Molecular mechanisms of ion permeation of membranes

Two mechanisms of selective ion permeation of membranes, that of carriers and that of channels, are discussed with emphasis on molecular conformation. The plasma membrane and excitation‐contraction coupling of cardiac cells are noted as examples implicating both carriers and channels. Correlations between carrier and channel conformations are utilized to develop descriptions of polypeptide channels. Two types of channels, with series‐ and parallel‐aligned repeating units, are discussed. Both types are shown to have the potential for electric field‐dependent conductance by utilizing the peptide dipole moment and both could be expected to adhere to Hodgkin‐Huxley kinetics, wherein the turning on of conductance is a high‐order process and the turning off is a first‐order process. In the case of a series alignment all units must be in a conducting conformation for conduction to occur and the change of a single unit to a nonconducting state would close the channel. The β‐helix‐β‐spiral conformation transition is an example of the field‐dependent series channel. In the case of a parallel alignment if, as Bauman and Mueller assumed, a critical number of units, say six, are required to form a conducting channel, this would then give the high order of kinetics and loss of a single unit could collapse the channel size, rendering it nonconducting. The helix rack model was developed where the peptide dipoles of an α‐helix, which all point in one direction along the helix axis, would couple with the electric field.