The photovoltage signals of bacteriorhodopsin in Langmuir–Blodgett films with different molecular orientations

The photoresponse of bacteriorhodopsin (bR) has been studied in Langmuir–Blodgett (LB) films bR was deposited by vertical dipping as x- or z-type monolayers and as z-type multi layers between stearic acid LB films. The electrodes used were a transparent vacuum-deposited ITO on a quartz substrate and an InGa metal alloy. The signals were observed from samples containing a single bR layer or five bR layers both at room humidity and in water-saturated conditions. The photoelectric signals of x- and z-type monolayers had opposite polarities and both had two exponential kinetics on a microsecond time scale with time constants of about 10 and 70 μs. The amplitude of the fast negative component of the signal of a bR monolayer was 0.4 mV, in the opposite direction to that in which protons were pumped. The amplitude due to five layers was five times as high as that due to a monolayer. The ratio between the amplitudes of the fast negative component and the slow positive part of the signal, in the proton-pumping direction, was 1:3. The photoresponse signal saturated with a light intensity of about 1.7 mJ cm−2, which is equivalent to a photon density of 0.5 A−2. The activation energy of the bR photovoltage was 75 ± 5 kJ mol−1 in the temperature range between 17 and 35°C.