Kenneth R. Jeffrey
NMR and neutron diffraction studies of model membrane systems, NMR of superionic conducting polymers and glasses, dynamic NMR microscopy.
Present Research Activities
Anaesthetics, sterols, herbicides, pesticides, some food preservatives and drugs such as the cannabinoids are all relatively small hydrophobic molecules which accumulate in biological membranes. In many cases the mode of action of these compounds is unknown and as a first step it is essential to know their location and orientation in the lipid bilayer which forms the structural matrix of biomembranes. Deuterium NMR combined with neutron and X-ray diffraction studies provide a unique method for the determination of both the average orientation and location of these hydrophobic solutes in the lipid bilayer. Neutron diffraction is particularly useful because the focus can be placed on the solute molecule by collecting results for both protonated and deuterated species and looking at the difference patterns. Experimental results have been obtained for tetrahydrocannabinoid, the principle psychoactive constituent of marijuana, and 2,4-D, a widely used herbicide.
Fast ionic conductors are interesting systems for research on mass transport in solid materials as well as for possible application to battery and fuel cell technology. Poly(propylene glycol), PPG, or poly(propylene oxide), PPO, doped with ionic salts such as LiCF3SO3 or NaClO4 have extraordinary high room temperature ionic conductivities, up to 10-2 S/cm. NMR has proven to be a very valuable tool for the study of the various sites that the mobile ion (Li or Na) can occupy with in the glassy framework. Relaxation time and spin-echo, magnetic field gradient, diffusion measurements are used to probe the motion of the ions through the glassy polymer matrix. Recently our attention has turned to proton conductors formed as a hydrogel by doping polypropylene carbonate with phosphoric acid. The conductivity of these materials in an order of magnitude greater than other polymer electrolytes and have real application in "smart window" technology. Deuterium NMR is being used to examine the dynamics of the mobile hydrogen atoms.
The glass transition has been a focus of recent experimental and theoretical studies but many aspect are not well understood. In addition this transition is important to food processing operations such as freezing, drying and extrusion. It affects such quality attributes as texture, stability, flavour release and spoilage. For example if a food matrix forms a glass at the storage temperature, crystallization of water and its subsequent effects on structure and texture are very much reduced. A relatively simple system which is of prime importance to the food industry is the sugar/water mixture. Deuterium NMR of the water and the sugar water molecules are being carried out to determine the correlation times for molecular reorientation throughout the glass transition region.
NMR imaging has become a standard diagnostic tool in many hospitals. The resolution of whole body imagers is about 1 mm which is perfectly adequate for medical diagnosis. NMR, however, has the sensitivity to be used as a microscope to look at small objects with a resolution of about 10 ?m. The true utility of NMR microscopy is its ability to spatially resolve not only nuclear spin density but other NMR parameters such as the spin-spin and spin-lattice relaxation times, chemical shift together with flow and diffusion in a non-invasive manner. Dynamic NMR microscopy combines NMR imaging techniques with velocity measurements to map velocities with a resolution of a few microns per second and a spatial resolution of a few tens of microns. Experiments are planned to study flow in both Newtonian and non-Newtonian fluids. In Newtonian fluids, where the Navier-Stokes equations are believed to contain all the basic physics, experimental determinations of the velocity profile provide a much needed check of numerical methods of solution of these equations for complex geometries. In rheological studies of non-Newtonian fluids, the constitutive equations describing the relationship between stress and strain must be validated by comparing theory with experiment. Measurements of the components of the velocity for relatively simple geometries provide excellent tests of the proposed constitutive quation.