Site menu:

Latest news:

February 2019:
Congratulations to Jihyun Kim for winning Best Poster Award at the Minerva-Gentner Symposium on MR Spectroscopy & Molecular Imaging

Read more »

Instrumentation and Methods Development

Drawing of multi-channel NMR probeA multiplexing 19F-NMR probe and spectrometer, built by our lab, are capable of measuring T2 spin relaxation rates simultaneously in two channels from a single hyperpolarized aliquot. The simultaneous measurement improves throughput and allows for a more efficient utilization of a hyperpolarized aliquot. It is applicable for example for experiments aiming at the determination of dissociation constant between a protein and ligand.

Rapid sample injection systemA fluid handling system allows for rapid and reproducible injection of DNP hyperpolarized samples. Shown in the image are the holding tanks for pressurized gas, as well as valves and associated electronics that allow for automated injection. This system is compatible with various solvents, including water and organic liquids. It can operate under exclusion of air, which renders it compatible with organometallic compounds such as those used for olefin polymerization reactions.

Low-cost NMR spectrometerA multi-channel NMR spectrometer making use of a low-cost, off-the-shelf field programmable gate array (FPGA) board is shown in this figure. The spectrometer, which we developed in-house, is programmable with arbitrary pulse sequences. It acquires signals using a four-step phase shifting quadrature scheme. In the image, it is shown in a configuration including a microcoil and a permanent magnet.

Pulse Schemes for Hyperpolarized NMR
Much of the power of NMR spectroscopy is realized by the ability to measure chemical shift correlations, which give information about molecular structure. Conventionally, chemical shift correlations are obtained from two- or multi-dimensional NMR experiments, where an indirect evolution time segment is incremented between successive scans. These experiments rely on the spin system to return to the equilibrium populations before every new, successive scan. When using hyperpolarized sample, a large amount of signal is achieved from one single NMR scan; however, the signal does not return to the polarized state after it has been converted into a coherence, and can be used only once. Often, a sample can be hyperpolarized only once, and conventional multi-dimensional NMR methods are thus not applicable.
Spectrum obtained using the off-resonance decoupling methodWe are developing alternative NMR techniques to obtain the maximum amount of information from a hyperpolarized sample. A robust method for determining two-dimensional heteronuclear chemical shift correlations is by off-resonance decoupling. A small number of scans can be recorded with a variable flip angle from a single hyperpolarized sample, while applying a decoupling field at different frequencies (see figure). The scaling of the apparent scalar coupling can then be used for determining the chemical shift of a coupled nucleus in reminiscence of an indirect spectral dimension.

A Unique Method for Correlating Reactant and Product
picture 4 Often, NMR spectroscopy is used to determine correlations between atoms through space. However, in exchange spectroscopy experiments, correlations between different chemical forms are measured. A powerful extension of this type of experiment makes use of the fact that in a hyperpolarized experiment, typically the only signals that are observable originate from the initially present magnetization.
picture 5 By manipulating this magnetization, for example with a selective pulse, it is possible to track atoms through reactions. Here, the spin state of the initially inverted spin ~195ppm in 3-methylbenzophenone transfers to the reaction product in a Grignard reaction.

Characterization of Hyperpolarized Spin Systems
Spontaneous emission of signals from a hyperpolarized spin system Hyperpolarization can give rise to unexpected behavior of the spin system. In the case of spins polarized to negative polarization, spontaneous emission of multiple signals is observed. This behavior cannot be explained by the commonly used Bloch equations with radiation damping. A simple model explains the observed features of the signal, based on the replenishment of fresh negatively polarized sample during and after the sample injection process into the NMR coil. Modified Bloch equations include an exponentially decaying term describing the magnetization added to the system during the measurement. The functional form of this added term represents the simplest possibility for including an initially large and later decaying contribution of polarization. Nevertheless, the numerical solutions to these equations are capable of describing all of the general features of the multiple emissions in the 1H signal.