Condensation Transition in DNA-Polyaminoamide Dendrimer Fibers Studied Using Optical Tweezers

When mixed together, DNA and polyaminoamide dendrimers form fibers that condense into a compact structure. We use optical tweezers to pull condensed fibers and investigate the decondensation transition by measuring force-extension curves (FECs). A characteristic force plateau (around 10 pN) and hysteresis between the pulling and relaxation cycles are observed for different dendrimer sizes, indicating the existence of a first-order transition between two phases (condensed and extended) of the fiber. Upon salt variation FECs change noticeably confirming that electrostatic forces drive the condensation transition. We propose a simple model for the decondensing transition that qualitatively reproduces the FECs and which is confirmed by atomic force microscopy images.

Figure 1

(a) Experimental setup and (b) the chemical structure of a dendrimer molecule $G2$ with 16 amino terminal groups. (c) Reproducibility of FECs using CFP for two DNA molecules (red and green colors) that were condensed with $G8$ dendrimers under the same conditions at a constant force of 4 pN. Dashed curves correspond to naked DNA. Inset: Relaxation curves during the condensation of the fiber. The arrow indicates the time at which condensation starts.

Figure 2

Upper panel: Size dependence of FECs using CFP for different dendrimer sizes. The dashed curves are naked DNA. All extensions have been normalized to a common contour length. Lower panel: Salt dependence of FECs in $G5$ fibers. From top to bottom 10 (orange), 50 (indigo), 100 (blue), 200 (green), 500 (red) mM NaCl. For the sake of clarity FECs corresponding to different salt concentrations have been shifted 10 pN upwards in the figure.

Figure 3

Main features of FECs and schematic model for the distinct phases ($C$, $E$) of the DNA-dendrimer fibers. The red curve corresponds to DNA condensed with $G6$ in the CFP at a force of 8.4 pN. The black curve corresponds to naked DNA. The cyan curve is the prediction of the theoretical model (see the text). Vertical arrows denote 100% of the corresponding phases. The schematic model has been checked by taking AFM images ($G5$) showing the formation of different structures ($C$, $E$ phases).