40 A. s. HOLT 



adds some confusion to the interpretation of the spectra, since it is 

 superimposed on the absorption band of the keto group at C9. 



We assign the band at 1615 cm. ~^ to the C=C bonds in the aromatic 

 nucleus; this assignment is supported by the strong absorption shown 

 in the same region by the phenylhydrazine derivatives and by the 

 presence of a similar band in the spectrum of bacteriochlorophyll, 

 which contains no vinyl C^C group. 



The spectra of chlorophyll a and ethjd chlorophyllide a show 

 marked differences, in the region of the C^O bands, from those of 

 the magnesium-free derivatives. These depend on the choice of the 

 solvent. Spectra taken in nonpolar solvents (GCI4 or CS2) or with 

 crystals of chlorophyll a dispersed in Xujol (mineral oil), show an 

 extra band at 1640 to 1650 cm.~i. This band is interpreted by us as 

 indicating a strongly hydrogen-bonded (internally chelated) form of 

 the /3-keto ester in ring V. In polar solvents, e.g., diethyl ether or 

 chloroform, chlorophyll a does not show any such band. 



Spectra of pheophytin a in pyridine show a band characteristic of 

 a hydrogen-bonded hydroxyl group, between 3100 and 3700 cm.~^ We 

 believe this could be due to enolization of the /3-keto ester, in position 

 9, and hydrogen bonding between the pyridine nitrogen atom and the 

 enolic O — H group. 



The aldehyde group of chlorophyll b (or pheophytin h) absorbs at 

 1663 to 1665 cm.~' ; this band is absent from the spectrum of pheo- 

 phytin a and from that of the 2,4-dinitrophenylhydrazine derivative 

 of ethyl pheophorbide 6. 



Spectra of chlorophyll h, taken in CS2 or CCI4 solutions and with 

 crystals of chlorophyll h dispersed in Nujol, do not — in contrast to 

 those of chlorophyll a — show a band indicating the occurrence of the 

 chelated enol form of the jS-keto ester in ring V. 



The absence of this band from the spectra of chlorophyll h must be 

 due to the presence of the oxygen atom in the formyl group in ring II. 

 This electronegative group in position 3 may neutralize the tendency 

 of magnesium to shift negative charge to the oxygen in the keto group 

 in position 9, and thus reduce the tendency of the /3-keto ester for 

 enolization. 



To summarize: infrared spectra indicate that in the crystalline state 

 and in nonpolar solvents the /3-keto ester group of chlorophyll a is 

 present as a chelated enol. In polar solvents, such as chloroform or 

 diethyl ether, there is no indication of similar enolization . Chlorophyll b, 



