Cuprous Complexes and Dioxygen. Part 12.. Rate law and mechanism of the copper‐catalyzed oxidation of ascorbic acid in aqueous acetonitrile

Abstract

The copper-catalyzed oxidation of ascorbic acid (AscH,) has been studied with a Clurk electrode in aqueous MeCN. Cu' or CU" may be equally used as the source of metal ion, without influence on the rate law. At sufficiently high [MeCN], the rate of the overall reaction is essentially given by the rate of Cu' autoxidation: the reaction is of first order with respect to [Cuk:'] [13-151. If Cu' is supposed to be essential in catalytic ascorbate oxidation, the very easy autoxidation of Cu& [l] [18] [24-261 should be taken into account, and any proposed mechanism should be compatible with the rate law of that latter reaction. With the exception of some preliminary results for media containing high amounts of C1-, this does not seem to be the case in general [2] [3]. It is the aim of the present work to firmly establish the role of Cu' autoxidation in the Cu oxidation of ascorbate and to show the full compatibility of the corresponding rate laws in aqueous MeCN. In addition, we hope to have laid a basis for a consistent mechanism of this reaction in the absence of MeCN or other Cu'-stabilizing ligands such as chloride and ally1 alcohol, too.

A Mets Motif Peptide Found in Copper Transport Proteins Selectively Binds Cu(I) with Methionine-Only Coordination

“…Ascorbic acid is stable to aerial oxidation in the absence of metal ions, but copper or iron ions catalyze its oxidative degradation via eqs 8 and 9 . The loss of its characteristic, pH-dependent absorption band between 245 and 265 nm is therefore a convenient spectroscopic signal for the presence of catalytic metal ions. ,, The pH-dependent oxidation rate is first-order with respect to both the ascorbate monoanion (HAsc - ) and the metal ion, and is known to decrease in the presence of metal chelators, with slower oxidation rates corresponding to more stable chelates. , Chelators that favor the high-valence oxidation state slow the rate by making eq 8 rate-limiting, whereas molecules that stabilize Cu(I) shift the rate-limiting step to eq 9, as has been shown for ascorbate oxidation in aqueous acetonitrile . The ascorbate UV−vis assay described here provides a convenient spectroscopic signal for monitoring the binding ability of the Mets peptides with spectroscopically silent Cu(I).…”

Inactivation of peptidylglycineα-hydroxylating monooxygenase by cinnamic acid analogs

“…None of the compounds tested within this study showed any O 2 consumption (above background) when screened for activity even though inactivation was O 2 - and ascorbate-dependent (Figure S3, Supplementary materials). A low background rate of O 2 consumption is observed for PHM in the absence of an oxidizable substrate resulting from the presence of Cu(II) and ascorbate in an aerobic solution 69 . Incubation of N -dansyl-4-aminocinnamate with PHM in the presence of ascorbate and O 2 does not result in any change in C 18 -reverse phase HPLC retention time for N -dansyl-4-aminocinnamate; additional evidence that the cinnamates are not PHM substrates.…”