A dumbbell double nicked duplex dodecamer DNA with a PEG6 tether.

A hairpin dodecamer DNA motif with a dangling end composed of four bases was studied in order to find conditions which promote a dumbbell structure as the sole form in solution. It could be used as a model of a DNA duplex with two nicks on opposite strands, mimicking a target for topo II poisons. We have established two alternative means of obtaining a dumbbell in solution as the only form present at 0 °C. The first one is to use a relatively high concentration of a hairpin motif, ca. 3.5 mM, at low ionic strength, and second is to use a moderate hairpin motif concentration of ca. 2 mM at high ionic strength, 200 mM and 15% of methanol. An NMR-derived structure in a buffered water solution is presented. A representative structure ensemble of 10 structures was obtained from MD calculations utilizing the AMBER protocol and using NOESY-derived experiment cross peak volumes transferred to experimental restraints by the MARDIGRAS algorithm.