Low-temperature Magic Angle Spinning Research Articles

Optimization of 15N–13C double-resonance NMR experiments under low temperature magic angle spinning dynamic nuclear polarization conditions

Dynamic nuclear polarization (DNP) enhanced magic angle spinning (MAS) solid-state NMR carried out at 25 K enables rapid acquisition of multi-dimensional 13C–15N correlation spectra for protein structure studies and resonance assignment. Under commonly used DNP conditions, solvent deuteration reduces 1H–15N cross polarization (CP) efficiencies, necessitates more careful optimization, and requires longer high-power 15N radio-frequency pulses. The sensitivity of 2D heteronuclear correlation experiments is potentially impaired. Here we show that 2D 15N-13C experiments based on 13C-15N transferred echo double resonance (TEDOR) methods outperform 2D experiments based on CP transfers in a fully deuterated solvent, and are competitive with CP-based experiments when the solvent is only partially deuterated. Additionally, we show that optimization of TEDOR-based 2D experiments is simpler than optimization of CP-based experiments under 25 K MAS conditions.

19 F Magic Angle Spinning Dynamic Nuclear Polarization Enhanced NMR Spectroscopy.

The introduction of high-frequency, high-power microwave sources, tailored biradicals, and low-temperature magic angle spinning (MAS) probes has led to a rapid development of hyperpolarization strategies for solids and frozen solutions, leading to large gains in NMR sensitivity. Here, we introduce a protocol for efficient hyperpolarization of 19 F nuclei in MAS DNP enhanced NMR spectroscopy. We identified trifluoroethanol-d3 as a versatile glassy matrix and show that 12 mm AMUPol (with microcrystalline KBr) provides direct 19 F DNP enhancements of over 100 at 9.4 T. We applied this protocol to obtain DNP-enhanced 19 F and 19 F-13 C cross-polarization (CP) spectra for an active pharmaceutical ingredient and a fluorinated mesostructured hybrid material, using incipient wetness impregnation, with enhancements of approximately 25 and 10 in the bulk solid, respectively. This strategy is a general and straightforward method for obtaining enhanced 19 F MAS spectra from fluorinated materials.

Apparatus for low-temperature magic-angle spinning NMR

Low-temperature magic-angle spinning (MAS) NMR experiments are of considerable interest since they permit the study of a variety of phenomena such as chemical exchange, phase transitions, and Knight shifts. In addition, the recent development of techniques for determining distances and distinguishing among molecular conformations in solids provides opportunities to examine the structure of reactions in intermediates (i.e., photolysis products and enzyme/inhibitor or substrate complexes) trapped at low temperatures. In this Note we describe a system that we have developed and successfully employed for performing MAS experiments for extended periods of continuous operation in the temperature range of 150-300 K. The desirable features of a low-temperature MAS system are severalfold. First, it should provide setable and stable spinning speeds for extended periods. For low temperatures, this dictates that at least the drive-bearing gas must be at room temperature and supplied to a spinner-speed controller before cooling in order to maintain constant speeds. Second, the spinning speed must be maintained when the temperature is changed. Third, facilities for replenishing the coolants and the drive and bearing gases must be provided to accommodate operation for extended periods. The first requirement above was initially addressed simply by circulating the drive and bearing gases through copper coils immersed in liquid nitrogen, the temperature being determined by adjusting the size of the coils and the flow rate. This is the method used at present in most laboratories and in most commercial equipment. However, with this arrangement the temperature and spinning speed cannot be adjusted independently. In addition, at the high pressures which are necessary to achieve high spinning speeds, condensation of the nitrogen occurs, resulting in pulsations of the spinning. Kendrick et al. (2) demonstrated that this problem could be circumvented by pressurizing the liquid nitrogen dewar above the pressure of the gas flow. However, this procedure has the disadvantages that it involves a large pressurized vessel (3) subject to strict safety rules in most operating environments, and that it is difficult to replenish the coolant. It is therefore unsatisfactory for samples requiring extended signal averaging periods.