Quantum chemical study of the autoxidation of ascorbate

Abstract

Reactions involved in the autoxidation of ascorbate have been investigated with quantum chemical first-principles and ab initio methods. Reaction energies and Gibbs energies of the reactions were calculated at the density functional theory level applying the gradient-corrected BP86 and the hybrid B3LYP functionals together with def2-TZVP basis sets. Results of single-point CC2, CCSD, and CCSD(T) calculations were used for calibration of the density functional theory data and show excellent agreement with the B3LYP values. Based on the Gibbs energy ascorbic acid AscH2 is found to be the energetically lowest species in aqueous solution, whereas the monoanion ascorbate AscH - is the most abundant one near pH = 7. Asc 2- was found to be the preferred reducing agent for autoxidation and oxidation processes. The results also support a metal-catalyzed synthesis of the reactive oxygen species H2 O2 according to a redox cycling mechanism proposed in literature. © 2016 Wiley Periodicals, Inc.

Ascorbate kills breast cancer cells by rewiring metabolism via redox imbalance and energy crisis

“…Those can be summed up in two major models, extracellular or intracellular [22,25]. The former suggests an abundance of hydrogen peroxide generated by the metal-catalysed auto-oxidation of ascorbate [73] selectivity against cancers depends upon the elevated oxidative burden in tumours [74]. The second model requires the cellular uptake of ascorbate, where it tips the redox homeostasis [22].…”

Isolation and Structure Determination of Echinochrome A Oxidative Degradation Products

“…The equilibrium is generally displaced in favor of the more thermodynamically stable ketol tautomer. The autooxidation of such enediols, as well as tetrahydroxy-l,4-benzoquinone, ascorbate, and Ech A, has been shown to involve the generation of intermediates such as carbon-centered free radicals and ROS including the superoxide radical anion, hydroxyl radical, and hydrogen peroxide [ 23 , 30 , 32 , 33 ]. The mechanisms of the primary attack of triplet and singlet oxygen molecules on Ech A ( 1 ), the result of which is bis- gem -diol 2 formation, were described in detail in previous studies [ 23 , 34 ].…”

Photoelectron Spectroscopic Study of Ascorbate and Deprotonated Ascorbate Anions Using an Electrospray Ion Source and a Cryogenically Cooled Ion Trap

“…This is supported by the fact that the radical anion, Asc •− , formed by removing an electron from deprotonated ascorbate, is more stable than the dianion; the radical anion is calculated to be lower in energy by 3.24 eV in the gas phase. 16 Since Asc •− is lower in energy than Asc 2− , the observed Asc 2− photodetachment transitions terminate on the excited states of the radical anion. This implies that the highest EBE peak in the fourth harmonic photoelectron spectrum of Asc 2− , i.e., the one with an onset at an EBE of ∼3.75 eV, is a transition from Asc 2− to a still higher energy excited state of Asc •− .…”

“…It should be noted the gas-phase ionization energy, 3.85 eV, is over 1 eV lower than the ionization energy of AscH − in an aqueous solution. 16 In either phase, the ascorbate anion will readily donate an electron to free radicals.…”

“…The most stable structures of ascorbic acid, the ascorbate anion, and deprotonated ascorbate have been determined using X-ray crystallography with the aid of calculations. , The vertical ionization energies and electron affinities (EA) have been theoretically calculated for ascorbic acid and the ascorbate anion, as well as deprotonated ascorbate and their radicals in both the gas phase and when solvated by water. − Another radical scavenger, vitamin E, has previously been studied via photoelectron studies . In this work, we report the anion photoelectron spectra of the ascorbate anion (AscH – ) and deprotonated ascorbate (Asc 2– ) generated by an electrospray ionization (ESI) source.…”