Studying the influence of macromolecular crowding at high ionic strengths on assemblies of biomolecules is of particular interest because these are standard intracellular conditions. However, up to now, no techniques offer the possibility of studying the effect of molecular crowding at the single molecule scale and at high resolution. We present a method to observe double-strand DNA under macromolecular crowding conditions on a flat mica surface by atomic force microscope. By using high concentrations of monovalent salt ([NaCl] > 100 mM), we promote DNA adsorption onto NiCl 2 pretreated muscovite mica. It therefore allows analysis of DNA conformational changes and DNA compaction induced by polyethylene glycol (PEG), a neutral crowding agent, at physiological concentrations of monovalent salt.
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