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First Steps of Retinal Photoisomerization in Proteorhodopsin

Martin O. Lenz ^*, Robert Huber ^*, Bernhard Schmidt , Peter Gilch , Rolf Kalmbach , Martin Engelhard  and Josef Wachtveitl ^*

^* Institut für Physikalische und Theoretische Chemie, Johann-Wolfgang-Goethe-Universität, Frankfurt, Germany; Universität München, Department für Physik, Lehrstuhl für BioMolekulare Optik, Munich, Germany; and Max-Planck-Institute of Molecular Physiology, Department of Physical Biochemistry, Dortmund, Germany

Correspondence: Address reprint requests to Josef Wachtveitl, E-mail: wveitl{at}theochem.uni-frankfurt.de.

The early steps (<1 ns) in the photocycle of the detergent solubilized proton pump proteorhodopsin are analyzed by ultrafast spectroscopic techniques. A comparison to the first primary events in reconstituted proteorhodopsin as well as to the well known archaeal proton pump bacteriorhodopsin is given. A dynamic Stokes shift observed in fs-time-resolved fluorescence experiments allows a direct observation of early motions on the excited state potential energy surface. The initial dynamics is dominated by sequentially emerging stretching (<150 fs) and torsional (300 fs) modes of the retinal. The different protonation states of the primary proton acceptor Asp-97 drastically affect the reaction rate and the overall quantum efficiencies of the isomerization reactions, mainly evidenced for time scales above 1 ps. However, no major influence on the fast time scales (150 fs) could be seen, indicating that the movement out of the Franck-Condon region is fairly robust to electrostatic changes in the retinal binding pocket. Based on fs-time-resolved absorption and fluorescence spectra, ground and exited state contributions can be disentangled and allow to construct a reaction model that consistently explains pH-dependent effects in solubilized and reconstituted proteorhodopsin.

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