First Steps of Retinal Photoisomerization in Proteorhodopsin

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

Abstract

The early steps (<1ns) 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 (<150fs) and torsional (∼300fs) 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 1ps. However, no major influence on the fast time scales (∼150fs) 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.
Original languageEnglish
JournalBiophysical Journal
Volume91
Issue number1
Pages (from-to)255-262
Number of pages8
ISSN0006-3495
DOIs
Publication statusPublished - 2006

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