MEASUREMENTS OF ABSORPTION SPECTRA OF PIGMENTS 



1811 



(e) Infrared Spectra of Chlorophtjll and Its Derivatives 



The early infrared absorption curves, reproduced in fig. 21.4, obtained 

 with solid deposits of unknown density, can be now replaced by better 

 data, obtained with solutions of known concentration by Weigl and 

 Livingston and by Holt and Jacobs. 



Weigl and Livingston (1953) published absorption curves for chloro- 

 phyll a, chlorophyll h, pheophytin a, bacteriochlorophyll, and allomerized 

 chlorophyll a. They gave a list of bands (a) common to all five compounds, 

 (b) common to several compounds and (c) unique to single compounds, 

 and suggested (in addition to several rocking and bending frequencies) 

 the following identifications of bond-stretching vibration frequencies: 



V (cm."') (in ecu or dry films) 



3400 (pheophytin a) 

 2862-2956 (all compounds) 



1740 (all compounds) 



1700 (all compounds) 



1660 (all phytol-containing compounds) 



1610 (all but bacteriochlorophyll) 



1380 (all phytol-containing compounds) 



No O — H band was noted in any of the five compounds, including chloro- 

 phyll a (where it could be produced by enolization), and allomerized 

 chlorophyll (where it would be present if oxidation in position 10 were 



I 1 



to lead to an HC(10)OH, rather than to an HC(10)OCH3 group). More 



i I 



remarkably, no "aldehyde" C=0 band could be noted in chlorophyll b. 



Holt and Jacobs (1954) obtained the infrared spectra of the same 



compounds, as well as of several other chlorophyll derivatives, dissolved 



in chloroform, carbon tetrachloride and pyridine. Some of the results 



are represented in fig. 37D.14. Most interesting are the changes in the 



C=0 and — H bands, indicating transformations in the cyclopentanone 



ring (enolization, chelation, allomerization), which characterize this ring 



as the most reactive center in the molecule. These transformations are 



sensitive to changes in solvent (for example, a basic solvent stabilizes 



the enol group, and prevents its chelation with the adjoining ester group); 



they are characteristically affected by the presence of the magnesium atom 



in the molecule (which favors enolization) : 



