Solvent-induced hydrogen tunnelling in ascorbate proton-coupled electron transfers

Proton-coupled electron transfer of catechin in tea wine: the enhanced mechanism of anti-oxidative capacity

Tea wine is a Chinese traditional alcoholic drink made by cereal and tea leaves. It is rich in tea polyphenols, caffeine, amino acids, and protons and possesses various healthcare functions. In this work, electrochemical methods, as well as density functional theory (DFT) calculations, were adopted to reveal the proton-coupled electron-transfer process of catechin in tea wine. The electrochemical results showed that the catechin preferred hydrogen-bonding with ethanol and formed molecular clusters. Thus, the direct electron-transfer process of catechin changed to proton-coupled electron transfer. This procedure reduced the energy barrier of the redox reaction and enhanced the anti-oxidative capacity. Subsequently, DFT calculations were employed to explore the bond length, bond energy, and HOMO-LUMO energy gap of catechin, which confirmed the above-mentioned mechanism. Our work offers some positive value for the scientific promotion of traditional food and a greater understanding of the health mechanisms in terms of chemistry.

Kinetic Isotope Effects and Hydrogen Tunnelling in PCET Oxidations of Ascorbate: New Insights into Aqueous Chemistry?

Recent experimental studies of kinetic isotope effects (KIE-s) and hydrogen tunnelling comprising three proton-coupled electron transfer (PCET) oxidations of ascorbate monoanion, (a) in aqueous reaction solutions, (b) in the mixed water-organic cosolvent systems, (c) in aqueous solutions of various salts and (d) in fairly diluted aqueous solutions of the various partial hydrophobes are reviewed. A number of new insights into the wealth of the kinetic isotope phenomena in the PCET reactions have been obtained. The modulation of KIE-s and hydrogen tunnelling observed when partially hydrophobic solutes are added into water reaction solution, in the case of fairly diluted solutions is revealed as the strong linear correlation of the isotopic ratios of the Arrhenius prefactors Ah/Ad and the isotopic differences in activation energies ΔE_a (D,H). The observation has been proposed to be a signature of the involvement of the collective intermolecular excitonic vibrational dynamics of water in activation processes and aqueous chemistry.

Hydrogen Tunnelling as a Probe of the Involvement of Water Vibrational Dynamics in Aqueous Chemistry?

Our study of tunnelling in proton-coupled electron transfer (PCET) oxidation of ascorbate with hexacyanoferrate(III) follows the insights obtained from ultrafast 2D IR spectroscopy and theoretical studies of the vibrational water dynamics that led to the proposal of the involvement of collective intermolecular excitonic vibrational water dynamics in aqueous chemistry. To test the proposal, the hydrogen tunnelling modulation observed in the PCET reaction studied in the presence of low concentrations of various partial hydrophobic solutes in the water reaction system has been analyzed in terms of the proposed involvement of the collective intermolecular vibrational water dynamics in activation process in the case. The strongly linear correlation between common tunnelling signatures, isotopic values of Arrhenius prefactor ratios ln AH/AD and isotopic differences in activation enthalpies ΔΔH‡ (H,D) observed in the process in fairly diluted water solutions containing various partial hydrophobic solutes (such as dioxane, acetonitrile, ethanol, and quaternary ammonium ions) points to the common physical origin of the phenomenon in all the cases. It is suggested that the phenomenon can be rooted in an interplay of delocalized collective intermolecular vibrational dynamics of water correlated with vibrations of the coupled transition configuration, where the donor-acceptor oscillations, the motions being to some degree along the reaction coordinate, lead to modulation of hydrogen tunnelling in the reaction.

Does the pressure dependence of kinetic isotope effects report usefully on dynamics in enzyme H-transfer reactions?

The temperature dependence of kinetic isotope effects (KIEs) has emerged as the main experimental probe of enzymatic H-transfer by quantum tunnelling. Implicit in the interpretation is a presumed role for dynamic coupling of H-transfer chemistry to the protein environment, the so-called 'promoting motions/vibrations hypothesis'. This idea remains contentious, and others have questioned the importance and/or existence of promoting motions/vibrations. New experimental methods of addressing this problem are emerging, including use of mass-modulated enzymes and time-resolved spectroscopy. The pressure dependence of KIEs has been considered as a potential probe of quantum tunnelling reactions, because semi-classical KIEs, which are defined by differences in zero-point vibrational energy, are relatively insensitive to kbar changes in pressure. Reported combined pressure and temperature (p-T) dependence studies of H-transfer reactions are, however, limited. Here, we extend and review the available p-T studies that have utilized well-defined experimental systems in which quantum mechanical tunnelling is established. These include flavoproteins, quinoproteins, light-activated enzymes and chemical model systems. We show that there is no clear general trend between the p-T dependencies of the KIEs in these systems. Given the complex nature of p-T studies, we conclude that computational simulations using determined (e.g. X-ray) structures are also needed alongside experimental measurements of reaction rates/KIEs to guide the interpretation of p-T effects. In providing new insight into H-transfer/environmental coupling, combined approaches that unite both atomistic understanding with experimental rate measurements will require careful evaluation on a case-by-case basis. Although individually informative, we conclude that p-T studies do not provide the more generalized insight that has come from studies of the temperature dependence of KIEs.

Proton tunnelling and promoting vibrations during the oxidation of ascorbate by ferricyanide?

A combination of the temperature- and pressure-dependencies of the kinetic isotope effect on the proton coupled electron transfer during ascorbate oxidation by ferricyanide suggests that this reference reaction may exploit vibrationally assisted quantum tunnelling of the transferred proton.