Direct measurement of energy landscapes of intermolecular and interfacial interactions using atomic force microscopy

Abstract

Energy barriers encountered in intermolecular and interfacial interactions determine the kinetics and equilibrium outcome of a multitude of key physical, chemical, and biological processes. The free energy landscape, i.e. the strength and physical extent of interaction energies between molecules, dictates the specificity and affinity of biological interactions. Knowledge of the energy landscape is also essential to understand the direction of a chemical reaction, the relative stability of intermediate and final states, and the corresponding reaction mechanisms. Despite advances in measuring interaction forces, determining energy landscapes at nanometer dimensions remains a challenge. Computer simulations do provide a useful approach for estimating free energy landscapes but only to the limit of the accuracy of the model potentials and the integration of the equations of motion. Indirect experimental approaches such as dynamic force spectroscopy measurements also do not completely determine the shape or curvature of the energy landscape, nor can they detect the presence of intermediate metastable states. The emergence of ultrasensitive force detection techniques such as atomic force microscope (AFM) could prove invaluable in direct determination of energy landscapes since they combine excellent force and distance resolution with the ability to probe local interactions at nanoscale levels. In this thesis, we have developed a AFM based technique to directly measure the free energy landscape of biological and interfacial interactions. The technique applies the Brownian (thermal) fluctuations to vibrate a sensitive AFM microcantilever through the energy profile between the tip and surface. By recording subtle deviations from the harmonic cantilever vibrations, and applying Boltzmann transformation techniques, the energy landscape is reconstructed. These techniques have been applied to measure the binding energy landscapes of ligand-receptor interactions in a biotin-avidin system. Through the use of stochastic excitations, we have also extended the applicability of our methods in measuring energy landscapes of strongly adhesive interfacial interactions with steep energy gradients, such as those encountered in silicon nitride and mica interfaces.