Tap150a

All Atomic Force Microscopy (AFM) samples were prepared in the AFM Buffer consisting of 1 mM Tris and 4 mM MgCl₂ (pH = 7.0). Concentrated aliquots of nicked DNA were mixed with AFM buffer to achieve final concentrations of 0.5 ng/μl and 2.2 ng/μl in 20 μl buffer volume. Twenty microlitres drops were then deposited on freshly cleaved mica for 5 min. Afterwards the mica was rinsed with 1 ml of double distilled water and dried under a gentle nitrogen flow.
AFM images were collected using a MultiMode Scanning Probe Microscope (SPM) with a Nanoscope III controller (Veeco Instruments, Santa Barbara, CA, USA) operated in tapping-mode in air. The AFM cantilevers used in air had a spring constant of 5 N/m (Veeco cantilevers, TAP150A) with resonance frequencies ranging between 120 and 160 kHz. All recorded AFM images consisted of 512 × 512 pixels with scan frequency ≤1 Hz. Images were simply flattened using the Gwyddion software (32 ) n (Version 2.25) and no further image processing was carried out.