Alignment of Lysine-Anchored Membrane Peptides under Conditions of Hydrophobic Mismatch: A CD, 15N and 31P Solid-State NMR Spectroscopy Investigation

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Abstract
The secondary structure and alignment of hydrophobic model peptides in phosphatidylcholine membranes were investigated as a function of hydrophobic mismatch by CD and oriented proton-decoupled 15N solid-state NMR spectroscopies. In addition, the macroscopic phase and the orientational order of the phospholipid headgroups was analyzed by proton-decoupled 31P NMR spectroscopy. Both, variations in the composition of the polypeptide (10−30 hydrophobic residues) as well as the fatty acid acyl chain of the phospholipid (10−22 carbons) were studied. At lipid-to-peptide ratios of 50, the peptides adopt helical conformations and bilayer macroscopic phases are predominant. The peptide and lipid maintain much of their orientational order even when the peptide is calculated to be 3 Å too short or 14 Å too long to fit into the pure lipid bilayer. A continuous decrease in the 15N chemical shift obtained from transmembrane peptides in oriented membranes suggests an increasing helical tilt angle when the membrane thickness is reduced. This response is, however, insufficient to account for the full hydrophobic mismatch. When the helix is much too long to span the membrane, both the lipid and the peptide order are perturbed, an indication of changes in the macroscopic properties of the membrane. In contrast, sequences that are much too short show little effect on the phospholipid headgroup order, but the peptides exhibit a wide range of orientational distributions predominantly close to parallel to the membrane surface. A thermodynamic formalism is applied to describe the two-state equilibrium between in-plane and transmembrane peptide orientations.