Oxygen-Centered Spin Adducts of 5,5-Dimethyl-1-pyrrolineN-oxide (DMPO) and 2H-Imidazole 1-Oxides

The hydroxyl and some alkoxyl spin adducts of 5,5-dimethyl-1-pyrroline 1-oxide (DMPO) are difficult to assign due to the remarkable similarity of their EPR spectra. The utility of resolving superhyperfine (SHF) structure followed by computer simulations has been demonstrated to assist in the assignment of EPR spectra with close values of hyperfine splitting constants, e.g., DMPO/^·OH and DMPO/^·OR. Here,^·OR is the alkoxyl radical derived from thermal decomposition of 2,2′-azobis (2-amidinopropane) hydrochloride (AAPH). In addition, two other spin traps, derivatives of 2H-imidazole 1-oxide, namely, 2,2,4-trimethyl-2H-imidazole 1-oxide (TMIO) and 2,2-dimethyl-4-phenyl-2H-imidazole 1-oxide (DMPIO), have been used in a model study to develop a procedure for distinguishing between oxygen-centered spin adducts. These results are compared with those for DMPO. TMIO and DMPIO spin traps provide more distinguishable individual spectra with^·OH and AAPH-derived^·OR radicals than the DMPO spin trap. The formation of DMPO/^·OR(AAPH) and DMPIO/^·OR(AAPH) spin adducts was confirmed by mass spectrometry. The comparison of spin trapping by DMPO and 2H-imidazole 1-oxides using typical biological sources of other oxygen-centered radicals reveals application limits of these spin traps. For example, 2H-imidazole 1-oxides do not form superoxide spin adducts in the xanthine/xanthine oxidase system. Also, for the first time, experimental evidence is presented for SHF structure in spectra of TMIO and DMPIO spin adducts with^·OH/^·OD and^·CH₃/^·CD₃radical species.