Tracing Dynamic Nuclear Polarization Pathways Using Transition Metal - Nuclear Spin Rulers

The ubiquitous technique of nuclear magnetic resonance (NMR) spectroscopy suffers from relatively low sensitivity due to the low polarization of nuclei. For decades, the technique of dynamic nuclear polarization (DNP) has been harnessed to increase the sensitivity of NMR, enabling detection of low abundance nuclei such as ¹⁷O and elucidation of protein structures. Yet, the catalogue of DNP agents today is limited to organic radical species, accompanied by a handful of metal ions (Cr³⁺, Mn²⁺, and Gd³⁺). This study significantly expands the scope and catalogue of DNP with the first demonstration of amplification of nuclear spin polarization at a set distance from a transition metal center (V⁴⁺) that has g-values significantly varied from 2 and anisotropic EPR line that is more than 3GHz broad.We showed that ¹H NMR signal enhancements of up to 33 can be achieved at 6.9T field and 4K temperature using a home-built DNP instrumentation that allows microwave irradiation over a frequency range of more than 10 GHz with pulse shaping capabilities by arbitrary waveform generator. A series of systematically designed vanadyl complexes, with V⁴⁺-¹H distances in range 4.0 Å to 13.6 Å, was used to trace the polarization pathway of DNP and determine the size of the spin-diffusion barrier.