This article is not directly related to DNP but it is about a small EPR resonator for nanoliter size samples. However, I find it fascinating how sow far you can push instrumentation to miniaturize EPR spectroscopy.
Sidabras, Jason W., Jifu Duan, Martin Winkler, Thomas Happe, Rana Hussein, Athina Zouni, Dieter Suter, Alexander Schnegg, Wolfgang Lubitz, and Edward J. Reijerse. “Extending Electron Paramagnetic Resonance to Nanoliter Volume Protein Single Crystals Using a Self-Resonant Microhelix.” Science Advances 5, no. 10 (October 2019): eaay1394.
Electron paramagnetic resonance (EPR) spectroscopy on protein single crystals is the ultimate method for determining the electronic structure of paramagnetic intermediates at the active site of an enzyme and relating the magnetic tensor to a molecular structure. However, crystals of dimensions typical for protein crystallography (0.05 to 0.3mm) provide insufficient signal intensity. In this work, we present a microwave self-resonant microhelix for nanoliter samples that can be implemented in a commercial X-band (9.5 GHz) EPR spectrometer. The self-resonant microhelix provides a measured signal-to-noise improvement up to a factor of 28 with respect to commercial EPR resonators. This work opens up the possibility to use advanced EPR techniques for studying protein single crystals of dimensions typical for x-ray crystallography. The technique is demonstrated by EPR experiments on single crystal [FeFe]-hydrogenase (Clostridium pasteurianum; CpI) with dimensions of 0.3 mm by 0.1 mm by 0.1 mm, yielding a proposed g-tensor orientation of the Hox state.