Interactions of von Willebrand factor on mica studied by atomic force microscopy

Abstract

Three-dimensional molecular level images of uncoated human von Willebrand factor (vWF), a plasma glycoprotein, have been obtained using atomic force microscopy (AFM). However, the structural detail obtained by AFM was found to depend on the interactive forces between the hydrophilic mica surface and the vWF, relative to the tracking force of the probe tip acting on the protein. vWF on mica was characterized by AFM in air, under two deposition conditions, which differed with respect to the amount of vWF on the surface and the degree of sample hydration. Under hydrated conditions, the vWF had weak adhesion to the mica and was moved easily by the AFM probe tip. This resulted in tip-induced organization of vWF into molecular aggregates oriented approximately perpendicular to the fast scanning direction. In contrast, under dehydrated conditions, three-dimensional images of individual vWF molecules were observed at approximately 10 nm lateral resolution. Extended vWF molecules up to 600 nm in length were imaged, but most molecules were in the 50-300 nm range with 15-45 nm globular subunits. AFM images obtained using the constant force imaging mode indicated height measurements of 3-5 nm for most of the vWF molecules.