Ph Dependence Of The Protein Orientation In Self-Assembled Bacteriorhodopsin/Polycation Multilayer Films

Bacteriorhodopsin (bR) multilayer films were successfully prepared by alternating depositions of cationic poly(diallyldimethylammonium chloride) (PDAC) polyelectrolyte and purple membrane (PM). The advantage of the method when compared to other thin film techniques is its simplicity to prepare large self-assembled films. The orientations of the bR molecules in the films were studied by a transient photovoltage method. The polarity of the photoresponse signals showed that in basic conditions the cytoplasmic side of PM was adsorbed onto the polyelectrolyte layer, whereas in acidic conditions the extracellular side was adsorbed onto the polyelectrolyte layer. The photocycle of the bR in the films was studied by using the conventional flash photolysis method.The photochemical behavior of bR in dry polycationic films was similar to that in other types of dry films and the intermediate lifetimes were almost independent of the protein orientation. The photovoltage signals introduce opposite proton transport directions for the formation of the K intermediate of the photocycle and for the successive relaxation to M intermediate. The charge displacement associated to the M intermediate was several times greater than that of the K intermediate. The fittings of the transient photovoltage signals resulted the values of 2, 15, and 70 microseconds for the time constants of the 3-exponential formation of the M intermediate. The kinetic data were compared to the results obtained with the transient absorption measurements.