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 wa s tested by measuring the nitroxide decay rates with a stopped-flow el ectron paramagnetic resonance technique in aqueous phosphate buffer so lution. The dependence of reactivity on the structures and pH of the m edium was found for both cyclic nitroxides and nitroxide adducts of ph enyl N-tert butyl nitrone (PEN). In cyclic nitroxides, the ring size i s a dominant factor in determining reaction rates but substituents hav e additional effects on the rate depending on their electronegativity. For alkyl and hydroxyalkyl adducts of PEN, at fixed ascorbate concent ration, half-lives increase with lengthening of the substituent, sugge sting that a long chain in the substituent sterically protects the nit roxide group and thus prevents its reduction by ascorbate.