The π-Cation Radical of Chlorophyll a
Borg, D.C., Fajer, J., Felton, R.H., and David Dolphin
Proc. Natl. Acad. Sci.
Chlorophyll a undergoes reversible one-electron oxidation in dichloromethane and butyronitrile. Removal of the electron by controlled potential electrolysis or by stoichiometric charge transfer to a known cation radical yields a radical (epr line width = 9 gauss, g = 2.0025 ± 0.0001) whose optical spectrum is bleached relative to that of chlorophyll. Upon electrophoresis this bleached species behaves as a cation. By comparison with the known properties of π-cation radicals of porphyrins and chlorins, the chlorophyll radical is also identified as a π-cation. Further correlation of optical and epr properties with published studies on photosynthesis leads to the conclusion that oxidized P700, the first photochemical product of photosystem I in green plants, contains a π-cation radical of the chlorin component of chlorophyll a. This radical is the likely source of the rapidly-decaying, narrow epr signal of photosynthesis.
Correction. In the article "The π-Cation Radical of Chlorophyll
a," by Donald C. Borg, Jack Fajer, Ronald H. Felton,
and David Dolphin, which appeared in the October 1970 issue
of Proc. Nat. Acad. Sci. USA, 67, 813-820, an editorial change
in the mode of abbreviation led to an error on page 818, line 6.
For ZnPh4 P+ , read ZnPh4C+’
(the cation radical of zinc tetraphenylchlorin). The sentence should read as follows:
"Other possibilities such as specific oxidation of a substituent
group of Chl a are ruled out by the above argument and by the
close similarity of the electronic spectra of Chl+- and ZnPh4C + (the cation radical of zinc tetraphenylchlorin)."
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