Atomic force microscopy study of DNA deposited on poly l-ornithine-coated mica

Analyses of individual biomolecules, like DNA, or DNA–protein complexes, via atomic force microscopy, require ‘gentle’ methods to immobilize DNA on surfaces, which allow the ensemble of molecules to adopt conformations dictated primarily by their physical characteristics, and which possibly permit the use of a wide selection of buffers. We show that poly‐l ‐ornithine‐coated mica is a good substrate for fast, reliable deposition of DNA for wet or dry imaging. The surface firmly secures DNA, which retains the B‐form helical rise (0.34 nm bp^?1). The conformations of DNA that result are reminiscent of three‐dimensional random coils projected on to a plane. The contrast is good, especially in solution, and buffers with physiological concentrations of salt with or without divalent cations may be used. This is important for comparison of scanning probe microscopy results with those obtained by different techniques.