Structural changes in bacteriorhodopsin induced by electric impulses

Electric impulses of high field intensity (2 x 105 to 3 × 106 Vm1, 1 to 20 #s duration) cause transient changes in the optical absorbance of suspended purple membranes ofHalobacterium halobium. The electric dichroism at 1 mM NaCl, pH ~ 6,and at 293K is dependent on field strength, pulse duration and wavelength of the monitoring, plane-polarized light in the range 400 to 650 nm. The optically detected processes are~ however, independent of bacteriorhodopsin concentration, of ionic strength and of the intensity of the monitoring light. These data together with the analysis of time course and steady state of the reduced dichroism, suggest electric field-sensitive, intramembraneous structural changes which lead to restricted orientation changes of the chromophore. A theoretical analysis of restricted orientation is developed and applied to the electro-optic data. As a result, it is .found that the electric dichroism of purple membranes is associated with a large polarizability anisotropy of 2.4 × I0 30 Fm 2 (2.2 x 10-14 cm3), the electric permanent dipole moment which is involved amounts to 4.7 x 1028 C m (140 Debye). The kinetic data suggest a cyclic reaction scheme with at leastfive different conformations. The high polarizability is probably due to displaceable ionic groups within the cooperative lattice ofbacteriorhodopsin molecules in purple membranes.