| Abstract
| - The secondary structure and alignment of hydrophobic model peptides in phosphatidylcholinemembranes were investigated as a function of hydrophobic mismatch by CD and oriented proton-decoupled15N solid-state NMR spectroscopies. In addition, the macroscopic phase and the orientational order of thephospholipid headgroups was analyzed by proton-decoupled 31P NMR spectroscopy. Both, variations inthe composition of the polypeptide (10−30 hydrophobic residues) as well as the fatty acid acyl chain ofthe phospholipid (10−22 carbons) were studied. At lipid-to-peptide ratios of 50, the peptides adopt helicalconformations and bilayer macroscopic phases are predominant. The peptide and lipid maintain much oftheir orientational order even when the peptide is calculated to be 3 Å too short or 14 Å too long to fitinto the pure lipid bilayer. A continuous decrease in the 15N chemical shift obtained from transmembranepeptides in oriented membranes suggests an increasing helical tilt angle when the membrane thickness isreduced. This response is, however, insufficient to account for the full hydrophobic mismatch. When thehelix is much too long to span the membrane, both the lipid and the peptide order are perturbed, anindication of changes in the macroscopic properties of the membrane. In contrast, sequences that aremuch too short show little effect on the phospholipid headgroup order, but the peptides exhibit a widerange of orientational distributions predominantly close to parallel to the membrane surface. Athermodynamic formalism is applied to describe the two-state equilibrium between in-plane andtransmembrane peptide orientations.
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