PROBING THE RESOLUTION LIMITS AND TIP INTERACTIONS OF ATOMIC-FORCE MICROSCOPY IN THE STUDY OF GLOBULAR-PROTEINS

It is suggested that the atomic force microscope, AFM, operating in re pulsive force mode with an insulating tip, is sensitive to charged obj ects on an insulating surface. This sensitivity is shown to significan tly affect the topographical images obtained. Theoretical calculations indicate that an electrostatic attractive force as large as 7.7 nN ex ists between a charged polystyrene sphere on mica and the AFM tip. Thi s result is consistent with the experimental measurements obtained. Gl obular domains of a blood plasma glycoprotein, von Willebrand factor, were also measured using the AFM to have elliptical cross sections wit h major axis = 106 (+/-22) nm and minor axis = 81 (+/-22) nm and heigh ts of 3.4 (+/-0.83) nm. These domains were modeled with the 14 nm diam eter polystyrene spheres. This approach allowed us to measure the inst rument response and account for lateral image distortion due to tip si ze thus enhancing the resolution of our data.