Filled pore approximation: a theoretical framework for solute-solvent coupling in narrow water channels

Area of Science:

• Biophysics
• Physical Chemistry
• Membrane Transport

Background:

• Water and solute transport through biological pores is crucial for cellular function.
• Existing models, like the single-file model, have limitations in describing transport in very narrow channels.
• Understanding these transport mechanisms is key to comprehending processes in kidney tubules and water channels.

Purpose of the Study:

• To present a new phenomenological model for water and solute transport in narrow pores (< 2 Å radius).
• To account for variations in unoccupied volume within pores, differing from conventional models.
• To relate pore volume, frictional coefficients, and transport permeabilities.

Main Methods:

• Developed a phenomenological model for water and solute transport.
• Incorporated variable unoccupied volume within the pore model.
• Related accessible unoccupied volume to mechanical frictional coefficients.

Main Results:

• The model accurately describes transport in channels like gramicidin A and aquaporin 1.
• Accessible unoccupied volume correlates with frictional coefficients and permeabilities.
• A solute's reflection coefficient can remain high (~0.5) even with low diffusive permeability.
• Predicted a minimum effective pore radius of 1.78 Å for water channels.

Conclusions:

• The model provides a framework for understanding transport in sub-nanometer pores.
• Variable unoccupied volume is critical for accurately modeling single-file transport.
• The findings are consistent with experimental observations in biological systems like proximal tubules.