Dipole Reversal in Bacteriorhodopsin and Separation of Dipole Components

Abstract

The electrostatics of purple membranes has been analyzed by measurements of the electric dichroism in dc and ac fields in a broad pH range. The dc data are mainly used to characterize the permanent dipole, whereas the ac data serve as control for changes of global structure and of the induced dipole. At pH values from 8 to 3.5, the dc dichroism is negative at low field strengths and turns to positive values at higher field strengths, in qualitative agreement with the orientation function for disks having a permanent dipole moment perpendicular to the plane and an induced dipole moment in the plane. The minimum value of the dichroism, , indicates the permanent dipole and shows a complex dependence on the pH but does not approach zero at any pH between 3.5 and 8, which would be expected for a simple reversal of the permanent dipole. However, the dependence of on pH shows a λ point at pH 4.9, which reflects reversal of the dipole. Fitting of the stationary dichroism for E ≤ 12 kV m-1 to the orientation function shows a decrease of the permanent dipole μ in the pH range 5 to about 50% of the value found at pH 7. In the same pH range, the limit dichroism, , derived from dc data also decreases to about 50% of the value at pH 7, whereas parallel measurements of the ac dichroism show almost constant values. The combined observations indicate reversal of the permanent dipole, changes of disk bending, and the existence of a fluctuating dipole moment, probably resulting from bending fluctuations. Reversal of the dipole moment at pH 4.9 is confirmed by combined pH and field jump experiments. The direction of the dipole is reverted in the same pH range as the direction of stationary pH changes upon illumination, indicating an important function of the dipole for vectorial proton transfer. Comparison of experimental data with simple calculations of the protein dipole from the crystal structure indicates the existence of a large dipole component, which is directed opposite to the protein dipole at pH 7 and is probably due to a nonsymmetric distribution of charges on lipid residues. The results indicate a high degree of compensation in the electric asymmetry, which seems to be necessary for stability of purple membranes.