Ulrtrafast Water Dynamics in Bacteriorhodopsin

Abstract

Protein bound water molecules play an important role for protein function. In the trans-membrane proton pump bacteriorhodopsin (BR) water molecules in the proton transport channel and in the retinal chromophor binding pocket have been shown by means of infrared vibrational spectroscopy to participate in the light driven proton transport mechanism (Garczarek et al., Nature 2006, 439, 109). Static low temperature FTIR experiments strongly suggest that the pentagonal water cluster involving three water molecules W401, W402 and W406 as well as the retinal Schiff-base, Asp85 and Asp212, is perturbed when the ground state BR570 is photoconverted to the K610 state (H. Kandori et al., J.Phys.Chem.B 1998, 102, 7899). We have now investigated the spectral range above 3.3 μm by means of femtosecond time resolved transient mid-IR spectroscopy. Here we find a broad IR absorption band which bleaches within (or even faster than) retinal isomerization, i.e. 0.5 ps. This finding is strongly corroborated by quantum-chemical QM/MM calculations that attribute this continuum band to a polarization coupling between the protonated retinal Schiff-base N-H stretch and water W402 in BR570 (M. Baer et al., ChemPhysChem 2008, 9, 2703). Our results indicate that the pentagonal water cluster is heavily perturbed already on this time scale.