| Abstract
| - Ion channel peptides have been prepared by solid-phase peptide synthesis, labeled with 15N atselected sites, and reconstituted into oriented lipid bilayers. The (Leu-Ser-Ser-Leu-Leu-Ser-Leu)3-CONH2peptide has previously been shown to exhibit well-defined and discrete ionic conductances when investigatedby single-channel measurements [Lear, J. D., et al. (1988) Science240, 1177]. Proton-decoupled 15Nsolid-state NMR spectroscopy indicates that (Leu-Ser-Ser-Leu-Leu-Ser-Leu)3-CONH2 preferentially alignsparallel to the membrane surface in excellent agreement with its amphipathic helical structure. However,by carefully choosing the conditions of the membrane environment, significant contributions that areindicative of transmembrane alignments become obvious in the 15N chemical shift solid-state NMR spectra.The data thereby provide experimental evidence for an equilibrium between in-plane and transmembrane-oriented helix configurations where the transmembrane and surface-oriented peptide fractions are in slowexchange. Similar topological equilibria are observed when the N-terminus of the LS21 peptide is acetylated.These observations provide experimental support for previous models, suggesting that the channels observedin single-channel conductance measurements are indeed formed by hexameric transmembrane helicalbundles. In contrast, the shorter peptide (Leu-Ser-Ser-Leu-Leu-Ser-Leu)2-CONH2 is oriented parallel tothe membrane surface under all conditions tested. This peptide exhibits erratic conductance changes wheninvestigated by electrophysiological methods, probably because it is too short to span the lipid bilayer.
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