Department of Chemistry, University of Washington, Box 351700, Seattle, WA 98107-1700, USA.
Proc Natl Acad Sci U S A. 2010 Mar 23;107(12):5282-7. doi: 10.1073/pnas.0910347107. Epub 2010 Mar 9.
Chemical reactions that involve net hydrogen atom transfer (HAT) are ubiquitous in chemistry and biology, from the action of antioxidants to industrial and metalloenzyme catalysis. This report develops and validates a procedure to predict rate constants for HAT reactions of oxyl radicals (RO(*)) in various media. Our procedure uses the Marcus cross relation (CR) and includes adjustments for solvent hydrogen-bonding effects on both the kinetics and thermodynamics of the reactions. Kinetic solvent effects (KSEs) are included by using Ingold's model, and thermodynamic solvent effects are accounted for by using an empirical model developed by Abraham. These adjustments are shown to be critical to the success of our combined model, referred to as the CR/KSE model. As an initial test of the CR/KSE model we measured self-exchange and cross rate constants in different solvents for reactions of the 2,4,6-tri-tert-butylphenoxyl radical and the hydroxylamine 2,2'-6,6'-tetramethyl-piperidin-1-ol. Excellent agreement is observed between the calculated and directly determined cross rate constants. We then extend the model to over 30 known HAT reactions of oxyl radicals with OH or CH bonds, including biologically relevant reactions of ascorbate, peroxyl radicals, and alpha-tocopherol. The CR/KSE model shows remarkable predictive power, predicting rate constants to within a factor of 5 for almost all of the surveyed HAT reactions.
涉及净氢原子转移 (HAT) 的化学反应在化学和生物学中无处不在，从抗氧化剂的作用到工业和金属酶催化。本报告开发并验证了一种预测各种介质中氧自由基 (RO(*)) 的 HAT 反应速率常数的程序。我们的程序使用马库斯交叉关系 (CR)，并包括对反应动力学和热力学的溶剂氢键效应的调整。通过使用 Ingold 模型包括动力学溶剂效应 (KSE)，并通过使用 Abraham 开发的经验模型来考虑热力学溶剂效应。这些调整对于我们的组合模型（称为 CR/KSE 模型）的成功至关重要。作为对 CR/KSE 模型的初步测试，我们在不同溶剂中测量了 2,4,6-三特丁基苯氧自由基和羟胺 2,2'-6,6'-四甲基哌啶-1-醇的自交换和交叉速率常数。计算出的交叉速率常数与直接测定的交叉速率常数非常吻合。然后，我们将模型扩展到超过 30 个已知的氧自由基与 OH 或 CH 键的 HAT 反应，包括抗坏血酸、过氧自由基和α-生育酚的生物学相关反应。CR/KSE 模型显示出出色的预测能力，对于几乎所有调查的 HAT 反应，预测的速率常数都在 5 倍以内。
