Lateral diffusion of ions near membrane surface.

Area of Science:

• Biophysics
• Computational Biology
• Membrane Biophysics

Background:

• Biological membranes control molecular transport, crucial for cellular function.
• The precise mechanism of ion diffusion at membrane interfaces remains poorly understood.
• Understanding ion dynamics is key to processes like neural conduction.

Purpose of the Study:

• To investigate the lateral diffusion of monovalent ions (Na+, K+, Cl-) at the surface of a zwitterionic phospholipid membrane.
• To elucidate the binding interactions between ions and membrane components.
• To determine the influence of these interactions on ion mobility.

Main Methods:

• Classical molecular dynamics (MD) simulations were employed.
• Realistic force fields for lipids (Amber Lipid17/21) and water (TIP4P-Ew) were utilized.
• Mass/charge densities and electrostatic potential across the POPC membrane were analyzed.

Main Results:

• Chloride ions form hydrogen bonds with the choline headgroup via water.
• Cations bind to phosphatic and carbonyl oxygens, shedding hydration water for headgroup atom binding.
• Cation binding to 3-4 headgroup atoms dictates immobilization or energization, controlling surface diffusion rates.

Conclusions:

• Interfacial ion binding significantly influences lateral diffusion rates.
• Cation diffusion at the membrane surface is only moderately slower than in bulk solution.
• K+ diffusion exhibits anomalous behavior, transitioning between sub- and super-diffusion regimes.