Dynamic nuclear polarisation enhanced N-14 overtone MAS NMR spectroscopy
Dynamic nuclear polarisation (DNP) has been used to obtain magic angle spinning (NOT)-N-14 (nitrogen-14 overtone) solid-state NMR spectra from several model amino acids, with both direct and indirect observation of the (NOT)-N-14 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 via H-1 spin diffusion from the highly-polarized surface H-1 nuclei, resulting in H-1 DNP signal enhancements of around two orders of magnitude. Cross polarisation from H-1 nuclei directly to the N-14 overtone transition is demonstrated under magic angle spinning, using a standard pulse sequence with a relatively short contact time (on the order of 100 ms). This method can be used to acquire N-14 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 D-2 (NOT)-N-14 heteronuclear correlation spectra from natural abundance powder samples. H-1-(NOT)-N-14 HETCOR and C-13-(NOT)-N-14 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 N-14 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 N-14 chemical shifts and quadrupolar interaction parameters.