Electric birefringence study of the purple membrane of <i>Halobacterium halobium</i>

Kahn, Leo D.; Tu, Shu‐I

doi:10.1002/bip.360230410

Cited by 9 publications

(4 citation statements)

References 27 publications

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“…These measurements have been used to characterize the electric moment of bacteriorhodopsin in its standard state. [5][6][7][8][9][10][11][12][13][14][15] Vectorial transport of protons by bacteriorhodopsin is controlled by a combination of different effects: light absorption induces a large pK change of the Schiff base and, thus, triggers a chain of proton-transfer reactions associated with changes in the protein structure. Changes of protein structure seem to be essential for the "relay" system, which directs the protons toward the extracellular side.…”

Section: Introductionmentioning

confidence: 99%

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Dipole Reversal in Bacteriorhodopsin and Separation of Dipole Components

Wang

Pörschke

2003

J. Phys. Chem. B

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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.

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Section: Introductionmentioning

confidence: 99%

“…A combined set of optical and electric parameters can be obtained by “molecular electrooptical” measurements. These measurements have been used to characterize the electric moment of bacteriorhodopsin in its standard state. − …”

Section: Introductionmentioning

confidence: 99%

Dipole Reversal in Bacteriorhodopsin and Separation of Dipole Components

Wang

Pörschke

2003

J. Phys. Chem. B

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“…Electro-optical studies including measurements of the electric dichroism, birefringence and light scattering [8][9][10][11][12][13][14][15][16][17][18] demonstrated that purple membranes have a large permanent dipole moment in the direction perpendicular to the disk plane and a large induced dipole moment along the disk plane (Fig. 1).…”

Section: Structure and Electric Parameters Of Purple Membranesmentioning

confidence: 99%

“…[1][2][3] A favored model system for analysis has been bacteriorhodopsin, [4][5][6][7] because it is easily available and relatively stable. Its high anisotropy both with respect to electric and optical parameters [8][9][10][11][12][13][14][15][16][17][18] simplifies experimental characterization by electro-optical methods. 19,20 A standard approach to determination of electric parameters requires measurements of electrooptical data (dichroism, birefringence or light scattering) using both high frequency and DC-pulses.…”

Section: Introductionmentioning

confidence: 99%

Slow modes of polarization in purple membranes

Pörschke

2004

Phys. Chem. Chem. Phys.

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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.

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Electric field induced conformational changes of bacteriorhodopsin in purple membrane films

Tsuji¹,

Hess²

1986

Eur Biophys J

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Electric birefringence study of the purple membrane of Halobacterium halobium

Cited by 9 publications

References 27 publications

Dipole Reversal in Bacteriorhodopsin and Separation of Dipole Components

Dipole Reversal in Bacteriorhodopsin and Separation of Dipole Components

Slow modes of polarization in purple membranes

Electric field induced conformational changes of bacteriorhodopsin in purple membrane films

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