Solvent Isotope Effects in Atomic Force Spectroscopy

Abstract

The utility of the atomic force microscope (AFM) for measuring forces is now well established. The AFM is capable of measuring forces from tens of piconewtons to tens of nanonewtons. Systems ranging from the adhesion of clean or functionalized tips interacting with different surface, to singlemolecule measurements of the force required to rupture protein-ligand interactions, to the forced unfolding of single protein molecules have been studied. The adhesion of a clean silicon nitride tip to a clean glass surface is one of the simplest systems to study and was the first system to demonstrate quantized adhesion. AFM cantilevers will adhere to glass surfaces in the presence of water and the force required to separate the surfaces is on the order of 1-6 nN at low pH. The adhesion mechanism is believed to be due to hydrogen bonds formed between the silanol groups present on both the glass and AFM tip. Deuterium oxide (D₂O) has physical properties significantly different from that of H₂O; the density, melting temperature and heat capacity are all greater than that of water. These and the differences between H₂O and D₂O can be attributed to the lower vibrational frequency of the OD bond and the fact that O-D…O bonds are about lkJ/mol stronger than O-H…O bonds.