Reaction of Amino Acids, Di- and Tripeptides with the Environmental Oxidant NO3. : A Laser Flash Photolysis and Computational Study

Abstract

Absolute rate coefficients for the reaction between the important environmental free radical oxidant NO₃^. and a series of N- and C-protected amino acids, di- and tripeptides were determined using 355 nm laser flash photolysis of cerium(IV) ammonium nitrate in the presence of the respective substrates in acetonitrile at 298±1 K. Through combination with computational studies it was revealed that the reaction with acyclic aliphatic amino acids proceeds through hydrogen abstraction from the α-carbon, which is associated with a rate coefficient of about 1.8×10⁶ m^-1 s^-1 per abstractable hydrogen atom. The considerably faster reaction with phenylalanine [k=(1.1±0.1)×10⁷ m^-1 s^-1 ] is indicative for a mechanism involving electron transfer. An unprecedented amplification of the rate coefficient by a factor of 7-20 was found with di- and tripeptides that contain more than one phenylalanine residue. This suggests a synergistic effect between two aromatic rings in close vicinity, which makes such peptide sequences highly vulnerable to oxidative damage by this major environmental pollutant.

Keywords: density functional calculations; kinetics; nitrate radical; oxidative damage; peptides; reaction mechanisms.