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 aN, aβ-H, and aγ-H. Conformational change around the substituents and their effect on the hfsc’s were also explored. Results indicate that aβ-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 aβ-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.