Solid-State NMR 31P Paramagnetic Relaxation Enhancement Membrane Protein Immersion Depth Measurements

Paramagnetic relaxation enhancement (PRE) is a widely used approach for measuring long-range distance constraints in biomolecular solution NMR spectroscopy. In this paper, we show that P-31 PRE solid-state NMR spectroscopy can be utilized to determine the immersion depth of spin-labeled membrane peptides and proteins. Changes in the P-31 NMR PRE times coupled with modeling studies can be used to describe the spin-label position/amino acid within the lipid bilayer and the corresponding helical tilt. This method provides valuable insight on protein lipid interactions and membrane protein structural topology. Solid-state P-31 NMR data on the 23 amino acid alpha-helical nicotinic acetylcholine receptor nAChR M2 delta transmembrane domain model peptide followed predicted behavior of P-31 PRE rates of the phospholipid headgroup as the spin-label moves from the membrane surface toward the center of the membrane. Residue 11 showed the smallest changes in P-31 PRE (center of the membrane), while residue 22 shows the largest P-31 PRE change (near the membrane surface), when compared to the diamagnetic control M2 delta sample. This PRE SS-NMR technique can be used as a molecular ruler to measure membrane immersion depth.