A pair of high resolution magnetic tweezers with illumination of total reflection evanescent field and its application in the study of DNA helicases

The resolution of conventional magnetic tweezers is limited by the Brown motion of magnetic beads. When the force is lower than similar to 10 pN, the resolution of magnetic tweezers decreases significantly because of the increased Brown motion. To improve the resolution of magnetic tweezers under low forces, we combine the total internal reflection fluorescence techniques with magnetic tweezers, and design a novel single molecule connection: "magnetic bead-DNA linker-fluorescent bead-single molecule". With the improved magnetic tweezers, we study the folding dynamics of a DNA hairpin. The results reveal that a nanometer-scale resolution is obtained. By analyzing these results, we calibrate the penetration depth of the total internal reflection field. Finally, we investigate the unwinding dynamics of a BLM helicase core protein. Some preliminary results of the helicase unwinding experiments confirm the practicability of the improved magnetic tweezers in the field of single molecular research.