Imaging two-dimensional crystals of catalase by atomic force microscopy

Two-dimensional (2D) crystals of a water-soluble protein, catalase, were imaged in water using atomic force microscopy (AFM). The catalase molecules formed a 2D ordered monolayer by binding to a charged polypeptide monolayer of poly(1-benzyl-L-histidine) (PBLH) spread at the air-water interface. The film of PBLH, bound to catalase molecules, was horizontally transferred onto an alkylated silicon wafer for AFM imaging. The AFM images revealed that the 2D catalase crystal is composed of an array of tetramers. We also found that the pH of the sample during preparation strongly affected the quality of the AFM image because of the imaging resolution influence upon the force acting between the cantilever and the sample. The optimum sample pH was 6.0; at which a weak adhesive force in the range of 10(-10)-10(-11) N was detected. From the results of investigation of the pH dependence of the sample during preparation on catalase-PBLH sample, it was demonstrated that the electrostatic force between the cantilever and the sample could be reduced as a result of charge cancellation at the catalase-PBLH interface (self-screening effect). We present 2D crystal images of catalase molecules acquired through AFM: with molecular resolution; and demonstrate that sample preparation in which pH is optimized to induce the self-screening effect of surface charges is advantageous for AFM imaging of water-soluble protein molecules, because the electrostatic force can be controlled by changing the binding condition of the protein.