Dynamic nuclear polarisation enhanced 14N overtone MAS NMR spectroscopy†
Dynamic nuclear polarisation (DNP) has been used to obtain magic angle spinning 14NOT (nitrogen-14 overtone) solid-state NMR spectra from several model amino acids, with both direct and indirect observation of the 14NOT signal. The crystalline solids were impregnated with biradical solutions of organic liquids that do not dissolve the crystalline phase. The bulk phase was then polarized via1H spin diffusion from the highly-polarized surface 1H nuclei, resulting in 1H DNP signal enhancements of around two orders of magnitude. Cross polarisation from 1H nuclei directly to the 14N overtone transition is demonstrated under magic angle spinning, using a standard pulse sequence with a relatively short contact time (on the order of 100 μs). This method can be used to acquire 14N overtone MAS powder patterns that match closely with simulated line shapes, allowing isotropic chemical shifts and quadrupolar parameters to be measured. DNP enhancement also allows the rapid acquisition of 2D 14NOT heteronuclear correlation spectra from natural abundance powder samples. 1H–14NOT HETCOR and 13C–14NOT HMQC pulse sequences were used to observe all single-bond H–N and C–N correlations in histidine hydrochloride monohydrate, with the spectra obtained in a matter of hours. Due to the high natural abundance of the 14N isotope (99.6%) and the advantages of observing the overtone transition, these methods provide an attractive route to the observation of C–N correlations from samples at natural isotopic abundance and enable the high resolution measurement of 14N chemical shifts and quadrupolar interaction parameters.