Theoretical and Experimental Studies of the Spin Trapping of Inorganic Radicals by 5,5-Dimethyl-1-Pyrroline N-Oxide (DMPO). 2. Carbonate Radical Anion

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2007-01, Vol.111 (2), p.384-391
• Main Authors: Villamena, Frederick A, Locigno, Edward J, Rockenbauer, Antal, Hadad, Christopher M, Zweier, Jay L
• Format: Article
• Language: English
• Subjects: Anions - chemistry; Chromatography, Gas; Cyclic N-Oxides - chemistry; Electron Spin Resonance Spectroscopy; Mass Spectrometry; Models, Molecular; Models, Statistical; Models, Theoretical; Molecular Conformation; Oxides - chemistry; Software; Spin Trapping; Thermodynamics; Time Factors; Ultraviolet Rays
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp065692d

Previous studies have shown that the enzyme-mediated generation of carbonate radical anion (CO3 •-) may play an important role in the initiation of oxidative damage in cells. This study explored the thermodynamics of CO3 •- 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 •- to DMPO yields an O-centered radical adduct (DMPO−OCO2) as governed by the spin (density) population on the CO3 •-. Electron paramagnetic resonance spin trapping with the commonly used spin trap, DMPO, has been employed in the detection of CO3 •-. 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 β - Η = 10.68 G, and a γ -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 HO• 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• such as in the case of enzymatically generated CO3 •-, 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 CO3 •- in the aqueous phase is predicted to be similar to that of the hydroperoxyl (HO2 •) radical.