174 



Simon Freed 



is apparently a single species at room temperature. The figure reproduces 

 the absorption spectra of chlorophyll b in ethyl ether and methanol (2). Our 

 first inclination is to ascribe the differences in the spectra to the perturbations 

 produced on the structure of the chlorophyll molecules by the two types of 

 solvent molecules. Figure 3b is a magnification of the Soret band in the blue 



400 



500 600 700 



WAVELENGTH IN mjJi 



Fig. 3a. Absorption spectra of chloro- 

 phyll b at room temperature. The thin- 

 lined curve with maxima at shorter wave- 

 lengths represents a solution of chloro- 

 phyll in ethyl ether; the thick-lined curve 

 gives the spectrum when the solvent is 

 methanol. 



4100 



5000A 



Wavelength 



Fig. 3b. The dependence of the absorp- 

 tion spectra of chlorophyll b on tem.pera- 

 ture. Only the Soret band in the blue is 

 shown. Enlarged scale of wave-lengths. 

 At 300"K the solvent is 20% propyl 

 ether, 80% hexane. At the lower tem- 

 perature it is 20% propyl ether, 40% 

 propane, and 40 % propene. The hexane 

 was substituted at 300°K for the hydro- 

 carbons propane-propene since they are 

 normally gases at room temperature. 



region and shows that a solution of chlorophyll b in ether is really a mixture 

 of two species (etherates) in equilibrium with each other in roughly equal 

 amounts and clearly resolved at 180°K. A study of the dependence on tempera- 

 ture of the absorption spectrum of chlorophyll b in methanol reveals that in 

 this solvent, chlorophyll b also exists as a mixture of solvates which are about 

 equal in concentration at room temperature and together they yield the com- 

 posite spectrum. However the spectrum of each alcoholate differs very little 

 in shape from that of each etherate. Fig. 4 illustrates a form stable at a lower 



