Robin Dykstra

A nuclear magnetic resonance study of Antarctic sea ice brine diffusivity

We have measured the diffusive motion of water molecules in the brine inclusions of Antarctic sea ice using a specially constructed nuclear magnetic resonance (NMR) apparatus. The method relies on the use of pulsed magnetic field gradients in precise analogy to well established laboratory procedures. One version of the apparatus utilised core samples extracted from the ice sheet which were subsequently analysed on site while a later version utilised a probehead which was inserted into the ice sheet, thus minimising any sample perturbation. The diffusive motion of water molecules in the brine inclusions is found to be strongly anisotropic, and, over short length scales, exhibits a rapidity greatly in excess of that expected for thermal equilibrium Brownian behaviour, an effect which we attribute to convective transport.

Rheo-NMR Studies of an Enzymatic Reaction: Evidence of a Shear-Stable Macromolecular System

Understanding the effects of shear forces on biopolymers is key to understanding how biological systems function. Although currently there is good agreement between theoretical predictions and experimental measurements of the behavior of DNA and large multimeric proteins under shear flow, applying the same arguments to globular proteins leads to the prediction that they should only exhibit shear-induced conformational changes at extremely large shear rates. Nevertheless, contradictory experimental evidence continues to appear, and the effect of shear on these biopolymers remains contentious. Here, a custom-built rheo-NMR cell was used to investigate whether shear flow modifies enzyme action compared with that observed quiescently. Specifically, (1)H NMR was used to follow the kinetics of the liberation of methanol from the methylesterified polysaccharide pectin by pectinmethylesterase enzymes. Two different demethylesterifying enzymes, known to have different action patterns, were used. In all experiments performed, Couette flows with shear rates of up to 1570 s(-1) did not generate detectable differences in the rate of methanol liberation compared to unsheared samples. This study provides evidence for a shear-stable macromolecular system consisting of a largely beta-sheet protein and a polysaccharide, in line with current theoretical predictions, but in contrast to some other experimental work on other proteins.

Computerised data acquisition and analysis in the biochemistry teaching laboratory

We have successfully implemented a simple computerised data acquisition system which has been used extensively in our biochemistry teaching laboratory classes. Our experience suggests that it is important to consider carefully what the computers are required to do in the laboratory well before their introduction. Finally, we believe that it is desirable to keep the system simple and sufficiently versatile that it can be adapted to several different uses without too much difficulty.

Rheo-NMR studies of a nematic worm-like micelle system in a high-shear-rate regime

Deuterium NMR spectroscopy has been used to study the director dynamics of the nematic liquid-crystal system cetyl trimethylammonium bromide (CTAB)/D2O under the action of applied viscous torques. Shear forces were applied using a custom-built Couette cell that was introduced into an NMR superconducting magnet so that its rotational axis was parallel to the magnetic field direction, along which the liquid-crystal director originally aligned. Subsequently, a series of experiments was undertaken in which the inner cylinder of the cell was rotated continuously, at different rates, using a stepper motor. Above a threshold shear rate it was found that the director of the entire sample reoriented until a steady-state orientation was reached, the value of which depended on the average applied shear rate. These experiments revealed the presence of two distinct regimes. At the lower shear rates used, the observed behaviour was successfully captured by modelling the competition between the applied viscous torque and magnetic terms, and, upon cessation of shear the resultant spectra evolved during the relaxation of the director orientation in a way that simply mirrored those observed after the initiation of shearing. However, upon increasing the shear rate further, another regime was found in which the time evolution of the spectra towards the steady state was clearly not the simple reverse of that observed during relaxation. It is hypothesised that at these faster shear rates the relevant timescale of induced reorientation becomes short compared with the characteristic timescale of the thermally-activated scission and recombination of the micelles. As such, shear forces in this regime not only reorient micelles but also provide a novel mechanism for micelles to reach the steady state reorientation via the “disassembly” of micelles. The time evolution of the angular distribution function of the director orientation was extracted in these cases via fitting of the complex experimental spectra using a genetic algorithm and indeed shows that while a proportion of micelles appear to simply reorient as intact entities, as seen at lower rates, a distinct population circumvent this route, appearing more rapidly at the steady state orientation.

A compact Class D RF power amplifier for mobile nuclear magnetic resonance systems

A 20 MHz Class D amplifier with an output of 100 W of RF power has been developed. The compact size printed circuit board area of 50 cm2 and efficiency of 73% make it suitable for mobile nuclear magnetic resonance (NMR) systems. Test results show that the rise and ring down times of the amplifier are less than 0.2 μs, and it is capable of producing constant amplitude pulses as short as 2 μs. Experiments using a Carr Purcell Meiboom Gill pulse sequence with a NMR MOUSE sensor confirm that the Class D amplifier is suitable for mobile NMR applications.

Magnetic resonance measurements of polar sea ice

A Nuclear Magnetic Resonance (NMR) sensor system has been developed to measure the brine fraction profile of polar sea ice. The system is based on a 0.3 T Halbach permanent magnet array and has significantly more sensitivity than earlier Earths field NMR systems deployed in Antarctica. Brine fraction experiments were performed in November 2009 near Ross Island, Antarctica and the results obtained agreed well with the traditional temperature/salinity determination method. The advantages of the NMR technique are that it is a direct method and it has the ability to obtain data within minutes compared to several hours with the traditional method.

Mobile Permanent Magnet NMR Systems for Field Studies on Antarctic Sea Ice

A permanent magnet based Nuclear Magnetic Resonance (NMR) system has been developed to measure various properties of the brine in sea ice. In contrast to systems utilizing the Earth’s field in previous expeditions, the comparatively high field of 0.28 T provided by permanent magnets allows for shorter measurement times and advanced NMR protocols. In November 2009 the instrument was deployed to Antarctica to investigate sea ice grown under natural conditions. In this paper the instrument is described, preliminary results are reported and future options for mobile NMR applications in Antarctica discussed.