What is DNP-NMR?

Overview

Dynamic Nuclear Polarization (DNP) is a technique used in Nuclear Magnetic Resonance (NMR) spectroscopy wherein the electron spin polarization is transferred to the nuclear spins. In the majority of DNP experiments, electron polarization is accomplished through excitation by microwave radiation at frequencies close to the electron paramagnetic resonance (EPR) frequency of a paramagnetic dopant.

DNP, originally proposed by Overhauser* and experimentally confirmed by Carver and Slichter** in 1953, was first investigated using metallic samples such as lithium. Since that time several other microwave-driven polarization mechanisms (solid effect, cross effect, and thermal mixing) as well as other electron spin alignment techniques (optical pumping, spin injection, and certain chemical reactions) have been discovered that have continued to expand the field of research. Since the signal strength in an NMR experiment is proportional to the nuclear polarization (which is normally quite small), the polarization enhancement given by DNP yields signal to noise ratios which are several orders of magnitude higher than traditional NMR experiments. To the researcher using this NMR technique this translates into a higher resolution scan for a given acquisition time or the ability to see what is happening on finer timescales for a given spectral resolution.

*Overhauser, A., "Polarization of Nuclei in Metals," Phys. Rev., 1953 92, 411–415.

**Carver, T.R., Slichter, C.P., "Polarization of Nuclear Spins in Metals," Phys. Rev., 1953 92, 212-213.

DNP-NMR with Biradicals

Electron-electron dipole coupling plays an important role in determining the efficiency of the highest signal enhancement mechanisms (the cross effect and thermal mixing). This coupling, highly dependent on the distance between the two dipoles, can be optimized through the use of carefully selected chemistry. In a traditional DNP experiment a mono-radical is used as the paramagnetic dopant but by precisely separating two radicals (a bi-radical) through a chemical linkage the dipole interactions of the two radicals can be optimized to produce larger signal enhancements when compared to the use of mono-radicals. Further developments have produced the highly effective, water-soluble bi-radical known as TOTAPOL which is compatible with salt solutions, gives much higher enhancement values compared to its monomeric constituents, can be used at much lower concentrations, and can be used with any brand of DNP-NMR spectrometer or suitably equipped MRI machine. Please check out our products page for more information about TOTAPOL or use our contact page to find out if you can benefit from using a bi-radical in your next DNP experiment.