Thinking Outside the Cage: A New Hypothesis That Accounts for Variable Yields of Radicals from the Reaction of CO₂ with ONOO–

In biology, the reaction of ONOO – with CO 2 is the main sink for ONOO – . This reaction yields CO 3 •–, NO 2 •, NO 3 –, and CO 2 . There is a long-standing debate with respect to the yield of the radicals relative to ONOO – . The reaction of ONOO – with CO 2 results at first in ONOOCO 2 – . According to one hypothesis, ONOOCO 2 – is extremely short-lived and devolves into a solvent cage that contains CO 3 •– and NO 2 • . Of these solvent cages, approximately two/thirds result in NO 3 – and CO 2, and approximately one/third release CO 3 •– and NO 2 • that oxidize the substrate. According to our hypothesis, ONOOCO 2 – is formed much faster, is relatively long-lived, and may also be an oxidant; the limited yield is the result of ONOOCO 2 – being scavenged by a second CO 2 under conditions of a high CO 2 concentration. We disagree with the first hypothesis for three reasons: First, it is based on an estimated K for the reaction of ONOO – with CO 2 to form ONOOCO 2 – of ∼1 M –1, while experiments yield a value of 4.5 × 10 3 M –1 . Second, we argue that the solvent cage as proposed is physically not realistic. Given the less than diffusion-controlled rate constant of CO 3 •– with NO 2 •, all radicals would escape from the solvent cage. Third, the reported ∼33% radical is not supported by an experiment where mass balance was established. We propose here a hybrid mechanism. After formation of ONOOCO 2 –, it undergoes homolysis to yield CO 3 •– with NO 2 •, or, depending on [CO 2 ], it is scavenged by a second CO 2; CO 3 •– oxidizes ONOO –, if present. These reactions allow us to successfully simulate the reaction of ONOO – with CO 2 over a wide range of ONOO – /CO 2 ratios. At lower ratios, fewer radicals are formed, while at higher ratios, radical yields between 30% and 40% are predicted. The differences in radical yields reported may thus be traced to the experimental ONOO – /CO 2 ratios. Given a physiological [CO 2 ] of 1.3 mM, the yield of CO 3 •– and NO 2 • is 19%, and lower if ONOOCO 2 – has a significant reactivity of its own.