No abstract

The static electric dipole moment persisting in bacteriorhodopsin was defined from electro-acoustic measurements of the dried films of purple membranes and compared with the value estimated from quantum chemical calculations. The projection of this value normal to the membrane surface is experimentally estimated to be equal to 40 D and oriented from the cytoplasmic side to the extracellular side of the membrane. This value is almost independent of the environment pH. QM/MM calculations were also performed for the known structures of the ground and intermediate states of bacteriorhodopsin. According to calculations the dipole moment is mainly determined by the cytoplasmic and extracellular coils, while the contribution from the transmembrane helices is smaller and of the opposite direction, and this value corresponding to the active centre is small. The calculated values of the dipole moment of bacteriorhodopsin in the intermediate states K, L, and M provide understanding about the origin of the driving force for the proton pumping. Employing the values of the dipole moments corresponding to the ground and intermediate states of bacteriorhodopsin, defined by means of QM/MM calculations, the experimentally determined photoelectric response of the dried films is explained.

Comparative analysis of the photoelectric response of dried films of purple membranes (PM) depending on their degree of orientation is presented. Time dependence of the photo-induced protein electric response signal (PERS) of oriented and non-oriented films to a single laser pulse in the presence of the external electric field (EEF) was experimentally determined. The signal does not appear in the non-oriented films when the EEF is absent, whereas the PERS of the oriented PM films demonstrates the variable polarity on the microsecond time scale. In the presence of the EEF the PERS of the non-oriented film rises exponentially preserving the same polarization. The polarization of the PERS changes by changing the polarity of the EEF with no influence on the time constant of the PERS kinetics. The EEF effect on the PERS of the oriented films is more complicated. By subtracting the PERS when EEF not equal 0 from the PERS when EEF = 0 the resulting signal is comparable to that of the non-oriented films. Generalizing the experimental data we conclude that the EEF influence is of the same origin for the films of any orientation. To explain the experimental results the two-state model is suggested. It assumes that the EEF directionally changes the pK(a) values of the Schiff base (SB) and of the proton acceptor aspartic acid D85 in bacteriorhodopsin. Because of that the SB-->D85 proton transfer might be blocked and consequently the L-->M intermediate transition should vanish. Thus, on the characteristic time scale tau( L --> M ) approximately 30 micros; both intermediates, the M intermediate, appearing under normal conditions, and the L intermediate as persisting under the blocked conditions when D85 is protonated, should coexist in the film. The total PERS is a result of the potentials corresponding to the electrogenic products of intermediates L and M that are of the opposite polarity. It is concluded that the ratio of bacteriorhodopsin concentrations corresponding to the L and M intermediates is driven by the EEF and, consequently, it should define the PERS of the non-oriented films. According to this model the orientation degree of the film could be evaluated by describing the PERS.

Two modifications of the Hall‐effect — for ohmic and for acoustoelectric currents — are combined and the problem is considered of the bipolar conductivity of semiconductors from this stand‐point. It is shown that 1) bipolarity exists if both the modifications of the Hall‐effect give different results at the same current; 2) the microscopic mobility of majority carriers is within the limits of mobilities obtained from these two modifications. Experiments are made using CdS single crystals under conditions of infrared quenching of photoconductivity.

The optical transmission modulation observed in the presence of acoustoelectric domains has been investigated in CdS and compared with the light scattering from the acoustic flux. Calculations of the decrease in the optical transmission due to light scattering show that light scattering may account for all the transmission modulation observed in the presence of acoustoelectric domains. The existing arguments for the presence of high ac‐electric fields connected with the domains are discussed, and it is concluded that these arguments probably are not correct.