The supercoiling of topologically constrained DNA molecules is altered by modifications of their original B structure. Contact mode, scanning force microscope (SFM) imaging requires a fairly tight binding of the DNA molecules to a flat surface, generally mica, and is often accomplished using divalent or trivalent cations and dehydrating the sample thoroughly immediately upon deposition, following procedures known to cause at least a partial B‐DNA to A‐DNA transition. We report a modification of the DNA deposition method presented by Koller etal. [Biopolymers 13, 995 (1974)] that is suitable to SFM imaging of DNA and allows complete control of the conditions of deposition (ionic strength, temperature). We have measured the contour length of linear DNA molecules in SFM images of depositions prepared with the reported method. Our data show that the B structure is preserved.

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