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Surface charge movements of purple membrane during light-dark adaptation.

Abstract：

:The difference in the surface charge distribution between light-adapted and dark-adapted purple membranes was investigated with electric dichroism measurements from approximately pH 5 to pH 11. Purple membrane sheets in solution are oriented in a weak electric field by their permanent dipole moment, which is due to the charge distribution of the membrane surfaces and/or within the membrane. The degree of orientation of purple membrane sheets was obtained from the measurement of "electrical anisotropy" of retinal chromophore in the membranes. At about pH 7, there was no difference in the "electric anisotropy" between light- and dark-adapted purple membranes. At about pH 9, the electric anisotropy of dark-adapted purple membrane was larger than that of light-adapted purple membrane. But at around pH 6 the difference was opposite. Linear dichroism experiments did not show any change of retinal tilt angle with respect to the membrane normal between the two forms from approximately pH 5 to pH 10. This result indicates that the changes in the "electric anisotropy" are not due to the change of retinal tilt angle, but due to the change in the permanent dipole moment of the membrane. To estimate the change in surface charges from the permanent dipole moment, we investigated the difference of the permanent dipole moment between the native purple membrane and papain-treated purple membrane in which negative charges in the cytoplasmic-terminal part are removed. This estimation suggests that this light-dark difference at around pH 9 can be accounted for by a change of approximately 0.5 electric charge per bacteriorhodopsin (bR) molecule at either of the two surfaces of the membrane. We also found from pH electrode measurements that at about pH 8 or 9 light adaptation was accompanied by an uptake of approximately 0.1 protons per bR. A possible movement of protons during light-dark adaptation is discussed. The direction of the permanent dipole moment does not change with papain treatment. The permanent dipole moment in papain-treated purple membrane is estimated to be 27 +/-2 debye/bR.

journal_name

Biophys J

journal_title

Biophysical journal

authors

Otomo J,Ohno K,Takeuchi Y,Ikegami A

doi

10.1016/S0006-3495(86)83454-3

subject

Has Abstract

pub_date

1986-08-01 00:00:00

pages

205-11

issue

eissn

0006-3495

issn

1542-0086

pii

S0006-3495(86)83454-3

journal_volume

pub_type

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Surface charge movements of purple membrane during light-dark adaptation.