Dynamic nuclear polarization surface enhanced NMR spectroscopy for atomic-level characterisation of materials surfaces
Résumé
When it can be applied, NMR spectroscopy is the richest source of structural information for non-crystalline materials. However, NMR sensitivity often prevents detection, especially for surfaces. We show how surface enhanced NMR spectroscopy (SENS) can be achieved by using dynamic nuclear polarization (DNP). In this approach, electron polarization is transferred from an organic radical to the rare nuclei (at natural isotopic abundance) at the surface, yielding at up to a hundred-fold signal enhancement for surface species in silica frameworks. By using incipient wetness impregnation of samples with a solution containing a polarizing radical, surface enhanced carbon-13, silicon-29, nitrogen-15 and aluminum-27 DNP NMR spectra were obtained. Furthermore, the remarkable gain in time provided by DNP SENS (up to a factor 10 000) allows the facile acquisition of two-dimensional correlation spectra, allowing access to detailed conformational features in a series of surface functionalized materials, ranging from catalysts to cements. inclusion of PREs reduces the RMSD for the structure bundle by 50%. Additionally, nuclear relaxation rates provide a powerful probe of protein dynamics. We have recently shown how to measure a complete set of rates for a site-specific motional determination and here we have measured the 15N R1r and 15N R1 measurements, as well as 15N R1r to characterize the conformational selection events in the active site associated with copper binding in SOD.