Chromophore reorientation during the photocycle of bacteriorhodopsin: experimental methods and functional significance

Light-induced isomerization leads to orientational changes of the retinylid ene chromophore of bacteriorhodopsin in its binding pocket. The chromophore reorientation has been characterized by the following methods: polarized a bsorption spectroscopy in the visible, UV and IR; polarized resonance Raman scattering; solid-state deuterium nuclear magnetic resonance; neutron and X-ray diffraction. Most of these experiments were performed at low temperat ures with bacteriorhodopsin trapped in one or a mixture of intermediates. T ime-resolved measurements at room temperature with bacteriorhodopsin in aqu eous suspension can currently only be carried out with transient polarized absorption spectroscopy in the visible. The results obtained to date for th e initial state and the It, L and M intermediates are presented and discuss ed. The most extensive data are available for the M intermediate, which pla ys an essential role in the function of bacteriorhodopsin. For this interme diate the various methods lead to a consistent picture: the curved all-tran s polyene chain in the initial state straightens out in the M intermediate (13-cis) and the chain segment between C-5 and C-13 tilts upwards in the di rection of the cytoplasmic surface. The kink at C-13 allows the positions o f beta-ionone ring and Schiff base nitrogen to remain approximately fixed. (C) 2000 Elsevier Science B.V. All rights reserved.