Effect of the Phosphoryl Substituent in the Linear Nitrone on the Spin Trapping of Superoxide Radical and the Stability of the Superoxide Adduct: Combined Experimental and Theoretical Studies

A new phosphorylated linear nitrone N-(4-hydroxybenzyliene)-1-diethoxyphosphoryl-1-methylethylamine
N-oxide (4-HOPPN) was synthesized, and its X-ray structure was determined. The spin trapping ability
of various kinds of free radicals by 4-HOPPN was evaluated. Kinetic study of decay of the superoxide
spin adduct (4-HOPPN-OOH) shows the half-life time of 8.8 min. On the basis of the X-ray structural
coordinates, theoretical analyses using density functional theory (DFT) calculations at the B3LYP/6-31+G(d,p)//B3LYP/6-31G(d) level were performed on spin-trapping reactions of superoxide radical with
4-HOPPN and PBN and three possible decay routes for their corresponding superoxide adducts. The
comparative calculations on the spin-trapping reactions with superoxide radical predicted that both spin
traps share an identical reaction type and have comparable potency when spin trapping superoxide radical.
Analysis of the optimized geometries of 4-HOPPN-OOH and PBN-OOH reveals that an introduction of
the phosphoryl group can efficiently stabilize the spin adduct through the intramolecular H-bonds, the
intramolecular nonbonding attractive interactions, as well as the bulky steric protection. Examination of
the decomposition thermodynamics of 4-HOPPN-OOH and PBN-OOH further supports the stabilizing
role of the phosphoryl group to a linear phosphorylated spin adduct.