beta-phosphorylated nitroxides in the pyrrolidine series: reduction by ascorbate.

Reduction-resistant nitroxides are particularly interesting for biomedical applications. beta-Phosphorylated pyrrolidinyl nitroxides, a new series of stable pyrrolidinoxyl radicals prepared in our laboratory, were tested toward ascorbate reduction in phosphate buffer at pH 7.4. The kinetics of decay were monitored by ESR and compared to those of two reference nitroxides, PCA and Proxyl. The introduction of a beta-phosphoryl group on a pyrrolidinoxyl structure resulted in a moderate increase of the reduction rate constant. However, inside the phosphorylated series, slight structural modifications can induce significant changes in the rate constants.