Direct observation of supramolecular structures of biorelated materials by atomic force microscopy

We have prepared DNA catenanes and studied the topological structures of DNA catenanes by atomic Force microscopy (AFM) and electrophoresis. Nicked DNA was synthesized by the addition of DNase I to a solution of plasmid pBR322. Catenated DNA molecules were prepared by the reaction of topoisomerase I with nicked DNA. Catenation reactions were monitored by the agarose gel electrophoresis. A droplet of solution, containing DNA catenanes extracted from the band of 1.0 % agarose gel electrophoresis by a centrifugal filter device, was applied to a freshly cleaved mica surface. After drying the specimens, AFM measurements were carried out by using a silicon cantilever. The single molecular images of DNA catenanes were clearly observed for the first time by AFM using a tapping mode at room temperature and in an ambient atmosphere.