Abstract
The flavonol quercetin undergoes both enzymatic and non-enzymatic reactions with nitroxyl (HNO/NO−), similar to analogous reactions with dioxygen, but in which N is regioselectively found in the ring-cleaved product. Here we report on kinetic and thermodynamic analysis of the non-enzymatic nitroxygenation reaction in water, which is orders of magnitude faster than the comparable dioxygenation. The second order rate constants were determined from variable temperature reactions, which allowed determination of the reaction activation enthalpy (ΔH≠=9.4kcal/mol), entropy (ΔS≠=−8.3cal/molK), and free energy (ΔG≠=11.8kcal/mol). The determined standard state energy (ΔGo) and activation free energy, as well as the low entropic energy of reaction, are consistent with a proposed single electron transfer (SET) rate determining step.
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