A low-E magic angle spinning probe for biological solid state NMR at 750 MHz.

Crossed-coil NMR probes are a useful tool for reducing sample heating for biological solid state NMR. In a crossed-coil probe, the higher frequency (1)H field, which is the primary source of sample heating in conventional probes, is produced by a separate low-inductance resonator. Because a smaller driving voltage is required, the electric field across the sample and the resultant heating is reduced. In this work we describe the development of a magic angle spinning (MAS) solid state NMR probe utilizing a dual resonator. This dual resonator approach, referred to as "low-E," was originally developed to reduce heating in samples of mechanically aligned membranes. The study of inherently dilute systems, such as proteins in lipid bilayers, via MAS techniques requires large sample volumes at high field to obtain spectra with adequate signal-to-noise ratio under physiologically relevant conditions. With the low-E approach, we are able to obtain homogeneous and sufficiently strong radiofrequency fields for both (1)H and (13)C frequencies in a 4mm probe with a (1)H frequency of 750 MHz. The performance of the probe using windowless dipolar recoupling sequences is demonstrated on model compounds as well as membrane-embedded peptides.