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 O-2(center dot-) adducts (also referred to as hydroperoxyl (HO2 center dot) 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 O-2(center dot-) 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(beta-H), and a(gamma-H). Conformational change around the substituents and their effect on the hfsc's were also explored. Results indicate that a(beta-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 sigma-orbital and the SOMO of the nitroxyl nitrogen plays a crucial factor in determining the magnitude of the a(beta-H). The hfsc values for the O2(center dot-) adducts were predicted with high accuracy by using a low-cost computational method at the PCM(water)/BH and HLYP/EPR-III//B3LYP/6-31G* level of theory without taking into account the explicit water interaction.