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 À1. 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 À1. The AC-polarizability a AC increases with decreasing frequency, whereas the saturation field strength decreases with decreasing frequency in most cases. DC-polarizabilities a 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. a DC is always larger than a AC-values; a-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 msinto the ms-time range. The increase of polarizability with decreasing frequency was observed in buffers containing Na + , Ca 2+ and Mg 2+ as counterions. The dichroism rise curves observed under AC-pulses could be described by single exponentials t r in the absence of Ca 2+ or Mg 2+ ; 1/t 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 Ca 2+ or Mg 2+. The slow amplitude observed in the presence of Mg 2+ is higher than in the presence of Ca 2+ , suggesting a contribution from Mg 2+-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.

Steady electro-optic characteristics of noninteracting colloidal particles

“…The above procedure is not consistent with the clearly revealed dependence of the shape of the frequency curves on field intensity [2,3]. Saturation of particle slow polarizability (related to mobility of counterions) is usually advanced as an explanation of the discrepancy [2,4]. In this paper we propose an alternative point of view on the low frequency anomalies in the electro-optic behavior of charged particles.…”

“…Electric polarizability obtained at kilohertz frequency is used in the analysis of data for DC pulses to determine the permanent dipole. The above procedure is not consistent with the clearly revealed dependence of the shape of the frequency curves on field intensity [2,3]. Saturation of particle slow polarizability (related to mobility of counterions) is usually advanced as an explanation of the discrepancy [2,4].…”

“…In the present paper we apply the proposed method to another colloidal system (monodisperse α-Fe 2 small axial ratio) and compare the results with literature data for particles of different geometry, structure, and mechanism of surface charging. The aim is to demonstrate similar tendencies in the frequency variation of the electro-optic responses in the hydrodynamic frequency range and the applicability of the proposed procedure for analysis of the effects in this range.…”

The universal electro-optic response of charged colloids in low electrolyte suspensions

“…Actually we did not find literature data indicating the absence of this effect for any anisotropic polarizable particles. Even for diskshaped particles with structure enhancing the existence of a transverse permanent dipole, the latter is not sufficient to explain the significant value of the negative effect at higher fields [17].…”

“…[2]. They are attributed to various causes: interaction forces [4,5], particle permanent dipoles [6,7], the presence of two types of particles [8], asymmetry of particle surface charge [9], field-induced particle deformation [10,11], interaction enhanced transverse conductivity [12,13], cluster orientation of self-assembled rod-shaped particles [14], ion-fluctuation dipole moments [15], saturation of electric polarizability [16,17], electro-* Fax: +359 2 9712688. E-mail address: marias@pchp.ipc.bas.bg.…”