Ac240 cantilevers

For surface deposition, nucleosome samples were diluted with TCS buffer to a final concentration of 2 nM. 50 µL of diluted samples were pipetted onto APTES-modified mica (Grade V, SPI). Samples were then incubated at room temperature for 5 min. The mica discs were then rinsed with purified 18.2-MΩ deionized water. For samples intended for AFM imaging in air, the washed mica disks were then dried using a gentle N₂ gas flow, perpendicular to the mica surface. For samples intended for imaging in liquid, the rinsed mica disks were quickly exchanged into imaging buffer (10 mM Tris-HCl pH 7.5, 3 mM NiCl₂). All samples were imaged by AFM immediately following preparation. Data acquisition was performed using a MFP-3D AFM or a Cypher ES Environmental Atomic-Force Microscope (Asylum Research). The samples were imaged in tapping mode using a commercial silicon cantilever with a spring constant of 46 N/m. For air imaging, Asylum AC240 cantilevers were used; for liquid imaging, Bruker SNL-10 cantilevers were used. Images were captured at 512×512 pixels in the trace direction, at a scan size of 2 µm and a scan rate of 1.0 Hz. Image processing was carried out using publicly available software (Gwyddion).