Chromophore motion during the bacteriorhodopsin photocycle: Polarized absorption spectroscopy of bacteriorhodopsin and its M-state in bacteriorhodopsin crystals

Abstract

The three-dimensional crystallization of bacteriorhodopsin was systematically investigated and the needle-shaped crystal form analysed. In these crystals the M-intermediate forms 10 times faster and decays 15 times more slowly than in purple membranes. Polarized absorption spectra of the crystals were measured in the dark and light adapted states. A slight decrease in the angle between the transition moment and the membrane plane was detected during dark adaptation. The crystallization of a mutated bacteriorhodopsin, in which the aspartic acid at residue 96 was replaced by asparagine, provided crystals with a long lived M-intermediate. This allowed polarized absorption measurements of the M-chromophore. The change in the polarization ratio upon formation of the M-intermediate indicates an increase in the angle between the main transition dipole and the membrane plane by 2.2° ± 0.5, corresponding to a 0.5 Å displacement of one end of the chromophore out of the membrane plane of the bacteriorhodopsin molecule.