Appl. Phys. A 68, 173–176 (1999) Applied Physics A Materials Science & Processing  Springer-Verlag 1999 AFM, a tool for single-molecule experiments M. Ludwig 1 , M. Rief 1 , L. Schmidt 1 , H. Li 2 , F. Oesterhelt 1 , M. Gautel 3 , H.E. Gaub 1 1 Lehrstuhl für Angewandte Physik, Ludwig-Maximilians-Universität München, Amalienstr. 54, 80799 München, Germany 2 Key Lab for Supramolecular Structure and Spectroscopy, Jilin University, Changchun, 130023, P.R. China 3 Biological Structures Division, European Molecular Biology Laboratory, Postfach 102209, 69012 Heidelberg, Germany Received: 27 March 1998 Abstract. Force versus elongation curves measured with AFM-related techniques reveal a detailed insight into material properties at the molecular level. Here an overview on dif- ferent implementations of this technique is given. The lateral distribution of a receptor on a sample surface was determined by mapping the adhesion force with a corresponding ligand immobilized at the tip. A dextran pattern on a gold surface was measured by mapping the rupture length of the polymer. Stretching single dextran molecules revealed a structural tran- sition in the polysaccharide backbone at forces of 750 pN. The unfolding of individual domains in the modular protein titin was observed and information about the rate dependence of the unfolding forces was obtained. PACS: 61.16.Ch; 87.15.-v During its maturation the atomic force microscope (AFM) [1– 3] has evolved into a versatile platform technology for ex- periments with individual molecules. Modern instruments combine high lateral resolution (< 1 Å) with very low forces (≈ 10 pN). This, together with the option to operate under Polymers with Binding Partners Force a) 200 pN 50 nm Repulsive Contact Elongation Force b) Typical Traces with „Fly Fishing“ Feedback 200 pN 50 nm c) Displacement Fig. 1. a Schematics of single-molecule force measurements. b Approach and re- tract curves showing discrete molecu- lar deformation and unbinding events. c Retract curves under conditions where only single molecular interaction occurs aqueous conditions at physiological temperatures, makes this technique ideally suited to tackle important questions in life sciences at the molecular level. Two very powerful new technologies meet in such experiments, nanotechnology and molecular biology, and give rise to a wealth of new dis- coveries and exciting applications [4–10]. In this paper an overview is given on mechanical experiments with individ- ual biomolecules. We report on specific molecular binding and unbinding experiments that reveal details of the mo- lecular recognition mechanisms of ligand–receptor pairs and we describe single-molecule force spectroscopy experiments where individual polymers are stretched and reversibly forced through conformational transitions revealing unique material properties. 1 Experimental results and discussion Figure 1a shows the experimental geometry of a single- molecule force experiment. The AFM tip (a small pyramidal needle at the end of a microscopic cantilever spring) is chem- ically modified with either a ligand or a receptor molecule.