Deuterium nuclear magnetic resonance spectroscopy and molecular dynamics studies of transmembrane polypeptides

2H NMR studies of synthetic polypeptides in bilayer membranes were performed. The results suggest that such transmembrane polypeptides undergo fast, axially symmetric reorientation about the bilayer normal and have a preferred average azimuthal orientation about the helix axis. Quadrupolar splittings and quadrupolar echo decay measurements indicate that the correlation time for reorientation is on the order of 10-7 s. At high temperatures the spectra are indicative of peptides that rotate mainly as monomers while broadening at lower temperatures is consistent with transient peptide-peptide interactions. The appearance of a broad, low amplitude feature in spectra at low concentrations seems to indicate that a certain fraction of peptides interact and rotate as short lived dimers. Further 2H NMR studies of two synthetic polypeptides with glycophorin A (GPA) motifs suggest that these model peptides are tilted in bilayers and rapidly reorient about the bilayer normal with a correlation time on the order of 10-7 s. Observed spectra and echo decay measurements indicate that the orientation and dynamics of each peptide depend on CPA motif position. This suggests that the position of the interaction motif in the peptide sequence may play an important role in determining peptide orientation in bilayers. To characterize helix motion and orientation, systems consisting of 64 molecules of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosph ocholine (POPC) and one α-helical polypeptide with the sequence acetyl-KK-(LA)11-KK-amide were examined by molecular dynamics simulation. The systems were allowed to evolve for up to 10 ns at 1 atmosphere of pressure and a temperature of 55oC. It is was found that the polypeptide orders lipid chains and thus increases bilayer thickness in accordance with previous simulations and experimental observations. Alanine methyl groups were found to be inequivalent which is consistent with 2H NMR splittings observed from specifically labelled polypeptides in POPC bilayers. The simulation results suggest that the polypeptide assumes a preferred orientation with respect to the bilayer normal and about the molecular long axis. (author)