Photoelectric properties of oriented bacteriorhodopsin/polycation multilayers by electrostatic layer-by-layer assembly

Organized, heterogeneous polyelectrolyte assemblies using poly(dimethyldiallylammonium chloride) (PDAC) as the polycation and bacteriorhodopsin (bR) (purple membrane fragments, wild-type (WT-PM) and D96N mutant) as the polyanion have been successfully constructed using alternating electrostatic adsorption. The differential photocurrent of the PDAC/bR multilayer assemblies showed a light-on maximum photocurrent of 52 nA/cm(2) for PDAC/WT-PM (eight bilayers) and 80 nA/cm(2) for PDAC/D96N (six bilayers). These results indicate that the biological integrity and activity of bacteriorhodopsin are maintained after electrostatic deposition and that the photoelectric sensitivity of the D96N mutant is higher than that of the WT-PM. It was determined that the photocurrent in these assemblies increases with the adsorption of only the first few layers of bR and then declines with the increase in the number of bilayers. It was also found that the magnitude and polarity of the photocurrent are greatly dependent on the pH, which suggests that the current is strongly correlated with the local change of proton concentration at the electrode/electrolyte interface. Addition of sodium azide to the electrolyte was found to instantly increase the magnitude of the photocurrent of the WT-PM and then the photocurrent attached saturation when [NaN3] > 100 mM, while an initial increase and then a decrease of the photocurrent was observed with the D96N mutant. These results show that the differential photocurrent of bR results originally from the action of formation and decay of the M intermediate, which then leads to the local change of proton concentration at the electrode/electrolyte interface. That is, the formation of M intermediate (which results in an increase of proton concentration at the electrode/electrolyte interface due to proton release of bR) produces the light-on photocurrent, while the decay of M intermediate (which results in a decrease of proton concentration at the interface due to proton uptake of bR) contributes to the light-off photocurrent.