1812 



SPECTROSCOPY AND FLUORESCENCE OF PIGMENTS CHAP. 37C 



H 



R 

 O 



^c— c— c 



R 

 O 



Mg 



I, 



^ C— CuT . „ 



A 



o 



^C— c— C 



R 

 O 



-- C— Ci'o 



Mg 



Ketone 



(stabilized by the 

 absence of Mg) 



1^ ^-b-C 



dH...NC5H5 '^H— O 



Enol Chelated enol 



(stabilized by base) 



In pheophytins, no enolization (with or without chelation) is noticeable 

 in CCI4 sohition. 



In contrast to Livingston and Weigl's observation, Holt and Jacobs' 

 curves show the expected extra "aldehyde" C=0 band in compounds of 

 the h series. 



Fig. 37C.15 shows the effect of allomerization (by exposure to air in 

 methanol) on the infrared absorption spectrum of ethyl chlorophyllide a 

 in chloroform. In the curve set (a), most striking is the practical dis- 

 appearance of the (ketone) C=0 band at 1680 cm.~^ in the main allo- 

 merized fraction, No. 2. This is in agreement with Fischer and Pfeiffer's 

 hypothesis of lactone formation (as indicated under the top curve in set 

 (b)) ; this transformation increases the frequency, since CO bonds generally 

 are stabilized by accumulation at one C atom (cf. p. 215). The band at 

 1720 cm.~"^ can therefore be attributed to the lactone (the ester band at 

 1740 cm.~^ showing only as a shoulder). The two bands are similarly 

 merged also in pyridine. In CCI4, the ketone C=0 band seems to re- 

 appear (at 1675 mfi), suggesting a dismutation equilibrium between the 



I I I 



lactone, CH3— 0— C(10)— 0— C(9), and the ketoester, 0=C(10)-f- 



I II 1 





 I 

 CH3O — C(9) (analogous to the equilibrium suggested by Fischer for the 

 II 

 O 



I I 



corresponding alcohol, HO— C(10)— O— C(9); cf. Fischer and Stern 1940, 



I II 



O 



p. 88). 



In CCI4 or CHCI3, none of the allomerized forms shows an OH-band, 

 confirming Fischer's view that allomerization ordinarily leads to an OCH3 

 (rather than an OH) group in position 10 (except when no alcohol is pres- 

 ent, as in the allomerization of methyl pheophorbide by dilute KOH in 

 pyridine). 



