Proton permeation of lipid bilayers

Area of Science:

• Biophysics
• Membrane Transport
• Physical Chemistry

Background:

• Proton permeation through lipid bilayers exhibits unusual high permeability and minimal pH-dependent variation.
• Existing models propose contaminant-induced conductance or proton translocation via transient hydrogen-bonded chains (tHBC).
• Contaminants explain some flux but not anomalous liposome behavior, necessitating further investigation of the tHBC model.

Purpose of the Study:

• To investigate the mechanism of anomalous proton conductance across lipid bilayers.
• To test the transient hydrogen-bonded chains (tHBC) model for proton translocation.
• To differentiate between contaminant-driven and tHBC-driven proton flux.

Main Methods:

• Measurements of relative proton and potassium permeability through the gramicidin channel.
• Analysis of proton flux as a function of pH gradient across liposome membranes.
• Quantitative assessment of tHBC contribution based on experimental data.

Main Results:

• Gramicidin channel studies revealed a four-order-of-magnitude difference in proton versus potassium permeability, indicating water chain selectivity.
• Calculations suggest that ~7% of bilayer water in a tHBC configuration could account for observed proton flux.
• Proton conductance across liposomes showed a superlinear relationship with pH gradient, supporting specific tHBC models.

Conclusions:

• The results strongly support the transient hydrogen-bonded chains (tHBC) model as the primary mechanism for anomalous proton conductance in lipid bilayers.
• Water molecule chains within membranes can exhibit significant ion selectivity, discriminating against cations like potassium.
• The findings provide crucial insights into proton transport dynamics in biological and artificial membranes.