Determination of the Surface Potential of Two-Dimensional Crystals of Bacteriorhodopsin by AFM

Abstract

The interaction forces between a silica sphere and two-dimensional bacteriorhodopsin crystals (bR) were measured in NaNO3 solutions at different pH values, and at two electrolyte concentrations. Using the known ζ potential for the silica sphere as a function of pH, the surface potential of the bR crystals could be determined. The isoelectric point (iep) was found to be pH 4.3 ± 0.5. The crystals had a uniform thickness on substrates of 5.2 ± 0.5 nm, and the Hamaker constant for the bR−H2O−SiO2 system was estimated to be 0.5−1.0 × 10-20 J at pH values near the iep. Analysis of the force curves suggested that the surface charge on the protein surface remained constant during approach of the silica colloid probe.