Ultimate cyclic voltammetry: an analytical examination of the reversible case

Multi-scan cyclic voltammetry may lead eventually to a repetitive current-versus-potential graph. Here such “ultimate” cyclic voltammograms are investigated mathematically for reversible electrode reactions. The voltammetric shapes, after an infinite number of cycles, are modelled and their characteristics are documented. It is predicted that the approach to truly repetitive behaviour, within an experimentally realistic length of time, requires that a specific condition—balancing the potential scan range to the initial concentrations—be satisfied.     

Combined Chemical Modification and Collision Induced Unfolding Using Native Ion Mobility-Mass Spectrometry Provides Insights into Protein Gas-Phase Structure

Native mass spectrometry is now an important tool in structural biology. Thus, the nature of higher protein structure in the vacuum of the mass spectrometer is an area of significant interest. One of the major goals in the study of gas-phase protein structure is to elucidate the stabilising role of interactions at the level of individual amino acid residues. A strategy combining protein chemical modification together with collision induced unfolding (CIU) was developed and employed to probe the structure of compact protein ions produced by native electrospray ionisation. Tractable chemical modification was used to alter the properties of amino acid residues, and ion mobility-mass spectrometry (IM-MS) utilised to monitor the extent of unfolding as a function of modification. From these data the importance of specific intramolecular interactions for the stability of compact gas-phase protein structure can be inferred. Using this approach, and aided by molecular dynamics simulations, an important stabilising interaction between K6 and H68 in the protein ubiquitin was identified, as was a contact between the N-terminus and E22 in a ubiquitin binding protein UBA2.     

Jacobian elliptic functions describe the properties of the diffuse charge region in narrow electrochemical cells

Diffuse double layers merge in narrow electrochemical cells. The distribution of ions, and the accompanying electrical properties, are predicted under static conditions that are mild enough not to engender electrochemical reactions. The predictions rest on assumptions that are not egregiously unreasonable. The potential and concentration profiles are respectively described by Legendre elliptic integrals and Jacobian elliptic functions. The special case of pure water is examined and the relevance of this study to water desalination is briefly addressed.     

Looking glass mechanism-based inhibition of peptidylglycine α-amidating monooxygenase

Carboxyl-terminal amidation, a required post-translational modification for the bioactivation of many peptide hormones, entails sequential enzymatic action by peptidylglycine α-monooxygenase (PAM, EC 1.14.17.3) and peptidylamidoglycolate lyase (PGL, EC 4.3.2.5). We have previously demonstrated that PAM and PGL exhibit strict tandem reaction stereospecificities, with PAM producing exclusively α-hydroxyglycine moieties of absolute configuration (S), and PGL being reactive only toward (S)-α-hydroxyglycines, and we have also shown that PAM exhibits strict P₂-subsite stereospecificity toward both peptide substrates and peptidyl competitive inhibitors. Herein, it is reported that the inhibitory stereochemistry of olefinic mechanism-based amidation inhibitors differs from the strict subsite stereospecificity exhibited by PAM toward substrates and reversible competitive inhibitors. Kinetic analyses of mechanism-based irreversible inhibition of PAM by the (S)- and (R)-enantiomers of 5-acetamido-4-oxo-6-phenyl-2-hexenoic acid were carried out using the rigorous progress curve method. The two enantiomers were found to exhibit very similar values of K_I and k_inact and in both cases kinetic analysis confirmed that irreversible inhibition occurs strictly at the substrate binding site with no ESI complex being formed during the catalytic processing of these irreversible inhibitors. Molecular docking studies were carried out to help rationalize the sharp contrast in the stereospecificity of PAM toward irreversible inhibitors versus substrates and competitive inhibitors. The results revealed that, in contrast to substrates, both docked enantiomers of the olefinic irreversible inhibitors are well-positioned to undergo catalytic processing at the Cu center that gives rise to irreversible inhibition. Taken together, these results provide one of the first clear examples where the stereospecificity of a particular enzyme toward mechanism-based irreversible inhibitors differs from that for substrates and competitive inhibitors.     

How does a reversible electrode respond in a.c. voltammetry? Part 2: solutions for the periodic current amplitudes

Journal of Solid State Electrochemistry - In a.c. voltammetry, a programmed electrical potential—a linear ramp modulated by a sine wave of frequency ω and modest amplitude—is...     

The propagation of radiation through a medium containing a component that absorbs the radiation and is steadily destroyed by it

The Beer–Lambert law is inadequate to describe the absorption of radiation by a medium if the absorbing component is being simultaneously destroyed by the radiation. A replacement law is derived and solved in terms of a family of polynomials. The solution is confirmed numerically and by simulation.     

Electrode “edge effects” analyzed by the Green function method

An exact method based on Green's equation is used to find the diffusion-controlled faradaic current for certain electrode geometries that incorporate edges and vertices. Thereby the magnitudes of the time-independent current density associated with angled electrode/electrode and electrode/insulator junctions are calculated. As well, the square-root-of-time-dependent currents associated with vertices, receive attention. These terms extend to longer times, the Cottrell formulation appropriate for short times. Though most of the problems solved here have been tackled previously, the novel Green function approach is shown to be straightforward and intuitive.     

Ultra‐Fast Degradation of Chemical Warfare Agents Using MOF–Nanofiber Kebabs

The threat associated with chemical warfare agents (CWAs) motivates the development of new materials to provide enhanced protection with a reduced burden. Metal–organic frame‐works (MOFs) have recently been shown as highly effective catalysts for detoxifying CWAs, but challenges still remain for integrating MOFs into functional filter media and/or protective garments. Herein, we report a series of MOF–nanofiber kebab structures for fast degradation of CWAs. We found TiO₂ coatings deposited via atomic layer deposition (ALD) onto polyamide‐6 nanofibers enable the formation of conformal Zr‐based MOF thin films including UiO‐66, UiO‐66‐NH₂, and UiO‐67. Cross‐sectional TEM images show that these MOF crystals nucleate and grow directly on and around the nanofibers, with strong attachment to the substrates. These MOF‐functionalized nanofibers exhibit excellent reactivity for detoxifying CWAs. The half‐lives of a CWA simulant compound and nerve agent soman (GD) are as short as 7.3 min and 2.3 min, respectively. These results therefore provide the earliest report of MOF–nanofiber textile composites capable of ultra‐fast degradation of CWAs.     

A general solution of the diffusion equation for semiinfinite geometries

The solution of the time-dependent diffusion equation in a semiinfinite planar, cylindrical, or spherical geometry with common initial and asymptotic boundary conditions is considered. It is shown that this boundary value problem may be described by a single equation which involves only a first order spatial derivative and a half order time derivative. The replacement is exact in the planar and spherical geometry cases but approximate in the cylindrical case. This replacement permits the solution of the original boundary value problem to be written for any boundary condition at the origin. It also leads to a simple relationship between the boundary flux and the boundary intensive variable, which does not require a calculation of the intensive variable at all positions and times.