Covalent Attachment of a Single Dextran Polymer Between the Tip of an Atomic Force Microscope and a Gold Surface
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ABSTRACT A method for covalent immobilization of a single dextran polymer between a gold surface and the tip of an atomic force microscope (AFM) is presented. Carboxymethylated dextran immobilized on gold by epoxythiol chemistry was activated with Nhydroxysuccinimide (NHS) and N-ethyl-N'-(dimethylaminopropyl) carbodiimide hydrochloride (EDC) in order to make the dextran polymer reactive for the amino groups present on the previously aminosilanized AFM tip. By measuring force vs extension curves we have shown that it is possible to catch such an activated dextran polymer with an AFM tip through the formation of a covalent bond. Dextran polymers were attached even without any detectable indentation of the tip in the dextran-coated gold surface. In this so-called fly-fishing mode, attachment of multiple dextran polymers, which typically occurs when the tip is indented into the surface, are efficiently avoided. INTRODUCTION Recently, a series of single molecule force spectroscopy experiments have been conducted, in order to investigate mechanical and structural properties of single polymer strands. These experiments have been realized by stretching a single polymer strand between a surface and the tip of an AFM. Force vs extension curves were successfully recorded for simple polymers like dextran (1, 2) polyethylene glycol (3, 4), xanthan gum (5), polyelectrolyte (6), amylose (7) or for biological macromolecules like the giant protein titin (8), DNA (9) and tenascin (10). The prerequesite of this technique is the attachment of the molecule between a surface and the tip of an atomic force microscope (AFM) cantilever. As a general protocol these experiments were performed by attaching chemically or physically the macromolecule of interest to a planar solid surface of glass or gold. The AFM tip is brought and kept in contact with the surface (for a few sec.) in order to allow the binding of the molecule onto the tip. This procedure allows highly reproducible force trace recording for a large set of macromolecules. In most of the experiments cited above, the molecule was tethered between the surface and the tip via non specific interaction. In one of these previous studies (2) the biotin-streptavidin pair was used as a handle to attach in a specific manner to the AFM tip one end of a dextran sugar polymer covalently immobilized on a gold support. This specific coupler allows one to pick a molecule at the surface in the so called "fly-fishing mode" and to minimize the number of multiple attachments, which typically occur for high indentation of the tip into the surface polymer brush. In this mode the tip approaches the surface without indenting into the polymer brush, until a binding event is observed. However, the very low rupture force of this biotin-streptavidin handle is by far too low to permit experiments in the high force regime. In a different study (7), we have made use 171 Mat. Res. Soc. Symp. Proc. Vol. 576 © 1999 Materials Research Society
of succinimide activated amylose to create covalent bonds between a gla
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