A Molecular Theory of Ion-Conducting Channels: A Field-Dependent Transition Between Conducting and Nonconducting Conformations

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Structural and conformational requirements for an electric field-dependent transition between conducting and nonconducting macromolecular systems are: two kinetically interconvertible and energetically similar conformations, one conducting and the other nonconducting, which have axes spanning the lipid layer of biological membranes, but which have different net dipole moments along those axes. Two examples are described. A previously defined helix, the π6LD-helix now termed the β63,3-helix, is proposed as the conducting species, and the linear peptide correlate of the cyclic hexapeptide conformation containing two β-turns and an inversion element of symmetry is proposed as a nonconducting species. The latter is termed an anti-β62-spiral and contains little or no net dipole moment per turn, whereas the β63,3-helix contains a net dipole moment along the helix axis of about 0.5 Debye per dipeptide unit. A related conducting and nonconducting pair with large net dipole moments of opposite sign, termed syn-β62-spiral and β62,4-helix, are also described. The spiral conformations are stabilized in a lipid layer by intermolecular hydrogen bonds, leading to a linear association of transmembrane structures. A conformational transition in one member of the array could lead to destabilization of an adjacent member of the array. The conformational analysis uses a concept of cyclic conformations with linear conformational correlates. The anti-β62-spiral and β63,3-helix are derivable from the conformations of the cyclic structure [unk], whereas the syn-β2-spiral and β62,4-helix may be derived from the cyclic structure [unk].

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