pH dependence of the protein orientation in self-assembled bacteriorhodopsin/polycation multilayer films

摘要

Bacteriorhodopsin (bR) multilayer films were successfully prepared by alternatingdepositionsof cationic poly(diallyldimethylammonium chloride) (PDAC) polyelectrolyte and purple membrane (PM). Theadvantage of the method when compared to other thin film techniques is its simplicity to prepare large selfassembledfilms. The orientations of the bR molecules in the films were studied by a transient photovoltagemethod. The polarity of the photoresponse signals showed that in basic conditions the cytoplasmic side of PMwas adsorbed onto the polyelectrolyte layer, whereas in acidic conditions the extracellular side was adsorbedonto the polyelectrolyte layer. The photocycle of the bR in the films was studied by usingthe conventionalflash photolysis method. The photochemical behavior of bR in dry polycationic films was similar to that inother 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 intermediateof the photocycle and for the successive relaxation to M intermediate. The charge displacementassociated to the M intermediate was several times greater than that of the K intermediate. The fittings of thetransient photovoltage signals resulted the values of 2, 15, and 70 microseconds for the time constants ofthe 3-exponential formation of the M intermediate. The kinetic data were compared to the results obtainedwith the transient absorption measurements.