The Gramicidin A Transmembrane Channel: Characteristics of Head-to-Head Dimerized π _(L,D) Helices

TLDR

Gramicidin A forms alternating L/D helices that can dimerize head‑to‑head to create transmembrane channels, with properties such as channel length, pore size, and ion selectivity characterized, and the π(L,D) helix with 6.3 residues per turn explains its action on thin lipid bilayers. The authors synthesized a malonyl dimer of gramicidin A to test the proposed head‑to‑head association for channel formation. NMR confirmed the malonyl dimer’s integrity, and lipid bilayer assays showed it mediates ion conductance with the predicted concentration dependence, confirming head‑to‑head dimerization as the mechanism for transmembrane channel formation.

Abstract

A series of helical structures for gramicidin A, with alternating L and D residues, are characterized as to number of residues per turn, atoms in hydrogenbonded rings, and dihedral angles. Because of alternating peptide C-O directions, these helices are capable of forming head-to-head hydrogen-bonded dimers with the capacity of functioning as transmembrane channels. The dimers are characterized as to channel length, pore size, and expected ion selectivity. In a test of the proposed head-to-head association for channel formation, the malonyl dimer [N,N'-(dideformyl gramicidin A)-malonamide] was synthesized. The chemical and conformational integrity of the product was verified by nuclear magnetic resonance; in lipid bilayer studies, the dimer was found to be a potent mediator of ion conductance with the predicted concentration dependence.Thus, the results on malonyl gramicidin A prove head-to-head association in formation of the transmembrane channel, and the results are consistent with the specific geometrical configuration involved in head-to-head dimerization of pi((L,D)) helices. At this stage, the action of gramicidin A on membranes with lipid-layer thicknesses of 30 A or less can best be understood in terms of the pi((L,D)) helix with 6.3 residues per turn.