Anisotropic electric properties of purple membrane and their change during the photoreaction cycle

Abstract

Purple membrane suspension shows two different orientations in electric fields of different frequencies. The orientation at low frequencies (less than or equal to approximately 10 Hz), with the membrane surface perpendicular to the electric field, is due to permanent dipole moment of the membrane and the orientation at high frequencies (greater than or equal to approximately 100 Hz), with the surface parallel to the electric field, is due to induced dipole moment. By quantitative analysis of these orientations, we determined the permanent dipole moment and the polarizability. Both values varied according to the membrane size: the permanent dipole moment ranged from 500 kD to 10 MD and was proportional to the square of the diameter of the membrane. The polarizability ranged from 1 X 10(-13) to 1 X 10(-11)cm3 and was proportional to the third to fourth power of the diameter. Because the permanent dipole moment was proportional to the area of the membrane, we could determine permanent dipole moment per bacteriorhodopsin. By determining the actual membrane size under electron microscopy, we got 98 D/bacteriorhodopsin. We also concluded that the direction of the permanent dipole moment was from the cytoplasmic to the extracellular side. These values, however, were strongly dependent on the ionic strength in the medium, suggesting a screening effect due to counter ions near the membrane surface. We evaluated the screening effect and showed about a four-charge difference between the two sides of the purple membrane. Under illumination, we found that the permanent dipole moment decreased from 98 to 63 D/bacteriorhodopsin. From the best-oriented sample, we also concluded that the angle of retinal against the axis normal to the membrane surface was greater than 68.6 degrees.

Estimation of the permanent dipole moment of bacteriorhodopsin

“…The internal electric DM detected by the electroacoustic technique qualitatively is not dependent on pH variation (Fig. 1), which strongly contradicts the experimental observations carried out on the PM suspensions where the value of the permanent DM of BR varies up to changing its direction depending on pH [12,[16][17][18][19].…”

“…From the sign of the photoresponse signal it has been deduced that the cytoplasmic side is charged more negatively when pH is higher than 5 and vice versa when pH is less than 5 [22], while at pH 5 both sides are almost equally charged [22,23]. However, in accord with predictions [12] the inverse DM of the BR molecule under the normal external conditions directed from the cytoplasmic side to the extracellular side has been identified from studies of the dried PM films [15]. Hence the dehydratation of the sample should have a crucial impact on the electric asymmetry of the PM.…”

“…Possibilities of orientation of the PM evidently confirm the existence of a permanent dipole moment at low frequencies and an induced dipole moment at high frequencies of the applied field [10][11][12][13][14]. This electric asymmetry seems to be responsible for the proton driving through the membrane [15].…”

“…However, the result can be conditioned by a large number of different factors. Indeed, the value of the DM of the PM is strongly dependent on the size of the membrane disc, the concentration of the PM fragments, and polarity of the environment perturbing the membrane surface [12,[16][17][18]. Hence, the conclusion about the nature of electric parameters of the PM is not a trivial task.…”

Fabrication and characterization of a radiation sensor based on bacteriorhodopsin

“…The direction of the dipole moment is also the same as the proton pumping direction, i.e. from inside to outside of the cell (Kimura et al, 1984). The proton pumping process in BR is related with diffusion phenomenon, however long range electrostatic interactions between the dipole moment and the released protons may help the process (Porschke, 1997).…”