10 R. LIVINGSTON 



REVERSIBLE PHOTOCHEMICAL REACTIONS OF CHLOROPHYLL 



Anaerobic solutions of chlorophyll in niothaiiol, (^ther, etc., but 

 not in hydrocarbons show rapid reversible changes in their ab- 

 sorption spectra when they are strongly illuminated. Under experi- 

 mentally realizable conditions, the maximum chan}:;e in the absorp- 

 tion of monochromatic light is less than 1%. Since the percentage 

 change is proportional to the square root of the intensity of the ab- 

 sorbed light, we may conchide that the labile product is a pair of 

 radicals or ions which reform chlorophyll by reeombining. The ab- 

 sorption spectrum of this intermediate has been determined (14) 

 only very crudely, but appears to resemble that of the triplet state 

 (7). Although this reaction is intrinsically interesting and may be 

 related to the primary act of photosensitized reactions, the last 

 publication dealing with the subject appeared in 1950 (16). 



It was observed by Linschitz (15) that chlorophyll, in a rigid 

 solvent (EPA at liquid nitrogen temperatures) containing a quinone, 

 is photochemically transformed into a product w^hich is stable at 

 low temperatures but reverts immediately to normal chlorophyll 

 when the glass is allowed to w^arm to its softening point. The ab- 

 sorption spectrum of this product is, likewise,, generally similar to 

 that of the triplet state. 



The earlier finding of Krasnovskii that chlorophyll can be revers- 

 ibly, photochemically reduced by ascorbic acid, when dissolved in 

 pyridine, has been confirmed by A. S. Holt and E. Rabinowitch and 

 by Evstigneev and Gavrilova (17b). The latter authors also observed 

 this reaction in toluene with phenylhydrazine as the reducing agent. 

 They have further shown that two labile intermediates are formed, 

 and have postulated that the maximum at 585 mu is due to a neutral 

 molecule or radical which ionizes in basic solution with the resulting 

 appearance of a maximum at 518 m/z. They measured the fluorescence 

 and low-temperature phosphorescence of these substances. 



Discussion 



Rabinowitch: How does the absorption curve of the triplet state compare 

 with that of ionized chlorophj'll? 



Livingston: The absorption spectra of the ions, of the triplet state, of Kras- 

 novskii's reduced form, and of Linschitz' chlorophyll-quinone intermediate, are 

 similar but not identical. They show distinct differences; but in each case the 

 two singlet peaks disappear, the Soret band is replaced by what looks Uke two 



