Biochemistry (1988) 27: 3720-3724.

ABSTRACT:

The influence of membrane supramolecular structure on the conformation and photodynamics of bacteriorhodopsin (bR) was studied by utilization of comparative absorption and circular dichroic (CD) spectroscopy of the native and a delipidated contracted lattice form of the purple membrane. The contracted form had similar absorption spectra in both the light-adapted, bR568 and the photocycle intermediate M412 states of the bacteriorhodopsin as compared to the native lattice form. The excitonic contribution to the visible CD spectrum was slightly greater in the contracted form than in the native lattice form. However, in both forms excitonic coupling, which is indicative of membrane order, was lost during the M412 photoconversion. Furthermore, as deduced from CD spectra, the conformation as well as the global structural changes associated with the bR568 to M412 transformation was essentially unaffected by the lattice supramolecular structure. In contrast, the light/dark-adaptation and the M decay rates were greatly decreased in the contracted form as compared to those in the native form. Also, the contracted form, and more importantly any purple membrane system with a slowed photocycle, was bleached much more rapidly than native purple membrane in the presence of light and hydroxylamine. These data can be related to protein structural changes consistent with a model for the reversible tilting of the a-helical segments of the bacteriorhodopsin away from the membrane normal during the photocycle.