Three-dimensional electrophoretic NMR correlation spectroscopy

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.

Counterion condensation and effective charge of poly(styrenesulfonate)

The effective charge of poly(styrenesulfonate) has been investigated by diffusion and electrophoresis nuclear magnetic resonance (NMR). While the electrophoretic mobility is determined in the electrophoresis NMR experiment, the hydrodynamic friction is determined from diffusion NMR using Einstein's formula. On the timescale of the NMR experiment a steady state is reached, which results from the force balance between the electric field and the hydrodynamic friction from that the effective charge is calculated without any further model. For the monomer and short polymers the effective charge is equal to the nominal charge, the difference increases with an increasing degree of polymerisation. Increasing the ionic strength of the solution leads to enhanced counterion condensation. If the dielectric constant of the solution is lowered, condensation of counterions is enhanced as well. A lowered effective charge results in reduced repelling forces along the polymer chain and thus in a more compact conformation of the polymer as reflected in the hydrodynamic size. The effective charge of poly(styrenesulfonate) has been studied experimentally as a function of the degree of polymerisation, of the ionic strength and the dielectric constant of the solution.

High flow-resolution for mobility estimation in 2D-ENMR of proteins using maximum entropy method (MEM-ENMR)

Multidimensional electrophoretic NMR (nD-ENMR) is a potentially powerful tool for structural characterization of co-existing proteins and protein conformations. By applying a DC electric field pulse, the electrophoretic migration rates of different proteins were detected experimentally in a new dimension of electrophoretic flow. The electrophoretic mobilities were employed to differentiate protein signals. In U-shaped ENMR sample chambers, individual protein components in a solution mixture followed a cosinusoidal electrophoretic interferogram as a function of its unique electrophoretic migration rate. After Fourier transformation in the electrophoretic flow dimension, the protein signals were resolved at different resonant frequencies proportional to their electrophoretic mobilities. Currently, the mobility resolution of the proteins in the electrophoretic flow dimension is limited by severe truncations of the electrophoretic interferograms due to the finite electric field strength available before the onset of heat-induced convection. In this article, we present a successful signal processing method, the Burg's maximum entropy method (MEM), to analyze the truncated ENMR signals (MEM-ENMR). Significant enhancement in flow resolution was demonstrated using two-dimensional ENMR of two protein samples: a lysozyme solution and a solution mixture of bovine serum albumin (BSA) and ubiquitin. The electrophoretic mobilities of lysozyme, BSA and ubiquitin were measured from the MEM analysis as 7.5x10(-5), 1.9x10(-4) and 8.7x10(-5) cm2 V-1 s-1, respectively. Results from computer simulations confirmed a complete removal of truncation artifacts in the MEM-ENMR spectra with 3- to 6-fold resolution enhancement.

Electrophoretic NMR studies of polymer and surfactant systems

The aim of this tutorial review is to introduce to a broader readership the emerging technique of electrophoretic NMR (eNMR). The "electrophoretic" element of the technique refers to the fact that charged particles are induced to flow by the application of an electric field. This flow is measured using pulsed-gradient spin-echo NMR (PGSE-NMR). The great potential of this experimental approach is the fact that NMR is chemically selective and non-invasive. eNMR, especially when combined with the more established PGSE-NMR experiment, may therefore be used to quantify the structure of multi-component systems via the dynamics and charge of each species within a complex mixture. Accordingly, eNMR is likely to be of great significance for colloid scientists, biologists, technologists and formulation scientists.

Constant-time multidimensional electrophoretic NMR

Multidimensional electrophoretic NMR (ENMR) has been introduced to determine structures of coexisting proteins and protein conformations in solution. Signals of different proteins are separated in a new dimension of electrophoretic flow according to their characteristic electrophoretic mobilities. The electrophoretic interferograms have been generated in the flow dimension in two approaches by incrementing either the amplitude or the duration of the electric field. The ENMR method of incrementing the duration of the electric field, however, introduces severe signal decays due to molecular diffusion and spin relaxation, limiting the effectiveness of the method. In this study, an improved method of constant-time multidimensional ENMR (CT-ENMR) has been proposed and successfully tested. The time delays between the magnetic field gradients and the RF pulses are kept constant in this new method so that the molecular diffusion and spin relaxation processes contribute to only a constant factor of signal amplitude. As an alternative approach of incrementing the amplitude of the electric field, this novel method significantly enhances our capability and potential in characterizing structural changes of interacting proteins during biological signaling processes. The CT-ENMR method is particularly useful in studies where the amplitude-incrementing of the electric field is not optimal. For example, the CT-ENMR method is superior when the electric field is applied in the direction not parallel to the static magnetic field B(0) to the xy-magnetization. The new method was successfully demonstrated with a sample solution containing 100 mM 4,9-dioxa-1,12-dodecanediamine and 100 mM L-aspartic acid in D(2)O.

Convection compensated electrophoretic NMR

A novel method of convection compensated ENMR (CC-ENMR) has been developed to detect electrophoretic motion of ionic species in the presence of bulk solution convection. This was accomplished using a gradient moment nulling technique to remove spectral artifacts from heat-induced convection and using the polarity switch of the applied electric field to retain spin phase modulations due to electrophoretic flow. Experiments were carried out with a mixture of 100 mM L-aspartic acid and 100 mM 4,9-dioxa-1,12-dodecanediamine to demonstrate this new method of ENMR. CC-ENMR enhances our previously developed capillary array ENMR (CA-ENMR) in solving the convection problem. The combined CA- and CC-ENMR approach strengthens the potential of multidimensional ENMR in simultaneous structural determination of coexisting proteins and protein conformations in biological buffer solutions of high ionic strength. Structural mapping of interacting proteins during biochemical reactions becomes possible in the future using ENMR techniques, which may have a profound impact on the understanding of biological events, including protein folding, genetic control, and signal transduction in general.