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Polarisation ultrarapide et mouvements vibrationnels dans la bactériorhodopsine étudiés par spectroscopie cohérente d'émission infrarouge.

Abstract : This work aims at elucidating primary proteins dynamics. More specifically, it deals with retinal proteins, which are in particular involved in vision processes. The model protein used is bacteriorhodopsin, a bacterial membrane protein that converts solar energy into a protons gradient. Upon absorption of a photon by the retinal cofactor, two phenomena appear, whose respective role and chronological order are still unknown: retinal isomerization in few hundreds femtoseconds and an ultrafast polarization of the retinal/protein system. The used spectroscopic method enables us to detect and quantify charge displacements with 13 fs time resolution, thus allowing temporal separation of both phenomena. It is based on a second order nonlinear process, optical rectification: following excitation with an oscillating electric field, an ultrafast directional polarization is created in a non centrosymetric medium. This medium consists of multilayer of oriented dry purple membranes, containing bacteriorhodopsin. The infrared beam emitted upon generation of the polarization is detected by letting it interfere with a broadband infrared reference beam created by optical rectification in a nonlinear crystal (GaAs or AgGaS2). The infrared beam emitted by bacteriorhodopsin directly reflects the dipole moment change between ground and excited states. In addition to this instantaneous response, a long lasting (up to several picoseconds) infrared emission is detected, reflecting charge displacements associated with vibrational motions of the retinal/protein complex. The separation of the two types of responses, and the detailed description of the overall signal in terms of frequency and phase, requires concerted use of several analysis methods. In this way, we have shown that an ultrafast (<13 fs) photoinduced transmembrane charges displacement takes place in bacteriorhodpsin. The associated dipole moment change (30 D) is higher in the protein complex, up to a factor 1.5 compared to retinal in solution. The simultaneous detection of both the electronic and vibrational responses paves the way to study the functional role of the initial polarization for the subsequent structural dynamics.
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Submitted on : Wednesday, July 28, 2010 - 9:01:54 AM
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Anne Colonna. Polarisation ultrarapide et mouvements vibrationnels dans la bactériorhodopsine étudiés par spectroscopie cohérente d'émission infrarouge.. Biophysique [physics.bio-ph]. Ecole Polytechnique X, 2005. Français. ⟨pastel-00001534⟩

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