Peroxynitrite does not decompose to singlet oxygen ((1)Delta (g)O(2)) andnitroxyl (NO(-))

Area of Science:

• Biochemistry
• Chemical Biology
• Spectroscopy

Background:

• Peroxynitrite (ONOO(-)) is a reactive nitrogen-oxygen species implicated in biological processes.
• Previous research suggested ONOO(-) decomposition yields singlet oxygen ((1)Delta(g)O(2)) and nitroxyl ((1)NO(-)) upon protonation.
• These proposed products were linked to nitrosyl hemoglobin formation, but potential contaminants were not fully addressed.

Purpose of the Study:

• To rigorously investigate the decomposition products of hydrogen peroxide-free peroxynitrite.
• To re-evaluate the claims of singlet oxygen and nitroxyl formation from peroxynitrite.
• To clarify the spectral assignment of hemoglobin species in the presence of peroxynitrite and potential contaminants.

Main Methods:

• UV-visible spectroscopy
• Electron Paramagnetic Resonance (EPR) spectroscopy
• Chemical trapping of singlet oxygen
• Quantum mechanical and thermodynamic calculations

Main Results:

• Hydrogen peroxide-free peroxynitrite did not produce nitroxyl ((1)NO(-)) when reacted with methemoglobin.
• No evidence of singlet oxygen ((1)Delta(g)O(2)) formation was observed with purified peroxynitrite, neither through light emission nor chemical trapping.
• Singlet oxygen trapping was only observed when hydrogen peroxide was present, indicating its role as a contaminant.
• Thermodynamic calculations indicated a high energy requirement for the proposed decomposition pathway.

Conclusions:

• The decomposition of peroxynitrite does not yield singlet oxygen or nitroxyl under physiological conditions.
• Previous findings attributing nitrosyl hemoglobin to peroxynitrite were likely due to hydrogen peroxide contamination.
• The reaction mechanisms proposed by Khan et al. are energetically unfavorable and inconsistent with experimental data from purified peroxynitrite.