Microinjection in combination with microfluorimetry to study proton diffusion along phospholipid membranes

Area of Science:

• Biophysics
• Membrane Biophysics
• Physical Chemistry

Background:

• Proton diffusion is crucial for biological processes.
• Understanding proton transport across lipid membranes is key to cellular function.
• Previous studies suggest proton diffusion occurs primarily in the bulk phase.

Purpose of the Study:

• To measure proton diffusion along a planar bilayer lipid membrane surface.
• To investigate the dimensionality of proton diffusion (2D vs. 3D).
• To explore the impact of membrane-bound protonation sites on diffusion dynamics.

Main Methods:

• Acid/base injection using a micropipette.
• Monitoring fluorescence changes of fluorescein-labeled lipids.
• Fitting kinetic data with 2D and 3D diffusion models.

Main Results:

• Lateral proton diffusion occurred via the bulk phase with buffer molecules as carriers (D = 600 µm²/s) at 1 mM buffer concentration.
• Introduction of proton binding sites on the membrane surface resulted in significant two-dimensional (2D) proton diffusion (D = 1,100 µm²/s).

Conclusions:

• Membrane surface proton binding sites can induce a substantial 2D diffusion component.
• The experimental system allows for studying proton diffusion rates influenced by membrane composition.
• This work provides insights into proton transport mechanisms at membrane interfaces.