CHLOROPHYLL AND CARBON DIOXIDE 



453 



Table 16.1 

 Reversible Carbon Dioxide Sorption at 0° C. 



(1 ATM. PARTUL PRESSURE) 



The existence of a certain affinity of chlorophyll for carbon dioxide is 

 thus established beyond question. This affinity may be "chemical" or 

 ''physical." Willstatter and Stoll (1918) suggested that the reversible 

 binding of carbonic acid constitutes the first step in the conversion of 

 chlorophjdl into pheophytin: 



H 



(16.3a) 



(16.3b) 



where Ph stands for the phytin radical, PhMg for chlorophyll, PhH2 for 

 pheophytin, and PhHMgHCOs for an intermediary "chlorophyll bi- 

 carbonate." Reaction (16.3) can be written as follows to show its ionic 

 mechanism : 



(16.4a) 

 (16.4b) 

 (16.4c) 



H 



I 

 > Ph— Mg+ 



H 



I I 



Ph— Mg+ + HCO3- > Ph— MgHCOa 



H H 



PhMg + H+ 

 H 



Ph— Mg+ + H+ 



-)■ Ph— H + Mg++ 



Willstatter and Stoll saw a confirmation of absorption mechanism 

 (16.3a) or (16.4b) in the fact that chlorophyll does not bind carbon 

 dioxide in organic solutions (which contain no carbonic acid molecules or 

 bicarbonate ions). However, if we assume the identity of the carbon 

 dioxide absorption by solid chlorophyll and by colloidal chlorophyll 

 solutions, the ionic mechanism is ruled out, and the lack of sorptive 

 power of chlorophyll solutions must be explained in a different way, for 

 example, by the saturation of the sorption centers by solvent molecules. 



