Thinking Outside the Cage: A New Hypothesis That Accounts for Variable Yields of Radicals from the Reaction of CO 2 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.