INTERPRETATION OF THE ABSORPTION AND CIRCULAR DICHROIC SPECTRA OF ORIENTED PURPLE MEMBRANE FILMS

The absorption and circular dichroic (CD) spectra of [Halobacterium halobium] purple membrane films in which the plane of the membranes is oriented perpendicular to the incident beam are compared with the solution spectra. Structural features of the purple membrane can be related to a coordinate system as defined by a normal to the membrane plane and 2 mutually perpendicular in-plane axes. The film and solution absorption spectra were similar except for a relative depression in the 200-225 nm region of the film spectrum. The CD spectra showed significant differences in the visible region, where the biphasic band in the solution spectrum was replaced by a single positive band at 555-nm in the film spectrum and in the far UV region, where the 208-nm band was deleted from the film spectra of the native and regenerated membranes. A small shoulder occurred at 208-nm in the film spectrum of the bleached membrane. The near UV spectra also showed differences, whereas the 317-nm band remained essentially the same for both spectra. Excitonic interpretations of the visible and far UV spectra led to the following conclusions: a relatively strong in-plane monomeric interaction occurs between the retinyl chromophore and apoprotein; the helical axes of the native and regenerated membrane proteins are tilted more in-plane than the axes of the native or regenerated membrane. Apparently an interaction occurs between an in-plane magnetic dipole moment of the retinyl chromophore and probably an in-plane electric dipole moment of a nearby aromatic amino acid(s), and although the membrane is anisotropic with respect to coupling between electric and magnetic moments of the aromatic amino acids, the transition dipole moments of the aromatic amino acids are not preferentially oriented in either direction.