First Steps of Retinal Photoisomerization in Proteorhodopsin

• Published in: Biophysical journal Vol. 91; no. 1; pp. 255 - 262
• Main Authors: Lenz, Martin O., Huber, Robert, Schmidt, Bernhard, Gilch, Peter, Kalmbach, Rolf, Engelhard, Martin, Wachtveitl, Josef
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 01.07.2006; Biophysical Society
• Subjects: Biophysics; Fluorescence; Isomerism; Light; Membranes; Photobiophysics; Photochemistry - methods; Proteins; Retinaldehyde - chemistry; Retinaldehyde - radiation effects; Rhodopsin - chemistry; Rhodopsin - radiation effects; Rhodopsins, Microbial; Spectrum analysis
• ISSN: 0006-3495; 1542-0086
• DOI: 10.1529/biophysj.105.074690

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.