Force Spectroscopy of Molecular Systems-Single Molecule Spectroscopy of Polymers and Biomolecules

How do molecules interact with each other? What happens if a neurotransmitter binds to a ligand-operated ion channel? How do antibodies recognize their antigens? Molecular recognition events play a pivotal role in nature: in enzymatic catalysis and during the replication and transcription of the genome; it is also important for the cohesion of cellular structures and in numerous metabolic reactions that molecules interact with each other in a specific manner. Conventional methods such as calorimetry provide very precise values of binding enthalpies; these are, however, average values obtained from a large ensemble of molecules without knowledge of the dynamics of the molecular recognition event. Which forces occur when a single molecular couple meets and forms a bond? Since the development of the scanning force microscope and force spectroscopy a couple of years ago, tools have now become available for measuring the forces between interfaces with high precision-starting from colloidal forces to the interaction of single molecules. The manipulation of individual molecules using force spectroscopy is also possible. In this way, the mechanical properties on a molecular scale are measurable. The study of single molecules is not an exclusive domain of force spectroscopy; it can also be performed with a surface force apparatus, laser tweezers, or the micropipette technique. Regardless of these techniques, force spectroscopy has been proven as an extraordinary versatile tool. The intention of this review article is to present a critical evaluation of the actual development of static force spectroscopy. The article mainly focuses on experiments dealing with inter- and intramolecular forces-starting with "simple" electrostatic forces, then ligand-receptor systems, and finally the stretching of individual molecules.