Kinetic studies revealed that nitrate radicals (NO3 ⋅), which are formed through reaction of the noxious air pollutants nitrogen dioxide (NO2 ⋅) and ozone (O3 ), very rapidly oxidize phenylalanine residues in an aqueous environment, with overall rate coefficients in the 108 -109 M-1 s-1 range. With amino acids and dipeptides as model systems, the data suggest that the reaction proceeds via a π-complex between NO3 ⋅ and the aromatic ring in Phe, which subsequently decays into a charge transfer (CT) complex. The stability of the π-complex is sequence-dependent and is increased when Phe is at the N terminus of the dipeptide. Computations revealed that the considerably more rapid radical-induced oxidation of Phe residues in both neutral and acidic aqueous environments, compared to acetonitrile, can be attributed to stabilization of the CT complex by the protic solvent; this clearly highlights the health-damaging potential of exposure to combined NO2 ⋅ and O3 .
Keywords: DFT calculations; kinetic studies; nitrate radicals; oxidative damage; phenylalanine; solvent effects.
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