Characterization of the High Resolution ESR Spectra of the Methoxyl Radical Adducts of 5-(diethoxyphosphoryl)-5-methyl-1-pyrroline N-oxide (DEPMPO)

Abstract

Spin-trapping investigators are largely limited by the instability of the radical adducts. Spin trap 5-(diethoxyphosphoryl)-5-methyl-1-pyrroline N-oxide (DEPMPO) forms very stable alkoxyl radical adducts. However, the presence of two chiral centers in the DEPMPO alkoxyl radical adduct results in two diastereomers with distinctive ESR spectra, which complicates the interpretation of the ESR spectra. We have analyzed the high resolution ESR spectra of the DEPMPO/•OCH3 radical adduct. DEPMPO/•OCH3 has been synthesized by the nucleophilic addition of alcohols to DEPMPO. The electron spin resonance (ESR) spectrum of DEPMPO/•OCH3 in oxygen-free methanol solution reveals superhyperfine structure with hyperfine coupling constants as small as 0.3 G. In order to simplify the analysis of the electron spin resonance (ESR) spectrum, we synthesized the DEPMPO/•OCD3 radical adduct. Computer simulation of the DEPMPO/•OCD3 ESR spectrum revealed two diastereomers. Hyperfine coupling constants of γ-protons and 17O from the –OCH3 group were also determined. ESR spectra of DEPMPO/•OCH3 in phosphate buffer have also been characterized. The presence of specific hyperfine couplings from the –OCH3 group can be used for the unambiguous identification of the DEPMPO/•OCH3 radical adducts. We suggest that the analysis of high resolution ESR spectra can be used for the unambiguous characterization of DEPMPO radical adducts.