and 

 of the partial pressure of the gas). At room temperature and in contact 

 with saturated water vapor, ethyl chlorophyllide takes up about one-half 

 mole of water per mole; but this is probably much less than the saturation 

 value; the latter may correspond to one or even two moles per mole. 



Additional data about the affinity of chlorophyll for water have been 

 contributed by Hanson (1939). As mentioned above, he found, in 

 studying chlorophyll monolayers on water, that their surface requirement 

 increases with increasing alkalinity, and attributed this to a "swelling" 

 of the molecules by water. Progress in swelling which follows a decrease 

 in [H+] can be interpreted as a tendency of the film to lose hydrogen 

 ions in water and to gather water dipoles around the negative charges. 

 Hanson concluded that negative groups must be responsible for hydration ; 

 this ruled out magnesium as the hydration center (which seemed at 

 first to be the most likely assumption, especially since the surface re- 

 quirement of magnesium-free compounds was found to be independent 



* Bibliography, page 468. 



