OXTDATIOX, ALLOMERIZATIOX AND REDUCTION OF riTLOROPHYLL 1773 



(as isomerization or soh'ation phenomena), or in Chapter 37C under the 

 heading of "Solvent Effects on Absorption Spectra." Since the latter ar- 

 rangement had been adopted in Chapter 20 we will adhere to it; the 

 description of Freed's low temperature studies will be therefore found in 

 Chapter 37C, section 2a. Only the part of these experiments related to the 

 mechanism of certain irreversible reactions of chlorophyll (with alkalis and 

 amines} will be described in section 4 below. 



4. Oxidation, AUomerization and Reduction 

 of Chlorophyll 



(Addendum to Chapter 16, section B3) 



In Chapter 10 (p. 459) the so-called aUomcrization of chlorophyll 

 was discussed — a transformation that occurs upon standing in alcoholic 

 solution, involves the uptake of oxygen, and can be brought about also by 

 addition of oxidants such as quinone. It was interpreted as oxidation of the 

 "lone" hydrogen atom at C(10). The allomerized chlorophyll differs 

 from the intact compound by its incapacity to give the "brown phase" 

 in alkalysis, and l)y its spectrum. A significant feature of this spectrum 

 (fig. 2 1.4 A) is the apparently complete absence of the short-wave satellite 

 of the main blue-violet band, that is present, with varying intensity, in 

 the spectra of all chlorophylls, chlorophyllides, pheophorbides, etc. {cf. 

 figs. 21.1 and 21.26). This satellite could be due to a vibration (of the 

 order of 750 cm.~^), or to the doublet structure of the excited electronic 

 term (perhaps, corresponding to charge oscillations in two mutually per- 

 pendicular directions in the porphin plane, cf. Chapter 37C, section 1) ; or to 

 a tautomeric form (as suggested tentatively on p. 027, Vol. 11,1). What- 

 ever the correct interpretation of the satellite band may be, its disaijpear- 

 ance upon allomerization must be significant. 



It has long been known that upon extraction from leaves and standing 

 in solution, homogeneous chlorophyll preparations very soon form several 

 fractions which are adsorbed in separate bands on the chromatographic 

 column. Some study was devoted to these changes by Strain (1949), 

 who placed mallow or barley leaves in methanol for over 24 hours, and 

 studied the products formed in the presence and in the absence of air. 

 Prolonged treatment with methanol turned chopped mallow leaves yel- 

 low; crystals of the methyl chlorophyllides a and h, and a small amount 

 of acidic chlorophyllides {i. e., free chlorophyllins) a and b (extractable 

 with 0.01 N KOH), were formed in them. In addition, new green and 

 yellow-green pigments were produced, the spectra of which were similar 

 to those of the chlorophylls a and h, but which gave no positive phase test 



