Orientationally Enhanced Photocurrent and Electrooptic Modulation Effects in Bacteriorhodopsin/Polyvinyl Alcohol Thin Films

Polymeric films of bacteriorhodopsin, formed by embedding purple membrane fragments into a poly(vinyl alcohol) matrix without field poling, exhibit modulated photocurrent and electrooptic effects when an AC modulating rectangular voltage or a sinusoidal voltage is applied across the films. When the modulating frequencies are lower than 100 Hz, both effects are enhanced by the orientation of the purple membrane fragments and exhibit two different orientational time constants. The orientational enhancement at low frequencies (∼7.1 Hz) arises from the permanent dipole moments of membranes, and the enhancement at higher frequencies (∼74.8 Hz) is attributed to the induced dipole moments of the purple membrane molecules. For quantitative analyses of these processes, a theoretical model is proposed. From the combination of model calculations and fitted experimental results, we determine that the permanent dipole moment of the membrane material is ∼33 kD and the induced dipole moment is ∼10-13 cm3. We also demonstrate that when a poling field is applied across the films, the electrooptic coefficients are increased linearly with increasing field strength.