Photoinduced Transformations in Bacteriorhodopsin Membrane Monitored with Optical Microcavities

Photoinduced molecular transformations in a self-assembled bacteriorhodpsin (bR) monolayer are monitored by observing shifts in the near-infrared resonant wavelengths of linearly polarized modes circulating in a microsphere cavity. We quantify the molecular polarizability change upon all-trans to 13-cis isomerization and deprotonation of the chromophore retinal (∼−57 Å3) and determine its orientation relative to the bR membrane (∼61°). Our observations establish optical microcavities as a sensitive off-resonant spectroscopic tool for probing conformations and orientations of molecular self-assemblies and for measuring changes of molecular polarizability at optical frequencies. We provide a general estimate of the sensitivity of the technique and discuss possible applications.