Cation diffusion selectivity in a pore model. The phosphatidylcholine/water lamellar phase

Area of Science:

• Physical Chemistry
• Biophysics
• Materials Science

Background:

• Understanding ion transport across lipid bilayers is crucial for cell membrane function.
• Phosphatidylcholine/water lamellar phases serve as model systems for biological membranes.
• Ion selectivity in biological pores is a complex phenomenon influenced by various factors.

Purpose of the Study:

• To quantify diffusion coefficients of monovalent (K+, Na+, Rb+, Cs+) and divalent (Ca2+) cations in phosphatidylcholine/water lamellar phases.
• To investigate the influence of phase hydration, temperature, and divalent cations on cation diffusion.
• To correlate observed diffusion sequences with Eisenman's theory of alkali ion selectivity.

Main Methods:

• Measurement of diffusion coefficients (D) using techniques sensitive to ion mobility within lamellar phases.
• Systematic variation of phase hydration and temperature to observe diffusion rate changes.
• Analysis of activation energies for cation diffusion.

Main Results:

• Diffusion rates for monovalent cations ranged from 10(-7) to 10(-6) cm2/s, and for Ca2+ from 10(-8) to 10(-7) cm2/s.
• Observed diffusion sequences with increasing water content matched predictions from Eisenman's theory.
• Activation energies for diffusion were relatively low (5-10 kcal/mol).

Conclusions:

• Cation diffusion in phosphatidylcholine/water lamellar phases is strongly dependent on hydration levels.
• Diffusional selectivity, governed by non-equilibrium mobility, supports Eisenman's selectivity theory.
• The study provides insights into ion transport mechanisms relevant to biological pore selectivity.