Mechanochemical properties of DNA origami nanosprings revealed by force jumps in optical tweezers.

By incorporating pH responsive i-motif elements, we have constructed DNA origami nanosprings that respond to pH changes in the environment. Using an innovative force jump approach in optical tweezers, we have directly measured the spring constants and dynamic recoiling responses of the DNA nanosprings under different forces. These DNA nanosprings exhibited 3 times slower recoiling rates compared to duplex DNA backbones. In addition, we observed two distinct force regions which show different spring constants. In the entropic region below 2 pN, a spring constant of ∼0.03 pN nm was obtained, whereas in the enthalpic region above 2 pN, the nanospring was 17 times stronger (0.5 pN nm). The force jump gave a more accurate measurement on nanospring constants compared to regular force ramping approaches, which only yielded an average spring constant in a specific force range. Compared to the reported DNA origami nanosprings with a completely different design, our nanospring is up to 50 times stiffer. The drastic increase in the spring constant and the pH responsive feature allow more robust applications of these nanosprings in many mechanobiological processes.