The decay of ONOOH near neutral pH has been examined as a function of isomerization to H+ and NO3−, and decomposition to NO2− and O2via O2NOO−. We find that in phosphate buffer kisomerization = 1.11 ± 0.01 s−1 and kdisproportionation = (1.3 ± 0.1) × 103 M−1 s−1 at 25 °C and I = 0.2 M. In the presence of 0.1 M tris(hydroxymethyl)aminomethane (Tris), the decay proceeds more rapidly: kdisproportionation = 9 × 103 M−1 s−1. The measured first half-life of the absorbance of peroxynitrite correlates with [Tris]0·([ONOO−]0 + [ONOOH]0)2, where the subscript 0 indicates initial concentrations; if this function exceeds 6.3 × 10−12 M3, then Tris significantly accelerates the decomposition of peroxynitrite.
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