Organization of model helical peptides in lipid bilayers: Insight into the behavior of single-span protein transmembrane domains

Selectively deuterated transmembrane peptides comprising alternating leucine-alanine subunits were examined in fluid bilayer membranes by solid-state nuclear magnetic resonance (NMR) spectroscopy in an effort to gain insight into the behavior of membrane proteins. Two groups of peptides were studied: 21-mers having a 17-amino-acid hydrophobic domain calculated to be close in length to the hydrophobic thickness of 1-paimitoyl-2-oleoyl phosphatidylcholine and 26-mers having a 22-amino-acid hydrophobic domain calculated to exceed the membrane hydrophobic thickness. H-2 NMR spectral features similar to ones observed for transmembrane peptides from single-span receptors of higher animal cells were identified which apparently correspond to effectively monomeric peptide. Spectral observations suggested significant distortion of the transmembrane a-helix, and/or potential for restriction of rotation about the tilted helix long axis for even simple peptides. Quadrupole splittings arising from the 26-mer were consistent with greater peptide "tilt" than were those of the analogous 21-mer. Quadrupole splittings associated with monomeric peptide were relatively insensitive to concentration and temperature over the range studied, indicating stable average conformations, and a well-ordered rotation axis. At high peptide concentration (6 mol% relative to phospholipid) it appeared that the peptide predicted to be longer than the membrane thickness had a particular tendency toward reversible peptide-peptide interactions occurring on a timescale comparable with or faster than similar to10(-5) s. This interaction may be direct or lipid-mediated and was manifest as line broadening. Peptide rotational diffusion rates within the membrane, calculated from quadrupolar relaxation times, T-2e, were consistent with such interactions. In the case of the peptide predicted to be equal to the membrane thickness, at low peptide concentration spectral lineshape indicated the additional presence of a population of peptide having rotational motion that was restricted on a timescale of 10(-5) s.