Thinking Outside the Cage: A New Hypothesis That Accounts for Variable Yields of Radicals from the Reaction of CO2 with ONOO.,Chemical Research in Toxicology

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Thinking Outside the Cage: A New Hypothesis That Accounts for Variable Yields of Radicals from the Reaction of CO2 with ONOO.
Chemical Research in Toxicology ( IF 3.7 ) Pub Date : 2020-05-07 , DOI: 10.1021/acs.chemrestox.9b00309
Willem H Koppenol ₁ , Sandra Serrano-Luginbuehl ₂ , Thomas Nauser ₃ , Reinhard Kissner ₃

Affiliation

In biology, the reaction of ONOO^– with CO₂ is the main sink for ONOO^–. This reaction yields CO₃^•–, NO₂^•, NO₃^–, and CO₂. There is a long-standing debate with respect to the yield of the radicals relative to ONOO^–. The reaction of ONOO^– with CO₂ results at first in ONOOCO₂^–. According to one hypothesis, ONOOCO₂^– is extremely short-lived and devolves into a solvent cage that contains CO₃^•– and NO₂^•. Of these solvent cages, approximately two/thirds result in NO₃^– and CO₂, and approximately one/third release CO₃^•– and NO₂^• that oxidize the substrate. According to our hypothesis, ONOOCO₂^– is formed much faster, is relatively long-lived, and may also be an oxidant; the limited yield is the result of ONOOCO₂^– being scavenged by a second CO₂ under conditions of a high CO₂ concentration. We disagree with the first hypothesis for three reasons: First, it is based on an estimated K for the reaction of ONOO^– with CO₂ to form ONOOCO₂^– of ∼1 M^–1, while experiments yield a value of 4.5 × 10³ M^–1. Second, we argue that the solvent cage as proposed is physically not realistic. Given the less than diffusion-controlled rate constant of CO₃^•– with NO₂^•, 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 ONOOCO₂^–, it undergoes homolysis to yield CO₃^•– with NO₂^•, or, depending on [CO₂], it is scavenged by a second CO₂; CO₃^•– oxidizes ONOO^–, if present. These reactions allow us to successfully simulate the reaction of ONOO^– with CO₂ over a wide range of ONOO^–/CO₂ 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^–/CO₂ ratios. Given a physiological [CO₂] of 1.3 mM, the yield of CO₃^•– and NO₂^• is 19%, and lower if ONOOCO₂^– has a significant reactivity of its own.

中文翻译：

在笼子外思考：一种新的假说，该假说解释了CO2与ONOO反应产生的自由基的可变产率。

在生物学中，ONOO的反应^-与CO ₂是用于ONOO主水槽^-。该反应产生CO ₃^•–，NO ₂^•，NO ₃^–和CO ₂。有相对于相对于ONOO自由基的产量争论由来已久^-。ONOO ^–与CO ₂的反应首先在ONOOCO ₂^–中产生。根据一个假设，ONOOCO ₂^-是极其短暂的和转予到含有CO溶剂笼₃^{• -}和NO ₂^•。这些溶剂笼，约两/三分之二导致NO ₃^-和CO ₂，和大约一/第三释放CO ₃^{• -}和NO ₂^•其氧化基材。根据我们的假设，ONOOCO ₂^-快得多形成，相对长住，也可能是一种氧化剂; 产量有限的原因是ONOOCO ₂^–在高CO ₂浓度的条件下被第二种CO ₂清除。我们不同意第一个假设，原因有以下三个：第一，它基于ONOO ^–与CO反应的估计K₂形成约1 M ^–1的ONOOCO ₂^–，而实验得出的值为4.5×10 ³ M ^–1。其次，我们认为所提出的溶剂笼在物理上是不现实的。给定CO ₃^•–小于NO ₂^•的扩散控制速率常数，所有自由基都会从溶剂笼中逸出。第三，报告的〜33％的自由基不受建立质量平衡的实验的支持。我们在这里提出一种混合机制。形成ONOOCO ₂^–后，将其进行均质分解以生成带有NO ₂^•的CO ₃^•–，或者取决于[CO₂ ]，它被第二个CO ₂清除；CO ₃^•–氧化ONOO ^–（如果存在）。这些反应使我们能够成功模拟ONOO ^–与CO ₂在广泛的ONOO ^– / CO ₂比率范围内的反应。在较低的比率下，形成的自由基较少，而在较高的比率下，预测的自由基产率为30％至40％。在报道基团的产率的差异因此可以追溯到实验ONOO ^- / CO ₂比。假设生理[CO ₂ ]为1.3 mM，则CO ₃^•和NO ₂^•的产率是19％，并降低如果ONOOCO ₂^-拥有自己的显著反应。

更新日期：2020-05-07

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