PHOTOXIDATION OF CHLOROPHYLL 1499 



sorption in the red and blue peaks, increase at 480-590 and 700-740 mju) 

 was slight, and immediately reversible even at this low temperature, indi- 

 cating that the photoproduct (metastable form of the chlorophyll molecule, 

 or of a chlorophyll-solvent complex) reacts back monomolecularly without 

 measurable activation energy. The light effect could be enhanced and 

 stabilized by the presence of a quinone or imine (c/. next section). 



Similar experiments were made by Kachan and Dain (1951), who froze chlorophyll 

 solutions in ethanol or ethanol-ether (1 :3) in liquid air and illuminated them with a 500- 

 watt incandescent lamp, a mercury arc, or a spark. However, they found no effect of 

 visible light; ultraviolet illumination caused the red absorption band to disappear in 

 20-30 min. The band returned upon melting; the reversible reaction was interpreted 

 as an oxidation, with the electron detached by light and held by the solvent (and a proton 

 transferred from chlorophyll to ether to form an oxonium ion, thus stabilizing the photo- 

 product). No effect was observed, even with ultraviolet irradiation, if methanol was 

 used as solvent. 



Uri (1952) used methyl methacrylate polymerization to indicate the 

 formation of free radicals in illuminated chlorophyll solution. Polymeriza- 

 tion was in fact observed in air-free 2 X 10~^ M solutions of chlorophyll in 

 methyl methacrylate, or 10% methyl methacrylate in ethanol or pyridine 

 (but not in benzene or acetone) exposed to red light. 



2. Photoxidation of Chlorophyll (Second Addendum to Section A2 of 



Chapter 18) 



The reversible decoloration of chlorophyll ( in methanol) by ferric or 

 eerie salts, first observed by Rabinowitch and Weiss, was described on p. 

 464. Evidence was presented there for considering this reaction a revers- 

 ible oxidation-reduction, leading to an equilibrium. This equilibrium 

 could be shifted back and forth by light (ef. page 488). Among the difficul- 

 ties of this interpretation mentioned on page 465 is the fact that the reversal 

 of the color change could be produced not only by Fe++ ions but also by 

 nonreducing salts, even by sodium chloride. However, the much faster 

 rate of the restoration of green color with FeCl2, the much lower ferrous salt 

 concentration required for that purpose, the complete (immediate) reversi- 

 bility of the reaction, and the possibility to repeat the light shift many 

 times, were quoted there (and can again be quoted here) as supporting the 

 hypothesis of reversible oxidation. The analogy with the chlorophyll- 

 quinone reaction (to be described below) points in the same direction. 



Ashkinazi, Glikman, and Dain (1950) opposed this interpretation because they 

 found that reversible color changes of chlorophyll can be produced also by nonoxidizing 

 salts. They suggested that metal complex formation is responsible for such changes in 

 all cases, including that of iron. 



Ashkinazi and Dain (1950) prepared a ferrous complex of chlorophyll by the action 



