Solvent drag across gramicidin channels demonstrated by microelectrodes

Area of Science:

• Biophysics
• Membrane Transport
• Ion Channels

Background:

• Gramicidin channels facilitate ion and water movement across lipid bilayers.
• Understanding the coupling between ion and water fluxes is crucial for membrane transport.
• Solvent drag, the movement of solutes due to water flow, influences ion transport.

Purpose of the Study:

• To investigate the competitive transport of ions and water across gramicidin channels.
• To quantify the ratio of water to ion transport through gramicidin.
• To demonstrate and measure the phenomenon of true solvent drag.

Main Methods:

• Simultaneous measurement of pore-impermeable and -permeable cation concentrations near a planar bilayer using double-barreled microelectrodes.
• Establishing a concentration difference across the membrane to induce hypertonic and hypotonic conditions.
• Measuring ion fluxes (Na+, K+) and membrane conductivity under controlled conditions.

Main Results:

• Water movement across the membrane caused solute accumulation and depletion, leading to both true and pseudo-solvent drag.
• By equalizing cation concentrations at membrane interfaces, true solvent drag was isolated.
• Approximately five water molecules were found to co-transport with a single ion through the gramicidin channel.
• A single-channel hydraulic permeability coefficient of 1.6 x 10^-14 cm^3 s^-1 was determined for diphytanoyl phosphatidylcholine membranes.

Conclusions:

• The study provides quantitative evidence for the co-transport of water and ions through gramicidin channels.
• The findings elucidate the mechanism of solvent drag in ion channel transport.
• The determined hydraulic permeability offers insights into the water transport capacity of gramicidin channels.