ONOOH

ONOOH, short for peroxynitrous acid, is the protonated form of peroxynitrite (ONOO−). It is a short-lived reactive nitrogen–oxygen species formed when nitric oxide (NO) reacts with superoxide (O2−). In water, ONOO− predominates at neutral pH, while ONOOH forms under acidic conditions; the equilibrium between ONOO− and ONOOH depends on pH, with a pKa around 6–7 reported in the literature. Because ONOOH is unstable, it is not isolated under normal conditions and exists only transiently in solution.

Upon formation or in the presence of carbon dioxide, ONOOH rapidly decomposes or reacts to yield reactive products such as nitrate (NO3−), nitrite (NO2−), hydroxyl radical (•OH), and nitrogen dioxide radical (NO2•). A key pathway is the reaction of ONOO− with CO2 to form nitrosoperoxocarboxylate (ONOOCO2−), which decays to NO2• and the carbonate radical (CO3−•). These reactive intermediates can nitrate tyrosine residues and initiate lipid peroxidation, contributing to cellular damage. The biological significance of ONOOH relates to oxidative and nitrosative stress and inflammatory signaling; peroxynitrite chemistry is linked to inflammatory conditions, ischemia-reperfusion injury, and some neurodegenerative processes.

Detection is typically indirect, based on markers such as 3-nitrotyrosine, nitrated lipids, or changes in NO2−/NO3− levels, because ONOOH itself is too short-lived to measure directly in most biological contexts. In chemical studies, ONOOH is treated as a highly reactive oxidant, and experiments are conducted under conditions that control its rapid reactivity and decomposition.