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Structural Changes in Bacteriorhodopsin Caused by Two-Photon-Induced Photobleaching

Authors :
Ivan Chizhik
Martin Imhof
Roelf-Peter Baumann
Norbert Hampp
Daniel Rhinow
Source :
The Journal of Physical Chemistry B. 116:7455-7462
Publication Year :
2012
Publisher :
American Chemical Society (ACS), 2012.

Abstract

Bacteriorhodopsin (BR) is the key protein of the halobacterial photosynthetic system. BR assembles into two-dimensional crystalline patches, the so-called purple membranes (PM), and acts as a light-driven proton pump converting light energy into the chemical energy of a proton gradient over the cell membrane. The two-photon absorption (TPA) of BR is so far not fully understood. Astonishingly high TPA cross sections have been reported, but the molecular mechanisms have not been elucidated. In this work, we address structural changes in BR and PM upon TPA, investigating its TPA photochemistry by spectroscopy, small-angle X-ray scattering, as well as electron and atomic force microscopy. We observe that TPA of BR leads to formation of an UV-absorbing N-retinyl-bacterioopsin state, which is accompanied by the loss of crystalline order in PM. FTIR and CD spectroscopy confirm that BR trimers as well as the secondary structure of the BR molecules are preserved. We demonstrate that excitation by TPA results in the photochemical reduction of the retinal Schiff base, which in turn causes a permanent asymmetric shape change of BR, similar to the one transiently observed during the photocycle-related opening and closing of the cytoplasmic proton half channel. This shape change causes PM sheets to merely roll up toward the extracellular side and causes the loss of crystallinity of PM. We present a model for the TPA photoresponse of BR, which also explains the irreversibility of the process in terms of a photochemical reduction of the Schiff base.

Details

ISSN :
15205207 and 15206106
Volume :
116
Database :
OpenAIRE
Journal :
The Journal of Physical Chemistry B
Accession number :
edsair.doi.dedup.....df6afc9cf4ea8ec74298782dfaf01e92
Full Text :
https://doi.org/10.1021/jp2112846