Oxidative Damage of Aliphatic Amino Acid Residues by the Environmental Pollutant NO3.: Impact of Water on the Reactivity

The rate of oxidative damage of aliphatic amino acids and dipeptides by the environmental pollutant nitrate radical (NO3 center dot) in an aqueous acidic environment was studied by laser flash photolysis. The reactivity dropped by a factor of about four for amino acid residues with secondary amide bonds and by a factor of up to nearly 20 for amino acid residues with tertiary amide bonds, compared with that in acetonitrile. According to density functional theory studies, the lower reactivity is due to protonation of the amide moiety, whereas in neutral water, hydrogen bonding with the amide should have little impact on the absolute reaction rate compared with that in acetonitrile. This finding can be rationalized by the high reactivity and broad reaction pattern of NO3 center dot. Although hydrogen bonding involving the amide group raises the energies associated with some electron transfer processes, alternative low-energy pathways remain available so that the overall reaction rate is barely affected. The undiminished high reactivity of NO3 center dot.( )toward aliphatic amino acid residues in a neutral aqueous environment highlights the health-damaging potential of exposure to the combined air pollutants nitrogen dioxide (NO2 center dot) and ozone (O-3).

Automated summary

This study investigated the rate of oxidative damage of aliphatic amino acids and dipeptides by the environmental pollutant nitrate radical (NO3 center dot) in an aqueous acidic environment. The reactivity of amino acid residues with secondary and tertiary amide bonds decreased compared to that in acetonitrile. Density functional theory studies showed that the lower reactivity was due to protonation of the amide moiety. In neutral water, hydrogen bonding with the amide had little impact on the reaction rate.