Structural variability and flexibility are crucial factors for biomolecular function. Here we have reduced the invasiness and enhanced the spatial resolution of atomic force microscopy (AFM) to visualize, for the first time, different structural conformations of the two polynucleotide strands in the DNA double helix, for single molecules under near-physiological conditions. This is achieved by identifying and tracking the anomalous resonance behavior of nanoscale AFM cantilevers in the immediate vicinity of the sample.

The properties and interactions of hydrophobic surfaces in water are determining factors in a wide range of industrial applications, and represent a fundamental scientific problem that is far from solved. Langmuir-Blodgett (LB) lipid monolayers have often been used as model hydrophobic surfaces, but are only metastable, which compromises the interpretation of experiments. Using frequency-modulation atomic force microscopy (FM-AFM), we find that LB-deposited monolayers of dioctadecyldimethylammonium bromide (DODAB) on mica undergo two transitions upon immersion in water: (i) a rapid molecular rearrangement from a complete monolayer coverage to a more densely packed monolayer with holes exposing the mica substrate, followed by; (ii) a gradual flipping of lipids in the monolayer to form bilayers, at a timescale of many days, orders of magnitudes slower than previously reported. The (meta)stability of the monolayer shows little dependence on the deposition pressure (5-25 mN m 21 ), but strongly depends on the cleanliness of the preparation and, in AFM experiments, is reduced from days to minutes when the force applied by the AFM tip is not kept to well below 1 nN. When properly prepared and analysed, the DODAB/mica surface thus yields a well-defined structure of sufficient stability to study intersurface forces, albeit with a heterogeneity that gives rise to very distinct forces above the bare mica on one hand, and on the monolayer and bilayer areas on the other.