Application of tunable quantum cascade lasers to monitor dynamics of bacteriorhodopsin in the mid-IR spectral range

The function of membrane proteins is highly impacted by their membrane environment. One suitable approach to get insights into the interaction-induced dynamics of membrane proteins and lipid membranes is time-resolved infrared (IR) spectroscopy. Conclusions about environmental influences to the system can be drawn by correlating the observed kinetics to the well-characterized photocycles of light-driven transmembrane proton pumps like bacteriorhodopsin (BR). For the investigation of photoreceptor-membrane interactions, also minor changes in the absorption spectra must be resolved. Therefore, we applied IR laser spectroscopy using tunable quantum cascade lasers (QCLs) as IR light source. Several QCLs were implemented in a home-built spectrometer and provide a tunability in a broad spectral region covering protein, chromophore and lipid vibrational modes. Kinetics of the BR photocycle were monitored at single wavenumbers. This study demonstrates the high potential of QCL-based spectroscopy for the application to membrane protein studies.