The induction by protons of ion channels through lipid bilayer membranes

The appearance of ion channels was induced in phospholipid bilayers by acidification of the bulk solution on one side Of the bilayer, by addition of HCl, acetic acid or by hydrolytic production of protons using purified acetylcholinesterase. Further acidification below an apparent critical pH range led to restoration of a low conductance state similar to that seen at neutral pH. Such experiments were performed with a heterogeneous soybean lecithin extract, with homogeneous synthetic diphytanoylphosphatidylcholine, and with a mixture of cholesterol and synthetic dioleoylphosphatidylcholine. It is proposed that the physical mechanism for this phenomenon involves fluctuations of lipid order induced by fluctuations in protonation of phospholipid head groups within a critical pH range; these, in turn, create conductive defects in the two-dimensional lattice of the lipid bilayer.