452 



CHLOROPHYLL 



CHAP. 16 



2.0 



a 

 o 



^1.5 



o 



E 



» 

 "o 1.0 



e 



"Z 

 o 



jO 



I- 



o 

 o 



400 



800 1200 



P; mm. 



1600 



Fig. 50. — Sorption isotherms for 

 carbon dioxide in solid ethyl chloro- 

 phyllide (after Rabinowitch 1938). 



that molecular solutions of chlorophyll are not affected by carbon dioxide, 

 but that colloidal solutions in water show a twofold interaction with 

 carbonic acid: a reversible absorption and a superimposed irreversible 

 chemical reaction — the latter being the well-known conversion of chloro- 

 phyll into the magnesium-free pheo- 

 phytin (page 447), caused by all 

 acids. The reversible binding of 

 carbon dioxide is increased, and the 

 velocity of the irreversible transfor- 

 mation is decreased, when the solu- 

 tions are cooled to 0°. Under these 

 conditions, approximately 0.25 mole 

 of carbon dioxide is reversibly ab- 

 sorbed by one mole of chlorophyll, 

 from an atmosphere of pure carbon 

 dioxide. 



Rabinowitch (1938) observed the 

 sorption of carbon dioxide gas by 

 degassed, dry, crystallized ethyl chloro- 

 phyllide in vacuo. Figure 50 shows 

 the sorption isotherms at 0° and — 

 80° C. They are smooth curves; the 

 sorption is reversible and equilibrium is established in a few seconds. 

 The data can be represented by equation (16.2), in which >Sp is the sorp- 

 tion under pressure p and *So the extrapolated maximum sorption under 

 high pressure: 



(16.2) o-^^ = const. X p 



The (extrapolated) saturation So corresponds to the uptake of approxi- 

 mately two moles of carbon dioxide by one mole of chlorophyllide. 



When chlorophyll was used instead of the crystalline ethyl chloro- 

 phyllide, a much weaker and slower sorption of carbon dioxide was 

 found. This can be attributed to the waxy consistency of the material, 

 which causes many crystal channels to be blocked and makes many 

 sorption centers unavailable. Smith (1940) also found that only 0.08 

 mole of carbon dioxide is taken up by one mole of solid chlorophyll 

 under a pressure of one atmosphere; the sorption is proportional to the 

 pressure, which shows that saturation is far away. 



Pheophytin absorbs about 50% less carbon dioxide than ethyl chloro- 

 phyllide, as shown by table 16.1, which summarizes the results of the 

 three investigations mentioned above. The table shows that the 

 reversible binding of carbon dioxide by colloidal chlorophyll is probably 

 identical in nature with the sorption of this gas by solid chlorophyllide. 



