Robert Griffin, MIT
Dynamic nuclear polarization (DNP) has become an invaluable tool to enhance sensitivity of magic angle spinning (MAS) NMR, enabling the study of biomolecules and materials which are otherwise intractable. In this presentation we explore some new aspects of time domain DNP experiments and their applications. One of the main thrusts of DNP was to provide increased sensitivity for MAS spectroscopy of
membrane and amyloid protein experiments. A problem frequently encountered in these experiments is the broadened resonances that occur at low temperatures when motion is quenched. In some cases it is clear that the proteins are homogeneously broadened, and therefore that higher Zeeman fields and faster spinning is required to recall the resolution. We show this is the case for MAS DNP spectra of Ab1-42 amyloid fibrils where the resolution at 100 K is identical to that at room temperature. Furthermore, we compare the sensitivity of DNP and 1H detected experiments and find that DNP, even with a modest ℇ=22, is ~x6.5 times more sensitive. We have also investigated the frequency swept-integrated solid effect (FS-ISE) and two recently discovered variants – the stretched solid effect (SSE) and the adiabatic solid effect (ASE). We find that the latter two experiments can give up to a factor of ~2 larger enhancement than the FS-ISE. The SSE and ASE experiments should function well at high fields. Finally, we discuss two new instrumental advances. First, a frequency swept microwave source that permits facile investigation of field profiles. It circumvents the need for a B0 sweep coil and the compromise of field homogeneity and loss of helium associated with such studies. This instrumentation has permitted us to elucidate the polarization transfer mechanism of the Overhauser effect, and also revealed interesting additional couplings (ripples) in field profiles of cross effect polarizing agents. Second, to improve the spinning frequency in DNP experiments, we have developed MAS rotors laser machined from single crystal diamonds. Diamond rotors should permit higher spinning frequencies, improved microwave penetration, and sample cooling.