Chemistry and Biochemistry at Low Temperatures 



175 



temperature reacting to produce reversibly a stable intermediate but at still 

 higher temperature ending in an irreversible reaction. 



The following specific observations may prove worthwhile in illustrating 

 what is probably a rather common phenomenon. Chlorophyll b dissolved in 



4000 5000 



WAVELENGTH % 



6000 



Fig. 4. Chlorophyll 6' in 15% mono-/-propyl amine in 1 : 1 propane-propene. 



To show the presence of the red-brown intermediate stable at 193''K which is in 



equilibrium with the original chlorophyll. At temperatures higher than about 



235''K, an irreversible reaction occurs. 



ether is deposited as a green powder by pumping off the ether at room tempera- 

 ture. When the temperature of the powder is reduced to that of dry ice (about 

 193°K) and propylamine is condensed upon it at this temperature, it dissolves 

 quickly, forming a red solution. Note in Fig. 4 the new absorption between 

 5000 A and 6000 A. A rise in temperature transforms the color into the green 

 of chlorophyll with its characteristic spectrum which reverts back reversibly 

 to the red substance when the temperature is reduced. However, if the tem- 

 perature is kept any length of time at about 235°K or higher, an irreversible 

 reaction sets in. For example, at room temperature the red color lasts only 

 a fraction of a second. This evanescent red color is produced in the well 

 known phase test for chlorophyll. 



Figure 5 represents a chemical reaction which appears rapid even between 

 167°K and 75°K. Chlorophyll h dissolved in di-/.so-propylamine is undergoing 



