Resolution and Limitations in Biological Applications of Atomic Force Microscopy

Abstract

Atomic force microscopy (AFM) has been applied to image DNA and a membrane protein: cholera toxin. By use of the Kleinschmidt method, DNA molecules were picked up on carbon-coated mica surfaces and imaged by AFM in air and in organic solvents. The resolution was found to be closely related to the adhesion force and a resolution of 3–6 nm was routinely obtained when the adhesion force was below 3 nN. The role of the adhesion force, the tip condition and the specimen preparation on resolution and imaging quality will be discussed. Polymerized diacetylene phosphatidylcholine (DAPC) bilayers provide a relatively stable matrix for studying membrane proteins. When cholera toxin (complete or B-subunit oligomer) was bound to mixed bilayers of DAPC and the receptor glycolipid GM1, the subunit structure was well resolved by AFM in buffer, without crystallization. The resolution was better than 2 nm with excellent reproducibility for a probe force of 0.3–0.5 nN. These results show that individual biomacromolecules under native conditions can be imaged by AFM with high spatial resolution.