Theoretical and experimental studies of the spin trapping of inorganic radicals by 5,5-dimethyl-1-pyrroline N-oxide (DMPO).: 2.: Carbonate radical anion

Previous studies have shown that the enzyme-mediated generation of carbonate radical anion (CO3 center dot-) may play an important role in the initiation of oxidative damage in cells. This study explored the thermodynamics of CO3 center dot- addition to 5,5-dimethyl-1-pyrroline N-oxide (DMPO) using density functional theory at the B3LYP/6-31+G**//B3LYP/6-31G* and B3LYP/6-311+G* levels with the polarizable continuum model to simulate the effect of the bulk dielectric effect of water on the calculated energetics. Theoretical data reveal that the addition of CO3 center dot- to DMPO yields an O-centered radical adduct (DMPO-OCO2) as governed by the spin (density) population on the CO3 center dot-. Electron paramagnetic resonance spin trapping with the commonly used spin trap, DMPO, has been employed in the detection of CO3 center dot-. UV photolysis of H2O2 and DMPO in the presence of sodium carbonate (Na2CO3) or sodium bicarbonate (NaHCO3) gave two species (i.e., DMPO-OCO2 and DMPO-OH) in which the former has hyperfine splitting constant values of a(N) = 14.32 G, a(beta-Eta) = 10.68 G, and a(gamma-H) = 1.37 G and with a shorter half-life compared to DMPO-OH. The origin of the DMPO-OH formed was experimentally confirmed using isotopically enriched (H2O2)-O-17 that indicates direct addition of HO center dot to DMPO. Theoretical studies on other possible pathways for the formation of DMPO-OH from DMPO-OCO2 in aqueous solution and in the absence of free HO center dot such as in the case of enzymatically generated CO3 center dot-, show that the preferred pathway is via nucleophilc substitution of the carbonate moiety by H2O or HO-. Nitrite formation has been observed as the end product of CO3 center dot- trapping by DMPO and is partly dependent on the basicity of solution. The thermodynamic behavior of CO3 center dot- in the aqueous phase is predicted to be similar to that of the hydroperoxyl (HO2 center dot) radical.