Electrostatics and electrodynamics of bacteriorhodopsin

Area of Science:

• Biophysics
• Structural Biology
• Membrane Proteins

Background:

• Bacteriorhodopsin is a light-driven proton pump.
• Its structure and function are crucial for understanding biological energy transduction.
• Electric dichroism offers insights into molecular orientation and dynamics.

Purpose of the Study:

• To quantitatively analyze the stationary electric dichroism of bacteriorhodopsin.
• To determine the contributions of permanent and induced dipoles to its orientation.
• To investigate the relationship between molecular structure and proton transport mechanisms.

Main Methods:

• Stationary electric dichroism measurements at varying electric field strengths.
• Analysis using an orientation function for disk-shaped molecules.
• Comparison of experimental data with Brownian dynamics simulations.

Main Results:

• Permanent and induced dipoles were characterized, with the permanent dipole moment increasing with disk radius squared and polarizability with radius to the fourth power.
• The permanent dipole moment was found to be 4x10^6 D at 0.5 micron radius, while the induced dipole reached 4x10^8 D at 40 kV/cm.
• A cooperative structural change in bacteriorhodopsin was observed at low electric fields (approx. 150 V/cm), not predicted by simulations.

Conclusions:

• Experimental dipole moments reflect the mu-potential at the plane of shear, analogous to the sigma-potential for translational diffusion.
• The permanent dipole likely aids proton transport by influencing proton attraction/repulsion.
• A cooperative structural change, induced by low electric fields, suggests a novel mechanism in bacteriorhodopsin function.