Dynamic nuclear polarisation enhanced 14N overtone MAS NMR spectroscopy

Abstract

Dynamic nuclear polarisation (DNP) has been used to obtain magic angle spinning ¹⁴N^OT (nitrogen-14 overtone) solid-state NMR spectra from several model amino acids, with both direct and indirect observation of the ¹⁴N^OT 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 ¹⁴N^OT heteronuclear correlation spectra from natural abundance powder samples. ¹H–¹⁴N^OT HETCOR and ¹³C–¹⁴N^OT 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.