Reaction of peroxynitrite with L-tryptophan

Area of Science:

• Biochemistry
• Chemical Kinetics

Background:

• Peroxynitrite is a reactive nitrogen species implicated in biological processes.
• L-tryptophan is an essential amino acid susceptible to oxidative modification.

Purpose of the Study:

• To investigate the reaction kinetics and products of peroxynitrous acid with L-tryptophan.
• To elucidate the mechanism of peroxynitrite decomposition.

Main Methods:

• Stopped-flow spectrophotometry to determine reaction rates.
• High-performance liquid chromatography (HPLC) for product identification.
• Kinetic analysis across a range of pH values.

Main Results:

• The reaction rate constant was determined to be 130 M⁻¹s⁻¹ at 25°C.
• A pH optimum for the reaction rate was observed at pH 5.1.
• 5- and 6-nitrotryptophan were identified as major products; hydroxytryptophans were absent.
• Thermodynamic parameters (enthalpy and entropy of activation) were calculated at pH 7.1.

Conclusions:

• The reaction mechanism favors nitration over hydroxylation of L-tryptophan by peroxynitrite.
• The findings support a mechanism for peroxynitrite isomerization to nitrate that does not involve hydroxyl radical formation.