Dynamic Nuclear Polarization

Abstract

Dynamic Nuclear Polarization (DNP) is a phenomenon by which high spin polarization, typically derived from a bath of free radical electrons, is transferred to a nuclear spin bath, enhancing the difference between the nuclear energy levels and thereby producing dramatically enhanced NMR signals for detection. The phenomenon was first predicted by Overhauser1, but was not observed experimentally until the work of Slichter in metals in 1953.2 It was soon understood that the same technique could be used to develop high polarizations of 1H, 2H, and 13C in non-conducting solids. This advance became foundational for production of solid targets for high energy physics research.3–5 High nuclear polarizations in the targets simplified the results of neutron scattering experiments. Subsequently, the DNP method migrated to chemistry, being used to study a variety of structural questions in the solid state.6,7 Robert Griffin of MIT has pioneered the use of DNP for signal enhancement in solid state NMR distance measurements for structural biology.8 In his method, a water soluble free radical is doped into a matrix containing H2O/glycerol and the solute molecule/ protein to be studied. This method has recently been used to study the K intermediate of bacteriorhodopsin in intact purple membrane.9 While DNP is also possible in the liquid state, it is much less efficient due to the diminishment of the intermolecular dipolar couplings by fast molecular tumbling.10