Buffer-induced changes of purple membrane surface electric properties: an electro-optical study

Area of Science:

• Biophysics
• Membrane Biophysics
• Biochemistry

Background:

• Purple membranes contain bacteriorhodopsin (bR), a light-driven proton pump.
• The electric dipole moment and surface charge of membranes are crucial for their function.
• Buffer solutions can influence membrane properties through interactions with surface charges.

Purpose of the Study:

• To investigate how different buffer molecules affect the electric dipole moments and electrokinetic charge of purple membranes.
• To understand the differential interaction of buffer molecules with purple membrane surfaces.

Main Methods:

• Electric light scattering was used to measure electric dipole moments.
• Microelectrophoresis was employed to determine electrophoretic mobility and surface charge.
• Analysis of light-induced charge displacement currents in the presence of different buffers.

Main Results:

• 'P'-type and 'N'-type buffer molecules induced opposing changes in the permanent dipole moment and electrophoretic mobility of purple membranes.
• 'P'-type and 'N'-type buffers introduced distinct 'positive' and 'negative' components to the bR light-induced charge displacement current.
• Differential distribution of buffer molecules on the cytoplasmic and extracellular membrane surfaces was observed, dependent on their protonation state and interaction.

Conclusions:

• Buffer molecules significantly alter the electric properties of purple membranes.
• The interaction of buffer molecules with membrane surfaces is dependent on their chemical nature and protonation state.
• Differential surface interactions of buffer molecules explain their distinct effects on membrane electrokinetics and charge displacement.