Photochromism of halorhodopsin. cis/trans isomerization of the retinal around the 13-14 double bond.

The isomeric composition of retinal in membrane-bound and in purified but detergent-free, dark-adapted halorhodopsin was found to be about 70% 13-cis and 30% all-trans. Any illumination increased the all-trans content relative to the dark-adapted state, but blue illumination shifted the isomeric composition more toward all-trans while red illumination of blue-adapted samples shifted it more toward 13-cis. In the presence of chloride this photoisomerization caused the kind of photochromic behavior reported earlier in Smith, S. O., Marvin, M. J., Bogomolni, R. A., and Mathies, R. A. (1984) J. Biol. Chem. 259, 12326-12329, i.e. blue light caused the absorption maximum to move toward longer wavelengths and red light reversed the shift. Only the all-trans chromophore exhibited the complete photocycle described earlier in detergent-solubilized halorhodopsin, and this was the form that could be associated with light-driven chloride transport activity in cell envelope vesicles. In the absence of chloride the spectroscopic changes caused by illumination were much smaller. Reconstitution of bleached preparations with 13-cis- and all-trans-retinal, in the presence and absence of chloride, confirmed that the difference between the absorption maxima of the two isomeric forms of the chromophore is affected by chloride: 13-cis-halorhodopsin absorbs at about 567-568 nm with and without chloride, and the all-trans pigment absorbs near 568 nm in the absence of chloride, but at 578 nm in its presence. The simplest explanation of this finding is that most of the red-shift which accompanies the 13-cis----all-trans transition originates from electrostatic interaction of the retinal with chloride bound in its vicinity.