Amudol

NMR has the resolution and specificity to determine atomic-level protein structures of isotopically-labeled proteins in complex environments and, with the sensitivity gains conferred by dynamic nuclear polarization (DNP), NMR has the sensitivity to detect proteins at their endogenous concentrations. Prior work established that DNP MAS NMR is compatible with cellular viability. However, in that work, 15% glycerol, rather than the more commonly used 10% DMSO, was used as the cellular cryoprotectant. Moreover, incubation of cells cryoprotected 15% glycerol with the polarization agent, AMUPol, resulted in an inhomogeneous distribution of AMUPol through the cellular biomass, which resulted in a spatial bias of the NMR peak intensities. Because 10% DMSO is not only the most used cryoprotectant for mammalian cells, but also because DMSO is often used to improve delivery of molecules to cells, we sought to characterize the DNP performance of cells that were incubated with AMUPol and cryoprotected with 10% DMSO. We found that, like cells preserved with 15% glycerol, cells preserved with 10% DMSO retain high viability during DNP MAS NMR experiments if they are frozen at a controlled rate. However, DMSO did not improve the dispersion of AMUPol throughout the cellular biomass. Cells preserved with 15% glycerol and with 10% DMSO had similar DNP performance for both the maximal DNP enhancements as well as the inhomogeneous dispersion of AMUPol throughout the cellular biomass. Therefore, 10% DMSO and 15% glycerol are both appropriate cryoprotectant systems for DNP-assisted MAS NMR of intact viable mammalian cells.

The sensitivity enhancements available from dynamic nuclear polarization (DNP) are rapidly reshaping the research landscape and expanding the field of nuclear magnetic resonance (NMR) spectroscopy as a tool for solving complex chemical and structural problems. The past decade has seen considerable advances in this burgeoning method, while efforts to further improve its capabilities continue along many avenues. In this report, we examine the influence of static magnetic field strength and temperature on the reported 1H DNP enhancements from three conventional organic biradicals: TOTAPOL, AMUPol, and SPIROPOL. In contrast to the conventional wisdom, our findings show that at liquid nitrogen temperatures and 700 MHz/460.5 GHz, these three bisnitroxides all provide similar 1H DNP enhancements, ε ≈ 60. Furthermore, we investigate the influence of temperature, microwave power, magnetic field strength, and protein sample deuteration on the NMR experimental results.

Dynamic Nuclear Polarization (DNP) enhanced solid state NMR increases experimental sensitivity, potentially enabling detection of biomolecules at their physiological concentrations. The sensitivity of DNP experiments is due to the transfer of polarization from electron spins of free radicals to the nuclear spins of interest. Here, we investigate the reduction of AMUPol in both lysed and intact HEK293 cells. We find that nitroxide radicals are reduced with first order reduction kinetics by cell lysates at a rate of ~ 12% of the added nitroxide radical concentration per hour. We also found that electroporation delivered a consistent amount of AMUPol to intact cells and that nitroxide radicals are reduced just slightly more rapidly (~15% per hour) by intact cells than by cell lysates. The two nitroxide radicals of AMUPol are reduced independently and this leads to considerable accumulation of the DNP-silent monoradical form of AMUPol, particularly in preparations of intact cells where nearly half of the AMUPol is already reduced to the DNP silent monoradical form at the earliest experimental time points. This confirms that the loss of the DNP-active biradical form of AMUPol is faster than the nitroxide reduction rate. Finally, we investigate the effect of adding N-ethyl maleimide, a well-known inhibitor of thiol (-SH) group-based reduction of nitroxide biradicals in cells, on AMUPol reduction, cellular viability, and DNP performance. Although pre-treatment of cells with NEM effectively inhibited the reduction of AMUPol, exposure to NEM compromised cellular viability and, surprisingly, did not improve DNP performance. Collectively, these results indicate that, currently, the most effective strategy to obtain high DNP enhancements for DNP-assisted in-cell NMR is to minimize room temperature contact times with cellular constituents and suggest that the development of bio-resistant polarization agents for DNP could considerably increase the sensitivity of DNP-assisted in-cell NMR experiments.