Low-field EPR measurements by field-cycled dynamic nuclear polarization

The techniques of proton-electron double-resonance imaging (PEDRI) ( 1) together
with field-cycled PEDRI (2) have been developed recently and have been used to
image the distribution of nitroxide free radicals in phantoms and in small animals (3,
4). PEDRI relies on dynamic nuclear polarization (DNP): a proton NMR image of
the sample is collected while an EPR resonance of a free radical solute is irradiated.
Under favorable conditions, the NMR signal is enhanced in those parts of the sample
where the free radical is influencing the proton relaxation rate, and these regions
exhibit greater intensity in the final image (I). A thorough knowledge of the EPR
characteristics of the free radical under study is necessary in order to optimize a PEDRl
experiment: in any experiment the observed enhancement will be maximized if the .
EPR irradiation occurs at the center of the EPR line of interest. While there exists a
wealth of published EPR spectral data at X band, very little information exists in the
literature pertaining to the low magnetic fields (and EPR frequencies) at which PEDRI
experiments are performed. In this study we have used a nonimaging variant of spectral
field-cycled PEDRI (Spectral FC-PEDRI) (5) to measure the EPR resonances of
TEMPOL (4-hydroxy-2,2,6,6-tetramethylpiperidine- 1-oxyl ) at field strengths of 12
mT and below. Fitting the experimental data to a model based on the Breit-Rabi
equations allows the EPR frequencies at any magnetic field to be calculated.