Structural dependence of nitroxide spin labels and nitroxide spin adducts on their reducibility by ascorbate ion

The reducibility of a series of nitroxides (aminoxyls) by ascorbate was tested by measuring the nitroxide decay rates with a stopped-flow electron paramagnetic resonance technique in aqueous phosphate buffer solution. The dependence of reactivity on the structures and pH of the medium was found for both cyclic nitroxides and nitroxide adducts of phenyl N-tert butyl nitrone (PBN). In cyclic nitroxides, the ring size is a dominant factor in determining reaction rates but substituents have additional effects on the rate depending on their electronegativity. For alkyl and hydroxyalkyl adducts of PBN, at fixed ascorbate concentration, half-lives increase with lengthening of the substituent, suggesting that a long chain in the substituent sterically protects the nitroxide group and thus prevents its reduction by ascorbate.