Effects of Discrete Protein-Surface Interactions in Scanning Force Microscopy Adhesion Force Measurements

The potential for measuring specific molecular recognition forces between probe-bound ligands and surface-bound proteins using a scanning force microscope (SFM) has recently gained much attention. Generally, observed discontinuities in the SFM force-displacement curves are attributed to the breaking of discrete, specific affinity bonds. The present study on the molecular recognition system composed of surface-immobilized antifluorescyl IgG molecules and SFM probe-bound fluorescein ligands has demonstrated that similar intermittent discontinuities in the SFM force-displacement curves may in fact be largely due to nonspecific discrete interactions between the protein and the SFM probe. The mechanical behavior of the cantilever-spherical bead system used in this study is discussed, as it appears to cause a false indication of the separation distance between the surface and probe. The strong lateral interactions which result in "stick and slip"-like discontinuities seen in the adhesion curves are likely the result of localized adhesion due to the heterogeneous nature of proteins and the lack of molecular mobility allowed in the experimental system. The effect is magnified with increasing contact time between the protein and probe. Factors which may cause such anomalous behavior in a specific ligand-protein system are discussed in order to avoid misinterpretation of SFM adhesion measurements.