Previous studies have shown that the enzyme-mediated generation of carbonate radical anion (CO3̇-) may play an important role in the initiation of oxidative damage in cells. This study explored the thermodynamics of CO3̇- 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 CO3̇- to DMPO yields an O-centered radical adduct (DMPO-OCO2) as governed by the spin (density) population on the CO3̇-. Electron paramagnetic resonance spin trapping with the commonly used spin trap, DMPO, has been employed in the detection of CO3̇-. UV photolysis of H2O2 and DMPO in the presence of sodium carbonate (Na2CO3) or sodium bicarbonate (NaHCO 3) gave two species (i.e., DMPO-OCO2 and DMPO-OH) in which the former has hyperfine splitting constant values of aN = 14.32 G, aβ-H = 10.68 G, and aγ-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 H2 17O2 that indicates direct addition of HȮ 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 HȮ such as in the case of enzymatically generated CO 3̇- 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 ̇- trapping by DMPO and is partly dependent on the basicity of solution. The thermodynamic behavior of CO3̇- in the aqueous phase is predicted to be similar to that of the hydroperoxyl (HO 2̇) radical.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry