Kinetics and mechanism of the Co(II)-assisted oxidation of l-ascorbic acid by dioxygen and nitrite in aqueous solution

Abstract

A detailed study of the oxidation of L-ascorbic acid by dioxygen and nitrite in water at pH 5.8 and 7.0, catalyzed by the octasulfophenyltetrapyrazinoporphyrazine complex of cobalt(II), was carried out using conventional spectrophotometric, low-temperature and high-pressure stopped-flow techniques. The Co(II) complex activates L-ascorbic acid through an intramolecular one-electron oxidation step that involves the reduction of the octasulfophenyltetrapyrazinoporphyrazine. The reaction rate strongly depends on pH due to the different redox behaviour of the L-ascorbic acid/ascorbate species present in solution. Kinetic parameters for the different reaction steps of the catalytic process were determined. The final product of the reaction between L-ascorbic acid and nitrite was found to be nitrous oxide.

Nitrite Reduction in Aqueous Solution Mediated by Amavadin Homologues: N2O Formation and Water Oxidation

“…The results for [V(HIDPA) 2 ] 2− protected from light are shown in Figure . Meanwhile, analysis of the literature on this topic suggests that the reaction may involve the formation of NO (and hyponitrite) as an intermediate, which is rapidly converted into N 2 O as mediated by the complex . Theoretical DFT calculations of the reaction mechanism confirmed this hypothesis (see below).…”

“…Most studies focused on iron or copper complexes that mimicked nitrite reductases. However, NO 2 − reduction with Ru, Mo, Ti, Cr, Co and U was also reported. Vanadium species are unconventional for this reaction, and there are only few publications on nitrite reduction by V III species from an analytical approach; none have described this reaction with V IV complexes.…”

“…[20][21][22][23][24][25][26] Soil bacteria metabolise nitrite to NO by means of nitrite reductases, which are enzymes of the EC 1.7 group [27] with iron-, [28] copper- [29] or molybdenum-based [26] active centres. [21] Despite such great importance,t he chemical homogeneous reduction of nitrites by inorganic transition metals and internal transition-metal compounds has rarely been investigated.M ost studies focusedo ni ron [33][34][35][36][37][38][39][40][41][42][43][44][45] or copper [46][47][48][49][50][51][52] complexes that mimicked nitrite reductases.H owever,N O 2 À reduction with Ru, [53] Mo, [54][55][56] Ti,C r, [56] Co [57][58][59] andU [60] was also reported. Various fungi, including A. muscaria,a lso contribute to the nitrogen cycle with nitrite as possible metabolic substrate.…”

Tetrapyrazinoporphyrazines and their metal derivatives. Part I: Synthesis and basic structural information

“…Evidence was obtained that the deprotonation of the coordinated water occurring in the basic medium leads to the electron transfer from the hydroxyl group to the TPyzPz macrocycle [198,202]. The facile reduction of the macrocycle, which also takes place in the case of Co II complex, is responsible for its catalytic activity in oxidation of different organic substrates by O 2 or nitrite [63][64][65].…”

“…Peripheral substituents endow TPyzPzs not only with solubility in organic or aqueous solvents, but also fine-tune spectral and other physico-chemical properties and biological activity. In the recent decade, these new peripherally substituted macrocycles have been especially studied as promising species for application in photodynamic therapy (PDT) [49][50][51][52][53][54][55] and plurimodal cancer therapy [56][57][58][59] and as active catalysts in redox processes [60][61][62][63][64][65]. In the following sections, different types of pyrazine-annulated porphyrazines and their metal derivatives will be considered with an emphasis primarily on the synthetic and structural aspects.…”