Superoxide Radical Anion Adduct of 5,5-Dimethyl-1-pyrroline N-Oxide. 4. Conformational Effects on the EPR Hyperfine Splitting Constants

Spin trapping has been commonly employed in the detection of superoxide radical anion in chemical and biological systems; hence, accurate interpretation of the hyperfine splitting constants (hfsc’s) arising from the O₂^•− adducts (also referred to as hydroperoxyl (HO₂^•) radical adducts) of various nitrones is important. In this work, the nature of the relevant hfsc’s was investigated by examining the effect of conformational changes in the hydroperoxyl moiety of the O₂^•− adducts of 5,5-dimethyl-1-pyrroline N-oxide (DMPO), 5-ethoxycarbonyl-5-methyl-1-pyrroline N-oxide (EMPO), 5-diethoxyphosphoryl-5-methyl-1-pyrroline N-oxide (DEPMPO), 5-carbamoyl-5-methyl-1-pyrroline N-oxide (AMPO), and 7-oxa-1-azaspiro[4.4]non-1-en-6-one N-oxide, (CPCOMPO) on the magnitude of a_N, a_β-H, and a_γ-H. Conformational change around the substituents and their effect on the hfsc’s were also explored. Results indicate that a_β-H is most sensitive to conformational changes of the hydroperoxyl and substituent groups relative to hfsc’s of other nuclei. The orbital overlap between the C−H σ-orbital and the SOMO of the nitroxyl nitrogen plays a crucial factor in determining the magnitude of the a_β-H. The hfsc values for the O₂^•− adducts were predicted with high accuracy by using a low-cost computational method at the PCM(water)/BHandHLYP/EPR-III//B3LYP/6-31G* level of theory without taking into account the explicit water interaction.