Surface-coupled proton exchange of a membrane-bound proton acceptor

Area of Science:

• Biophysics
• Membrane Biophysics
• Physical Chemistry

Background:

• Proton-transfer reactions are crucial for cellular energy conversion via transmembrane proton gradients.
• Understanding these reactions at the membrane surface is key to cellular bioenergetics.

Purpose of the Study:

• To investigate local protonation and deprotonation rates at biological membrane surfaces.
• To determine the influence of lipid composition, ion concentration, and pH on proton exchange kinetics.
• To elucidate the conditions under which membranes function as proton-collecting antennae.

Main Methods:

• Utilized fluorescence correlation spectroscopy to measure proton exchange rates.
• Employed pH-sensitive fluorophores with different pK(a)s conjugated to liposomes.
• Varied lipid composition, ion concentration, and pH to study proton exchange dynamics.

Main Results:

• Identified two distinct proton exchange regimes based on pH.
• Observed membrane surface acting as a proton-collecting antenna at high pH (> 8).
• Found protonation rates are dictated by the local membrane environment at low pH (< 7).
• Quantified membrane surface potentials and fluorophore-surface distances based on ion concentration and surface charge.

Conclusions:

• Biological membranes can function as proton-collecting antennae under specific conditions.
• Established a relationship between membrane surface charge density and local proton exchange kinetics.
• Provided fundamental insights into proton transfer mechanisms at biological interfaces.