U. Akbey, Dynamic Nuclear Polarization of Deuterated Proteins, Angew. Chem. Int. Ed., 2010, 49(42), 7803-7806
Magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy has evolved as a robust and widely applicable technique for investigating the structure and dynamics of biological systems. It is in fact rapidly becoming an indispensable tool in structural biology studies of amyloid, nanocrystalline, and membrane proteins. However, it is clear that the low sensitivity of MAS experiments to directly detected 13C and 15N signals limits the utility of the approach, particularly when working with systems which are difficult to obtain in large quantities.
This limit provides the impetus to develop methods to enhance the sensitivity of MAS experiments, the availability of which will undoubtedly broaden the applicability of the technique. Remarkable progress towards this goal has been achieved by incorporating high-frequency dynamic nuclear polarization (DNP) into the MAS NMR technique. The DNP method exploits the microwave-driven transfer of polarization from a paramagnetic center, such as nitroxide free radical, to the nuclear spins, and has been demonstrated to produce uniformly polarized macromolecular samples. In principle signal enhancements, e = 660 can be obtained for 1H and recently signal enhancements of e = 100–200 were observed in model compounds. However, in applications of DNP to MAS spectra of biological systems, including studies of lysozyme, and bacteriorhodopsin, the enhancements have been smaller, e=40–50. An exception is the amyloidogenic peptide GNNQQNY7–13 which forms nanocrystals for which the proton T1 time is long and e ~ 100.