Molecular insight into the electrostatic membrane surface potential by 14N/31P MAS NMR spectroscopy:: Nociceptin-lipid association

Exploiting naturally abundant N-14 and P-31 nuclei by high-resolution MAS NMR (magic angle spinning nuclear magnetic resonance) provides a molecular view of the electrostatic potential present at the surface of biological model membranes, the electrostatic charge distribution across the membrane interface, and changes that occur upon peptide association. The spectral resolution in P-31 and N-14 MAS NMR spectra is sufficient to probe directly the negatively charged phosphate and positively charged choline segment of the electrostatic P--O-CH2-CH2-N+(CH3)(3) headgroup dipole of zwitterionic DMPC (dimyristoylphosphaticlylcholine) in mixed-lipid systems. The isotropic shifts report on the size of the potential existing at the phosphate and ammonium group within the lipid headgroup while the chemical shielding anisotropy (P-31) and anisotropic quadrupolar interaction (N-14) characterize changes in headgroup orientation in response to surface potential. The P-31/N-14 isotropic chemical shifts for DMPC show opposing systematic changes in response to changing membrane potential, reflecting the size of the electrostatic potential at opposing ends of the P--N+ dipole. The orientational response of the DMPC lipid headgroup to electrostatic surface variations is visible in the anisotropic features of N-14 and P-31 NMR spectra. These features are analyzed in terms of a modified molecular voltmeter model, with changes in dynamic averaging reflecting the tilt of the C beta-N+(CH)(3) choline and PO4- segment. These properties have been exploited to characterize the changes in surface potential upon the binding of nociceptin to negatively charged membranes, a process assumed to proceed its agonistic binding to its opoid G-protein coupled receptor.