Penetration Of Aromatic Residues Into Membrane Bilayers: A New Approach

Penetration of Aromatic Residues into Membrane Bilayers: A New Approach Lipid bilayers are characterized by a unique molecular motional regime that makes it possible to apply both solid-state NMR and solution-state NMR methods together for structural studies. The combination of magic angle spinning (MAS) with the high-resolution 1H NOESY NMR experiment is an established method for measuring through-space 1H¼1H dipolar couplings in biological membranes, and has been applied extensively in the past to biological membranes to determine the location of bound drugs and peptides. 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 introduce the use of solid-state radiofrequency driven dipolar recoupling (RFDR) pulses during the mixing time. We compare the established 1H MAS NOESY experiment with a new 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 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.