.pi. Cation radicals of isobacteriochlorins. Implications for nitrite and sulfite reductases

Abstract

Stable cation radicals of free base and metalloisobacteriochlorins have been generated chemically and electrochemically and characterized by optical, magnetic, and electrochemical techniques. The ESR spectra of the radicals have been assigned on the basis of ENDOR results, selective deuteration, and molecular orbital calculations. Evidence for a cation radical of an iron(II) isobacteriochlorin-pyridine-carbon monoxide complex is also presented. The facile oxidations of the isobacteriochlorin class and the existence of iron ..pi.. cations suggest that such radicals may mediate electron transport in nitrite and sulfite reductases, enzymes which catalyze the six-electron reductions of nitrite to ammonia and of sulfite to hydrogen sulfide, and which contain iron isobacteriochlorin prosthetic groups.