High-resolution 1H MAS RFDR NMR of biological membranes.

The combination of magic angle spinning (MAS) with the high-resolution 1H NOESY NMR experiment is an established method for measuring through-space 1H…1H dipolar couplings in biological membranes. The segmental motion of the lipid acyl chains along with the overall rotational diffusion of the lipids provides sufficient motion to average the 1H dipolar interaction to within the range where MAS can be effective. One drawback of the approach is the relatively long NOESY mixing times needed for relaxation processes to generate significant crosspeak intensity. In order to drive magnetization transfer more rapidly, we use solid-state radiofrequency driven dipolar recoupling (RFDR) pulses during the mixing time. We compare the 1H MAS NOESY experiment with a 1H MAS RFDR experiment on dimyristoylphosphocholine, a bilayer-forming lipid and show that the 1H MAS RFDR experiment provides considerably faster magnetization exchange than the standard 1H MAS NOESY experiment. We apply the method to model compounds containing basic and aromatic amino acids bound to membrane bilayers to illustrate the ability to locate the position of aromatic groups that have penetrated to below the level of the lipid headgroups.