Membrane insertion and orientation of polyalanine peptides:: A 15N solid-state NMR spectroscopy investigation

Polyalanine-based peptides were prepared by solid-phase peptide synthesis, labeled with N-15 at selected sites, reconstituted into oriented phosphatidylcholine bilayers, and investigated by proton-decoupled N-15 solid-state NMR spectroscopy. The anisotropic N-15 chemical shift is a direct indicator of helix alignment with respect to the membrane normal. The in-plane to transmembrane equilibrium is the focus of this study. Time- and solvent-dependent transmembrane alignments of K(3)A(18)K(3) have been obtained, and these are stabilized when a few alanine residues are replaced with leucine. The results are discussed in the context of a model where polyalanines adopt a variety of configurations, which are interconnected by multiple equilibria. The data indicate hydrophobicity values of alanine close to zero when studied in the context of helical polypeptides (greater than or equal to 24 residues) and phospholipid bilayers.