Proton/hydroxide conductance through lipid bilayer membranes.

Area of Science:

• Biophysics
• Membrane Biophysics
• Electrochemistry

Background:

• Proton and hydroxide ion (H+/OH-) transport across biological membranes is crucial for cellular functions.
• Accurate measurement of H+/OH- conductance (GH/OH) in model systems is essential for understanding these processes.
• Existing methods often yield pH-dependent results, complicating interpretation.

Purpose of the Study:

• To develop and validate a simple method for measuring proton/hydroxide conductance (GH/OH) in planar lipid bilayer membranes.
• To investigate the pH dependence of GH/OH in phosphatidylethanolamine (PE) lipid bilayers.
• To explore the influence of membrane surface charge and water activity on H+/OH- transport.

Main Methods:

• Electrical measurement of total membrane conductance (Gm).
• Estimation of H+/OH- transference number (TH/OH) from diffusion potential (Vm) across a transmembrane pH gradient.
• Calculation of GH/OH using the relationship GH/OH = TH/OH * Gm.

Main Results:

• GH/OH in PE lipid bilayers was found to be nearly independent of pH, ranging from 10^-9 S cm^-2 at pH 1.6 to 10^-8 S cm^-2 at pH 10.5.
• Calculated permeability coefficients for H+/OH- showed strong pH dependence, reconciling previous discrepancies.
• GH/OH was unaffected by membrane surface charge but decreased 10-fold with a 33% reduction in water activity.

Conclusions:

• The developed method provides a reliable way to measure proton/hydroxide conductance in lipid bilayers.
• The pH independence of GH/OH and the pH dependence of permeability coefficients offer a new perspective on H+/OH- transport across membranes.
• Water molecules likely play a role in the H+/OH- transport mechanism, as suggested by the reduction in conductance with decreased water activity.