The Cation Radicals of Free Base and Zinc Bacteriochlorin, Bacteriochlorophyll, and Bacteriopheophytin
Fajer, J., Borg, D.C., Forman, A., Felton, R.H., David Dolphin, and Vegh, L.
Proc. Natl. Acad. Sci. U.S.A.
One-electron oxidation of Zn tetraphenylbacteriochlorin and its metal-free base yielded stable cation radicals. ESR hyperfine splittings were assigned by selective deuterations. These results indicate that the protons of the saturated rings of the bacteriochlorins carry large spin densities, in accord with molecular orbital calculations. Comparison in vitro of the optical spectra of bacteriochlorins and their cation radicals with those of bacteriochlorophyll show close correspondence and suggest that the ESR data from the former may also prove a guide to the biological system. The surprising similarity in properties between the radicals of free base and Zn bacteriochlorins is maintained in the chlorophylls: cation radicals of bacteriopheophytin and Me pheophorbide (the free bases of bacteriochlorophyll and Me chlorophyllide, respectively) exhibit ESR properties similar to those of their Mg-containing derivatives. The possibility that metal-free chlorophylls participate in photosynthesis is discussed.