The Development, Characterization, and Demonstration of a Versatile Immobilization Strategy for Biomolecular Force Measurements

Biomolecular force measurements, obtained for example using atomic force microscopy (AFM), can provide valuable molecular-level information on the interactions between biological receptor−ligand pairs. However, in such experiments, a significant level of care must be taken to ensure that adhesion events due to these specific biomolecular interactions are not confused with unwanted interactions that can originate from the underlying surfaces. To this end, we have developed a versatile immobilization strategy of general applicability for biomolecular force measurements. Specifically, dextrans were modified by O-succinoylation to produce a range of partially succinoylated dextrans of different molecular weights and degrees of esterification. The introduced monosuccinate group, upon chemical activation, is able to form covalent bonds with a wide range of functional groups on substrate surfaces and biomolecular species of interest. X-ray photoelectron spectroscopy measurements and AFM tapping-mode images were employed to demonstrate that modified dextran molecules were able to form a surface-immobilized layer that could be hydrated under aqueous conditions. The general utility of this immobilization method for the investigation of a range of receptor−ligand complexes was then demonstrated through force measurements recorded for the streptavidin−biotin system and an antigen−antibody system. Importantly, our results show that specific biomolecular recognition was retained in these systems, indicating that the dextran allows preservation of the native structure of the biomolecules.