Improved accuracy and precision in electrophoretic NMR experiments. Current control and sample cell design.

Electrophoretic NMR has the capacity to provide unique physico-chemical information but is limited by a variety of experimental artifacts, such as thermal convection and electrolytic products in the sample. Here we present some simple modifications to the experimental hardware and protocol that, in a significant number of cases, can much improve experimental accuracy and precision. We show that one can strongly reduce artifacts in a symmetric sample cell with an appropriate feeding of current and with a porous plug suitably inserted. This latter feature requires that the electric field pulses across the sensitive volume are implemented as current-controlled pulses applied to the sample. Measurements with current-controlled pulses have the additional advantage of not requiring calibration with samples of known electrophoretic mobility.

Assessing 2D electrophoretic mobility spectroscopy (2D MOSY) for analytical applications.

Electrophoretic displacement of charged entity phase modulates the spectrum acquired in electrophoretic NMR experiments, and this modulation can be presented via 2D FT as 2D mobility spectroscopy (MOSY) spectra. We compare in various mixed solutions the chemical selectivity provided by 2D MOSY spectra with that provided by 2D diffusion-ordered spectroscopy (DOSY) spectra and demonstrate, under the conditions explored, a superior performance of the former method. 2D MOSY compares also favourably with closely related LC-NMR methods. The shape of 2D MOSY spectra in complex mixtures is strongly modulated by the pH of the sample, a feature that has potential for areas such as in drug discovery and metabolomics. Copyright © 2016 The Authors. Magnetic Resonance in Chemistry published by John Wiley & Sons Ltd.

Molecular complexation and binding studied by electrophoretic NMR spectroscopy.

Electrophoretic mobilities obtained on a molecularly selective manner by electrophoretic NMR can be used to provide a quantitative characterization of the composition and stoichiometry of molecular complexes. This is demonstrated in complexes formed by uncharged cyclodextrins which attain an electrophoretic mobility upon inclusion of charged surfactants.

Sensitive and robust electrophoretic NMR: instrumentation and experiments.

Although simple as a concept, electrophoretic NMR (eNMR) has so far failed to find wider application. Problems encountered are mainly due to disturbing and partly irreproducible convection-like bulk flow effects from both electro-osmosis and thermal convection. Additionally, bubble formation at the electrodes and rf noise pickup has constrained the typical sample geometry to U-tube-like arrangements with a small filling factor and a low resulting NMR sensitivity. Furthermore, the sign of the electrophoretic mobility cancels out in U-tube geometries. We present here a new electrophoretic sample cell based on a vertically placed conventional NMR sample tube with bubble-suppressing palladium metal as electrode material. A suitable radiofrequency filter design prevents noise pickup by the NMR sample coil from the high-voltage leads which extend into the sensitive sample volume. Hence, the obtained signal-to-noise ratio of this cell is one order of magnitude higher than that of our previous U-tube cells. Permitted by the retention of the sign of the displacement-related signal phase in the new cell design, an experimental approach is described where bulk flow effects by electro-osmosis and/or thermal convection are compensated through parallel monitoring of a reference signal from a non-charged species in the sample. This approach, together with a CPMG-like pulse train scheme provides a superior first-order cancellation of non-electrophoretic bulk flow effects.

Solute-solvent contact by intermolecular cross-relaxation. 2. The water-micelle interface and the micellar interior.

The intermolecular dipole-dipole cross-relaxation is measured between 19F nuclei of sodium perfluorooctanoate in micelles and 1H nuclei of the water solvent. The cross-relaxation rates for fluorines in the different moieties along the surfactant vary strongly by the resonance frequency in the investigated range of 188-470 MHz. This frequency dependence indicates that the cross-relaxation between water and amphiphilic aggregates is not controlled solely by the fast local water dynamics but significantly contributed to by the long-range translational diffusion of water. The cross-relaxation rates, analyzed in the framework of a model (Nordstierna, L.; Yushmanov, P. V.; Furó, I. J. Chem. Phys. 2006, 125, 074704), provide information about the dynamic retardation of water molecules by the micellar headgroup region and the location of the various moieties along the hydrophobic tail with respect to the water-micelle interface. Both intermolecular cross-relaxation and aggregation-induced 19F chemical shift changes indicate no direct water contact to fluorines except for those closest to the head group.

Solute-solvent contact by intermolecular cross relaxation. I. The nature of the water-hydrophobic interface.

Intermolecular cross-relaxation rates between solute and solvent were measured by {1H} 19F nuclear magnetic resonance experiments in aqueous molecular solutions of ammonium perfluoro-octanoate and sodium trifluoroacetate. The experiments performed at three different magnetic fields provide frequency-dependent cross-relaxation rates which demonstrate clearly the lack of extreme narrowing for nuclear spin relaxation by diffusionally modulated intermolecular interactions. Supplemented by suitable intramolecular cross-relaxation, longitudinal relaxation, and self-diffusion data, the obtained cross-relaxation rates are evaluated within the framework of recent relaxation models and provide information about the hydrophobic hydration. In particular, water dynamics around the trifluoromethyl group in ammonium perfluoro-octanoate are more retarded than that in the smaller trifluoroacetate.

A temperature-jump design for conventional NMR probes.

The design and performance of a simple probe insert for temperature-jump experiments in conventional NMR probes is described. The insert uses the output from conventional NMR amplifiers for heating conductive aqueous samples with a rate of 30-80 K/s for 200 W radiofrequency power. The observed dependence of the heating rate on sample conductivity is explained by the dominance of dielectric heating. Factors governing the temperature gradient within the sample are discussed.

Micellar kinetics of a fluorosurfactant through stopped-flow NMR.

19F NMR chemical shifts and transverse relaxation times T2 were measured as a function of time after quick stopped-flow dilution of aqueous solutions of sodium perfluorooctanoate (NaPFO) with water. Different initial concentrations of micellar solution and different proportions of mixing were tested. Previous stopped-flow studies by time-resolved small-angle X-ray scattering (TR-SAXS) detection indicated a slow (approximately 10 s) micellar relaxation kinetics in NaPFO solutions. In contrast, no evidence of any comparable slow (>100 ms) relaxation process was found in our NMR studies. Possible artifacts of stopped-flow experiments are discussed as well as differences between NMR and SAXS detection methods. Upper bounds on the relative weight of a slow relaxation process are given within existing kinetic theories of micellar dissolution.

A rapid-mixing design for conventional NMR probes.

A simple stopped-flow design for rapid mixing of two liquids within the NMR probe is presented. The device uses no switches or relays but exploits instead the torque exerted by the magnetic field on a current-leading-coil to open and close the start and stop valves. Two serially arranged tangential jet mixer blocks provide a homogeneous mixture with, depending on conditions and requirements, a filling time in the 50-100 ms range and a subsequent stabilization time in the range of 10-40 ms as tested by mixing various combinations of liquids and observing their 1H NMR spectrum. Factors influencing the mixing process are analyzed.