Electrochemical, Electrocatalytic, and Magnetic Properties of Vanadium-Containing Polyoxometalates

Mono-substituted vanadium-containing Dawson-type polyoxometalates having the general formula α1-[VIVW17X2O62]8− and α2-[VIVW17X2O62]8−, with X = As or P, were synthesised and subject to a comprehensive electrochemical study comprising the pH dependency. These POMs exhibit an electrocatalytic behaviour towards the oxidation of thiols (namely cysteine), rendering them interesting species for mimicking oxidative stress situations, at physiological pH values. The efficiency of the electro-oxidation was assessed with thiols of different nature, and the substrate that responded best was used to compare the electrocatalytic capabilities of the POM series. The magnetic behaviour of these POMs was also evaluated and compared to their analogues, α1- and α2-[VVW17X2O62]7− (X = As or P), at low temperatures and showed, as expected, a paramagnetic behaviour of VIV based compounds.

A simplified methodology: pH sensing using an in situ fabricated Ir electrode under neutral conditions

Herein, a simplified fabrication method for the producing of a pH-sensitive iridium electrode is developed. The in situ electrochemical fabrication of an iridium oxide film is optimized and shown to be achievable under neutral conditions rather than the acidic conditions hitherto employed. The formation of a pH sensitive Ir(III/IV) hydrous film is confirmed via XPS. The amperometric pH-sensing properties of this electrochemically generated material were investigated using square wave voltammetry. In the pH range 2–13, the iridium oxide redox signal has a pH dependency of 86.1 ± 1.1 mV per pH unit for midpoint potentials with uncertainties being ± 0.01–0.05 pH. Finally, the newly developed pH sensor was used to measure the pH of a natural water sample with excellent results as compared to a conventional glass pH probe.

Antimicrobial peptides with pH dependent activity and alkaline optima: their origins, mechanisms of action and potential applications

: A number of disorders and diseases are associated with conditions of high pH and many conventional antibiotics lose their efficacy under these pH conditions, generating a need for novel antimicrobials, and a potential solution to fulfil this need is antimicrobial peptides (AMPs) with high pH optima. This review shows that a variety of anionic and cationic AMPs with this pH dependency are produced by creatures across the eukaryotic kingdom, including humans, rabbits, cattle, sheep, fish and frogs. These AMPs exhibit activity against viruses, bacteria and fungi that involves membrane interactions and appear to be facilitated by a variety of mechanisms that generally promote passage across membranes to attack intracellular targets, such as DNA or protein synthesis, and / or membrane lysis. Some of these mechanisms are unknown but those elucidated include the use of bacterial pores and transporters, the self-promoted uptake pathway and established models of membrane interaction, such as the carpet mechanism, toroidal pore formation, the adoption of tilted peptide and the SHM model. A variety of potential roles have been proposed for these AMPs, including use as antivirals, antibacterials, antifungals, adjuvants to antimicrobial therapy, biomarkers of disease and probes for pathogenic microbes. In this review, these properties are described and discussed, with an emphasis on the antimicrobial mechanisms used by these AMPs and the pH dependency of these mechanisms.

Kinetics of Non-Enzymatic Synthesis of Dipeptide Cbz-Phe-Leu with AOT Reversed Micelles

The non-enzymatic synthesis of N-benzyloxycarbonyl-L-phenylalanyl-L-leucine (Cbz-Phe-Leu) from lipophilic N-benzyloxycarbonyl-L-phenylalanine (Cbz-Phe) and hydrophilic L-leucine (Leu), by N, N’-dicyclohexylcarbodiimide (DCC) as a condensing agent, was carried out using a reversed micellar system composed of bis(2-ethylhexyl) sodium sulfosuccinate (AOT) as a surfactant and isooctane. We successfully synthesized Cbz-Phe-Leu in a short time and investigated the effects of its operational conditions, the DCC concentration, w0, and the pH on the kinetic parameters and the maximum yields. For dipeptide synthesis, we had to add an excess of DCC with the substrates because of the side reactions of Cbz-Phe. From the pH dependency of the reactivity, a partially cationic form of Leu was better for a synthesis reaction because of the enrichment of Leu at the interface by anionic AOT. The optimum water content on the dipeptide synthesis was w0 = 28 due to the competition of the peptide synthesis and the side reactions. The maximum yield of Cbz-Phe-Leu was 0.565 at 80 h under optimum experimental conditions.

A Single Residue on the WPD-Loop Affects the pH Dependency of Catalysis in Protein Tyrosine Phosphatases

<p>Catalysis by protein tyrosine phosphatases (PTPs) relies on the motion of a flexible protein loop (the WPD-loop) that carries a residue acting as a general acid/base catalyst during the PTP-catalyzed reaction. The orthogonal substitutions of a non-catalytic residue in the WPD-loops of YopH and PTP1B results in shifted pH-rate profiles, from an altered kinetic p<i>K</i>_a of the nucleophilic cysteine. Compared to WT, the G352T YopH variant has a broadened pH-rate profile, similar activity at optimal pH, but significantly higher activity at low pH. Changes in the corresponding PTP1B T177G variant are more modest and in the opposite direction, with a narrowed pH profile and less activity in the most acidic range. Crystal structures of the variants show no structural perturbations, but suggest an increased preference for the WPD-loop closed conformation. Computational analysis confirms a shift in loop conformational equilibrium in favor of the closed conformation, arising from a combination of increased stability of the closed state and destabilization of the loop-open state. Simulations identify the origins of this population shift, revealing differences in the flexibility of the WPD-loop and neighboring regions. Our results demonstrate that changes to the pH dependency of catalysis by PTPs can result from small changes in amino acid composition in their WPD-loops affecting only loop dynamics and conformational equilibrium. The perturbation of kinetic p<i>K</i>_a values of catalytic residues by non-chemical processes affords a means for nature to alter an enzyme’s pH dependency by a less disruptive path than altering electrostatic networks around catalytic residues themselves. </p>

On the Mechanistic Origins of the pH-Dependency in Au-Catalyzed Glycerol Electro-Oxidation: Insight from First Principles Calculations

We have performed a comprehensive computational study to understand and explain the experimentally observed pH- and potential-dependency of alkaline electro-oxidation of glycerol on gold. It is shown that the adsorbed OH groups have a decisive role on the reaction mechanism, thermodynamics, and kinetics. Overall, our findings highlight that computational studies should explicitly account for pH and coverage effects under alkaline conditions for electrocatalytic oxidation reactions to reliably predict electrocatalytic behaviour.