A new electron spin resonance spectrum for an exchange- and dipole-dipole coupled superoxocobalamin ⋯ free-radical pair occurring in adenosylcobalamin-containing systems

Abstract

Laser-induced photolysis of highly purified, polycrystalline adenosylcobalamin generates, under anaerobic conditions, an e.s.r. spectrum characteristic of a strongly exchange-coupled Co^II⋯ free-radical pair. On exposure to oxygen, a new spectrum appears, concurrent with the disappearance of the previous spectrum. The band structure of the new spectrum is that of a triplet, and can be shown to be the result of exchange and dipole–dipole coupling between two unpaired spins separated by a distance of ca. 8.7 Å. We suggest that the new biradical state is a Co–O₂^–⋯ R˙ pair (R˙= adenosyl radical).