Surface dependent structures of von Willebrand factor observed by AFM under aqueous conditions

von Willebrand factor (VWF) is a large multimeric plasma glycoprotein that adheres rapidly to biomaterial surfaces upon exposure to blood. The adsorbed structure influences subsequent functional interactions with other blood components that mediate surface induced thrombosis. To examine the surface-dependent properties of VWF, we compared the adsorbed structures of VWF molecules on two different surfaces: Mica, which is hydrophilic; and octadecyltrichlorosilane (OTS) modified glass, which is hydrophobic. Atomic force microscopy (AFM) was used to image adsorbed VWF under aqueous conditions at physiologic pH and ionic strength. Individual VWF molecules were clearly discernible on both surfaces. On the hydrophobic surface, VWF displayed compact tertiary structures with rare examples of extended molecules. In addition, these data revealed intramolecular structural arrangements of the repeat units within VWF multimers. On the hydrophilic mica surface, VWF displayed extended structures in which intramolecular repeat units were exposed. The lateral dimensions of VWF on mica (640+/-161x303+/-113 nm) were larger than on the hydrophobic OTS (256+/-74x152+/-62 nm, P<0.005). Our results demonstrate how surface properties mediate the molecular level structure and probable function of VWF, and provide some essential groundwork to develop a mechanistic understanding of surface-induced thrombosis.