Infrared Spectroscopy of Chlorophyll and 



Derivatives 



A. STANLEY HOLT,* Photosynthesis Laboratory, Department oj Botany, 

 University of Illinois, Urbana, Illinois 



Several reversible chemical and photochemical reactions of chloro- 

 phyll in vitro are known (1-3), but little is known about the groups in 

 the molecule which are involved in these reactions. The study sum- 

 marized below, and presented in detail elsewhere (4), was undertaken 

 in the hope that infrared absorption data may give specific answers. 

 Various derivatives of chlorophyll and ethyl chlorophyllide have been 

 prepared to permit vnieciuivocal assignment of the absorption bands. 

 Of the atomic groups present in the chlorophyll molecule, only the 

 C=0, C=C, — H, N — H, and C — H groups gave infrared absorption 

 bands which could be assigned with reasonable certainty. These bands 

 lie between 3600 and 1600 cm.~'; the spectrum below 1600 cm.~^ 

 proved too complex for definite interpretation. 



Table I and Fig. 1 summarize the data. Of the three C=0 groups 

 present in pheophytin a and ethyl pheophorbide a, the ester groups 

 at Ct and Cio absorb near 1740 cm.-^, which is the usual frequency 

 found for the C=0 group in an ester (5). The keto group of the cyclo- 

 pentanone ring absorbs at 1697 cm.~^ (in CHCls-solution) or 1706 

 cm.~i (in CCU-solution). This agrees with the assignment given to 

 this band in the earlier work of Weigl and Livingston (6). The decline of 

 the frequency from 1740 cm.~\ the absorption frequency of the keto 

 group of cyclopentanone itself, to about 1700 cm.~^ must be due to 

 the attachment of the isocyclic ring to the aromatic nucleus. That the 

 1697 cm.~i band is in fact due to the keto group of the cyclopentanone 

 ring is indicated by its absence from the spectrum of the oxime of 

 ethyl pheophorbide a, and from those of the 2,4-dinitrophenylhydra- 

 zine derivatives of pheophorbide a and pyropheophorbide a. The C=0 

 in the carboxyl group of the propionic side chain in position 7, present 

 in the two latter compounds, absorbs at 1705 to 1706 cm."~^, and thus 



* Present address: Division of Applied Biology, National Research Labora- 

 tories, Ottawa, Canada, 



37 



