Martin Badertscher

Potential Drifts of Solid‐Contacted Ion‐Selective Electrodes Due to Zero‐Current Ion Fluxes Through the Sensor Membrane

A thin aqueous layer is formed between the polymeric ion-selective membrane and an inner gold electrode. Its composition can be altered upon sample changes, which leads to drifting potentials. It is shown by theory and experiment that interfering ions enter the inner layer much faster than primary ones. Measurement protocols with equal contact times for the sample and the reconditioning solution can therefore not eliminate the effect of changes in the composition of the inner solution. The formation of an inner aqueous film and corresponding drifts for solid-contacted membrane electrodes can be avoided by creating a lipophilic self-assembled monolayer on the surface of the inner electrode.

13C NMR Spectroscopy

The characterization of an organometallic complex involves obtaining a complete understanding of the same right from its identification to the assessment of its purity content, to even elucidation of its stereochemical features. Detailed structural understanding of the organometallic compounds is critical for obtaining an insight on its properties and which is achieved based on the structure-property paradigm.

Effects of controlled current on the response behavior of polymeric membrane ion-selective electrodes

The influence of external currents on the potential response of polymeric membrane ion-selective electrodes (ISEs) is analyzed theoretically. The treatment is based on the well-established potential model of ISE membranes, extended by contributions from current flow and corresponding electrodialytic ion fluxes through the membrane. A general implicit solution is derived that describes the potential response in terms of the external current, the selectivity characteristics of the membrane, and the compositions of the membrane, the sample solution, and the inner solution, respectively. Explicit relationships are given for limiting cases, e.g. when only one kind of cation is present in the solution or when the disturbance by interfering cations is small. For the latter case, theoretical curves are compared to experimental ones and are shown to give a good correspondence.

Monitoring Pb2+ with optical sensing films

Abstract A new lipophilic ionophore, N,N,N′,N′ -tetradodecyl-3,6-dioxaoctane-1-thio-8-oxodiamide (ETH 5493), is characterized in optical sensing films with a view to being used for environmental monitoring. It sufficiently discriminates all alkali and alkaline earth metal ions and, contrary to what is observed with the corresponding dithioamide, does not suffer from irreversible changes upon contact with Ag + and Hg 2+ salts. Different response functions are found for bulk optodes with ratios of chromoionophore/ion exchanger above and below 1.0. The data obtained fit the theoretical response function with a standard deviation of the residuals of 0.0012 absorbance units (AU) without any systematic error. When comparing different membranes, the precision is still high (±0.0029 AU) but the residuals show a certain systematic trend.

A novel formalism to characterize the degree of unsaturation of organic molecules.

The existing formalism to calculate the degree of unsaturation from the molecular formula of organic molecules cannot be applied to charged and/or disconnected species. Moreover, the calculated value depends on the assumed formal valence of each of the elements. In this work, we introduce a new formalism that eliminates these problems. The suggested property, degree of unsaturation, can be calculated from the molecular formula as well as from any structural representation of a molecule corresponding to that molecular formula.

Determination of protein-ligand binding constants of a cooperatively regulated tetrameric enzyme using electrospray mass spectrometry.

This study highlights the benefits of nano electrospray ionization mass spectrometry (nanoESI-MS) as a fast and label-free method not only for determination of dissociation constants (KD) of a cooperatively regulated enzyme but also to better understand the mechanism of enzymatic cooperativity of multimeric proteins. We present an approach to investigate the allosteric mechanism in the binding of inhibitors to the homotetrameric enzyme fructose 1,6-bisphosphatase (FBPase), a potential therapeutic target for glucose control in type 2 diabetes. A series of inhibitors binding at an allosteric site of FBPase were investigated to determine their KDs by nanoESI-MS. The KDs determined by ESI-MS correlate very well with IC50 values in solution. The Hill coefficients derived from nanoESI-MS suggest positive cooperativity. From single-point measurements we could obtain information on relative potency, stoichiometry, conformational changes, and mechanism of cooperativity. A new X-ray crystal structure of FBPase tetramer binding ligand 3 in a 4:4 stoichiometry is also reported. NanoESI-MS-based results match the current understanding of the investigated system and are in agreement with the X-ray structural data, but provide additional mechanistic insight on the ligand binding, due to the better dynamic resolution. This method offers a powerful approach for studying other proteins with allosteric binding sites, as well.

Design features of ionophores for ion selective electrodes

ustract - Ionophores as components for analytically relevant ion selective electrodes have to exhibit a specified lipophilicity and a free energy of activation of < 45-65 kJ mol-l for the ligand exchange reaction. The selectivity of membranes may be tuned by membrane-technological interventions (dielectric constant of membrane phase, ionophore concentration, concentration of ionic sites confined to membrane phase). Molecular modelling techniques can guide the design of ionophores of a given inherent selectivity and reduce the number of ionophore candidates to be synthesized. The selectivity enhancement expected for a covalent link of ionophore subunits is analyzed. Significant effects are predicted only if suitable ligand preorganization is realized.

Self-Aliquoting Microarray Plates for Accurate Quantitative Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry

Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) is a fast analysis tool employed for the detection of a broad range of analytes. However, MALDI-MS has a reputation of not being suitable for quantitative analysis. Inhomogeneous analyte/matrix co-crystallization, spot-to-spot inhomogeneity, as well as a typically low number of replicates are the main contributing factors. Here, we present a novel MALDI sample target for quantitative MALDI-MS applications, which addresses the limitations mentioned above. The platform is based on the recently developed microarray for mass spectrometry (MAMS) technology and contains parallel lanes of hydrophilic reservoirs. Samples are not pipetted manually but deposited by dragging one or several sample droplets with a metal sliding device along these lanes. Sample is rapidly and automatically aliquoted into the sample spots due to the interplay of hydrophilic/hydrophobic interactions. With a few microliters of sample, it is possible to aliquot up to 40 replicates within seconds, each aliquot containing just 10 nL. The analyte droplet dries immediately and homogeneously, and consumption of the whole spot during MALDI-MS analysis is typically accomplished within few seconds. We evaluated these sample targets with respect to their suitability for use with different samples and matrices. Furthermore, we tested their application for generating calibration curves of standard peptides with α-cyano-4-hdydroxycinnamic acid as a matrix. For angiotensin II and [Glu(1)]-fibrinopeptide B we achieved coefficients of determination (r(2)) greater than 0.99 without the use of internal standards.

Making use of ion fluxes through potentiometric sensor membranes: ISEs with step responses at critical ion activities

Note: 246 Reference SAMLAB-ARTICLE-2001-027 Record created on 2009-05-12, modified on 2016-08-08

Assemble 2.0: a structure generator

Abstract Assemble 2.0, a new structure generator with a user-friendly interface is presented. The pieces of structural information that can be input are reviewed and an example of the achievable results in a fully automatic mode of operation is given. The generated structures can be ranked automatically on the basis of their 1 H and/or 13 C NMR spectra. The program is available for Windows 95, Windows 98, and Windows NT.