EFFECTS OF DISCRETE PROTEIN SURFACE INTERACTIONS IN SCANNING FORCE MICROSCOPY ADHESION FORCE MEASUREMENTS

The potential for measuring specific molecular recognition forces betw een probe-bound ligands and surface-bound proteins using a scanning fo rce microscope (SFM) has recently gained much attention. Generally, ob served discontinuities in the SFM force-displacement curves are attrib uted to the breaking of discrete, specific affinity bonds. The present study on the molecular recognition system composed of surface-immobil ized antifluorescyl IgG molecules and SFM probe-bound fluorescein liga nds has demonstrated that similar intermittent discontinuities in the SFM force-displacement curves may in fact be largely due to nonspecifi c discrete interactions between the protein and the SFM probe. The mec hanical behavior of the cantilever-spherical bead system used in this study is discussed, as it appears to cause a false indication of the s eparation distance between the surface and probe. The strong lateral i nteractions which result in ''stick and slip''-like discontinuities se en 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 w ith increasing contact time between the protein and probe. Factors whi ch may cause such anomalous behavior in a specific ligand-protein syst em are discussed in order to avoid misinterpretation of SFM adhesion m easurements.