1802 



SPECTROSCOPY AND FLUORESCENCE OF PIGMENTS CHAP. 37C 



7000 6000 t 



ioocA 



Wavelength 



Fig. 37C.8. Changes with tem- 

 perature of the absorption spectrum of 

 the mixture of isomers of chlorophyll h 

 in solution (Freed and Sancier 1951). 



Wovelength 



Fig. 37C.7. Visible absorption spec- 

 tra of solutions of chlorophyll a at 230° 

 K (dashed line) and at 75° K (solid 

 line) (Freed and Sancier 1951). Con- 

 centrations are different at the two 

 temperatures. 



the replacement of "room temperature bands" by "low temperature 

 bands," generally situated at somewhat longer wave lengths (c/. par- 

 ticularly fig. 37C.8). 



The changes shown in figs. 37C.7 and 8 are completely reversible. 

 The same spectra could be obtained by approaching a certain temperature 

 from below or from above; they are therefore to be considered as belonging 

 to equilibrium mixtures of different solvates, formed practically \\dthout 

 any activation energy. The chlorophylls a and h are half-converted into 

 their low-temperature, solvated forms at —93° C, chlorophyll h' , at 

 — 43°C. Linschitz and co-workers (1952) noted that spectral changes 

 similar to those described by Freed and Sancier for the chlorophylls 

 occurred also upon cooling of ethyl chlorophyllide solutions (a or h) in 

 EPA (ether -\- pentane + ethanol, 8:5:3) to -193° C. The rigidity of 

 the solvent over a part of this temperature range did not interfere with 

 the completion of the changes. As noted also by Freed and Sancier, 

 allomerization does not prevent the transformation, which thus cannot 

 depend on the keto-enol isomerism in the cyclopentanon ring. 



Strain (1952) suggested that the effects observed by Freed and Sancier 

 could be due to the formation of colloidal (amorphous or crystalline) 

 particles. Freed and Sancier (1952) argued against this interpretation, 

 pointing out that what they had observed was substitution of new bands 

 for the original ones, and not a gradual shift of the latter. However, we 



