The ability for springs to store mechanical energy can be intrinsic to the material, such as in a rubber band, or result from geometry, such as the helical springs seen in mechanical devices from watches to mousetraps. Jinsong Ren and colleagues from the Changchun Institute of Applied Chemistry in China1 have taken inspiration from springs to prepare DNA nanostructures that extend and contract in solution depending on the concentration of protons.

DNA is a versatile molecule for assembling devices from the bottom up due to its unique structural features and powerful recognition capabilities. Ren and her team generated their molecular springs in solution from rings of DNA connected by three different linker molecules. The DNA springs extended in response to a change in the pH of the surrounding solution. Also present in the solution were repeat copies of additional DNA strands with a nucleotide sequence complementary to segments in the linker molecules. Increasing the pH allowed this strand to hybridize to the linker molecules and form a rigid extended structure. At low pH, the linker molecules dehybridized and folded into a closed ‘I’ motif structure.

Fig. 1: AFM images and schematics of DNA spring in extended state at high pH (left) and compressed state at low pH (right).From Ref. 1. Reproduced with permission. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA.

The researchers confirmed their findings through gel electrophoresis, atomic force microscopy (AFM) (Fig. 1) and Forster resonance energy transfer (FRET) measurements. The FRET data was particularly compelling. A fluorescent fluorophore and a quencher unit were attached to the same linking strand. As the pH of the solution varied, the flexible strand transformed from a random linear structure to a compact, folded structure. The DNA thus extended and retracted, causing the emission of the fluorophore to strengthen and weaken as its proximity to the quencher molecule changed. By attaching nanoparticles to the hybridizing DNA strand, the researchers could also reversibly assemble nanoparticles onto the springs simply by varying the pH of the solution.

Ren and her colleagues suggest that the variety of DNA structural motifs available offer opportunities for designing functional devices with different motions and functionalities. “The pH-responsive feature of this system makes it particularly attractive for drug delivery, such as targeting certain pathological conditions that are characterized by relatively high extracellular acidity,” says Ren.