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1.D.113.  The Synthetic Channel-forming (LSSLLSL)3-CONH2 Peptide (LS21) Family 

Ion channel proteins are composed of bundles of alpha-helices that associate to allow ion-conduction. Three model peptides containing only leucine and serine residues were synthesized. A 21-residue peptide, H2N-(Leu-Ser-Ser-Leu-Leu-Ser-Leu)3-CONH2, LS21, is a membrane-spanning amphiphilic α-helix that formed well-defined ion channels with ion permeabilities and lifetime characteristics resembling those of the acetylcholine receptor. In contrast, a 14-residue version of this peptide, LS14, which is too short to span the phospolipid bilayer as an α-helix, failed to form stable channels. A third peptide, H2N-(Leu-Ser-Leu-Leu-Leu-Ser-Leu)3-CONH2, in which one serine per heptad repeat was replaced by leucine, produced proton-selective channels (Lear et al. 1988).

Proton-decoupled 15N solid-state NMR spectroscopy indicated that LS21 preferentially aligns parallel to the membrane surface as expected from its amphipathic helical structure, although a transmembrane H+-conducting orientation can also be assumed. An equilibrium exists between in-plane and transmembrane-oriented helix configurations where the transmembrane and surface-oriented peptide fractions are in slow exchange (Sudheendra and Bechinger 2005). Similar topological equilibria are observed when the N-terminus of the LS21 peptide is acetylated. The channels observed are possibly formed by hexameric transmembrane helical bundles, but the shorter peptide (Leu-Ser-Ser-Leu-Leu-Ser-Leu)2-CONH2 is oriented parallel to the membrane surface under all conditions tested. This peptide exhibits erratic conductance changes when investigated by electrophysiological methods, probably because it is too short to span the lipid bilayer.

References associated with 1.D.113 family:

Lear, J.D., Z.R. Wasserman, and W.F. DeGrado. (1988). Synthetic amphiphilic peptide models for protein ion channels. Science 240: 1177-1181. 2453923
Sudheendra, U.S. and B. Bechinger. (2005). Topological equilibria of ion channel peptides in oriented lipid bilayers revealed by 15N solid-state NMR spectroscopy. Biochemistry 44: 12120-12127. 16142910