Radial compression elasticity of single DNA molecules studied by vibrating scanning polarization force microscopy

The radial compression properties of single DNA molecules have been studied using vibrating scanning polarization force microscopy. By imaging DNA molecules at different vibration amplitude set-point values, we obtain the correlations between radially applied force and DNA compression, from which the radial compressive elasticity can be deduced. The estimated elastic modulus is $\sim 20–70\mathrm{MPa}$ under small external forces $(<0.4\mathrm{nN})$ and increases to $\sim 100–200\mathrm{MPa}$ for large loads.

Figure 1

(Color) A schematic description of the tip/DNA interaction at different $A_{\mathit{sp}}$ values ($A_{\mathit{sp}\left(\mathit{\text{top}}\right)}$, $A_{\mathit{sp}\left(i\right)}$, and $A_{\mathit{sp}\left(\mathit{\text{bottom}}\right)}$).

Figure 2

(Color) Images showing the processes of compressing a single DNA molecule deposited on a mica surface at different $A_{\mathit{sp}}$. A1–A4 and B1–B4 are, respectively, topographic and phase images taken at $A_{\mathit{sp}}=0.934$, 0.922, 0.910, and $0.934\mathrm{V}$. Image size: $200\mathrm{nm}\times 200\mathrm{nm}$. The horizontal bars at points 1, 2, and 3 specify positions along the DNA molecule at which height-force curves discussed in later figures were taken.

Figure 3

(Color) (a) Height-force curves taken at three different locations along a DNA molecule as indicated in Fig. 2 (A3). The open and the corresponding solid square, triangle, and circle denote approaching and retracting processes, respectively. Data (a) on the top is for a colloidal gold particle. (b) The height-force curve usually reaches a relatively flat region under a larger load.

Figure 4

(Color) (a) Topography of two crossed DNA molecules with $A_{\mathit{sp}}=0.810\mathrm{V}$. Image size: $200\mathrm{nm}\times 200\mathrm{nm}$. (b) The height-force curves taken at three different locations as indicated in (a). The open and the corresponding solid square, triangle, and circle denote approaching and retracting processes, respectively.