Slow modes of polarization in purple membranes

Abstract

The polarization of bacteriorhodopsin discs has been characterized by measurements of the electric dichroism induced by sinusoidal electric field pulses in the frequency range from 0.2 to 100 kHz with field strengths up to 40 kV m⁻¹. Analysis of the stationary dichroism by a disk model with a saturating induced dipole moment in the direction of the plane shows saturation of the induced dipole at low field strengths in the range around 10 kV m⁻¹. The AC-polarizability α_AC increases with decreasing frequency, whereas the saturation field strength decreases with decreasing frequency in most cases. DC-polarizabilities α_DC were obtained from the stationary dichroism induced by DC pulses and analyzed by the orientation function for discs with a permanent dipole perpendicular to the plane and an induced dipole in the plane. α_DC is always larger than α_AC-values; α-values increase from 100 kHz to DC by factors of 5 to 9. These data demonstrate the existence of a spectrum of slow polarization processes extending over the μs- into the ms-time range. The increase of polarizability with decreasing frequency was observed in buffers containing Na⁺, Ca²⁺ and Mg²⁺ as counterions. The dichroism rise curves observed under AC-pulses could be described by single exponentials τ^r in the absence of Ca²⁺ or Mg²⁺; 1/τ^r increases linearly with the square of the electric field strength, as expected for induced dipoles. The polarizabilities obtained from the slope of this dependence increase with decreasing frequency; the absolute values are higher by factors of ∼2 than those derived from stationary dichroism data. The dichroism risecurves induced by AC-pulses show an additional slow exponential in the presence of Ca²⁺ or Mg²⁺. The slow amplitude observed in the presence of Mg²⁺ is higher than in the presence of Ca²⁺, suggesting a contribution from Mg²⁺-inner sphere complexes. Thus, polarization data provide information about the coordination state of counterions. A simple criterion for the limit mechanisms of polarization by “biased dissociation” and “motion along surface” is discussed.