Chiral Spin Traps. The Spin Trapping Chemistry of 5-Methyl-5-phenylpyrroline-N-oxide (MPPO)

Abstract The use of 5,5-dimethylpyrroline- N -oxide (DMPO) as a versatile spin trap was first published in this journal (E. G. Janzen and J. I.-P. Liu, J. Magn. Reson. 9, 510–512 (1973). In this paper, the general use of an improved DMPO-type spin trap, namely 5-methyl-5-phenylpyrroline- N -oxide (MPPO), is proposed. MPPO is more stable than DMPO and has an excellent shelf life. Commonly known artifacts of DMPO are not present in MPPO. The EPR spectra of MPPO spin adducts have the same patterns as DMPO spin adducts which users have become familiar with. The lifetimes of spin adducts are longer for MPPO than for DMPO and the rate constants of spin trapping are similar. An interesting additional feature is associated with the detection of two spin-adduct spectra in some cases. The major component is assigned to the trans addition product (with respect to the phenyl group) in the case of carbon-centered radicals. The minor component is assigned to the cis adduct. In the superoxide/peroxyl radical adduct, however, the reverse appears to be the case. Only one EPR spectrum is detected in the hydroxyl radical adduct of MPPO.