AFM of an Ionic Switch [10], courtesy of WILEY-VCH

Dateline: April 8, 2001

Nanotechnology is abundant in living systems [1]. One of the primary components of all biomolecular nanotechnology is DNA, a molecule with physical properties that make it ideal for both nanoscale construction [2] and information storage [3]. Thus, it's natural that scientists are finding ways to use DNA for biomimetic control systems. In particular, researchers are learning to take advantage of the nanomechanical properties of DNA. Scientists have found ways to make DNA twist and extend [4], to bend DNA coated cantilevers by exposure to DNA of the appropriate sequence [5] and to create a nanoscopic tweezer-like device that can be closed or opened with the addition of auxiliary DNA fragments [6].

Yet another method takes advantage of the natural compaction mechanism of DNA known as supercoiling, a mechanism in which DNA strands wrap around each other in a tightly packed manner. Theory [7] and experiments [8] demonstrate that the stability of supercoiled DNA depends on the presence of positively charged ions, which balance the close interactions of the negatively charged DNA helices. Atomic force microscopy of self-assembled DNA-protein nanostructures [9] demonstrate that the supercoiling can be regulated by controlling the ionic strength of the solution, thus resulting in an "ionic switch" [10].

The nanostructures in this work all had the same length of DNA separating the nanoparticle connectors (in this case, streptavidin protein). Since eukaryotic DNA is organized by histones bound to the DNA at regularly spaced intervals, it is likely that similar ionic switching takes place in vivo. For instance, ionic concentrations within cells are carefully regulated by ion pumps and channels, which are known to be vital for a myriad of physiological functions. Discrete switching of nucleic acid supercoiling might play crucial roles in allowing enzymes to access the DNA, affecting nuclear processes ranging from transcription to replication. The vast majority of our knowledge about the ubiquitous biomolecule that contains our blueprints focuses on the information contained within its sequence. Like the electronic properties of DNA [11], less is known about how the nanomechanical properties of DNA are used in vivo.

Furthermore, DNA is only one of the numerous nanomechanical devices within biological systems that might be controlled by such means. For instance, consider the axon of a nerve cell. These wire-like components of neurons use molecular motors to transport vesicles over distances as long as meters [12]. At the same time, the axon also propagates electrical signals by rapidly (on the order of 2 milliseconds) exchanging sodium and potassium ions. One can only imagine how these rapid changes in ionic strength might affect the nanomechanics of the vesicle transport system within the axon. Thus, improvements in our understanding of NEMS not only helps nanoengineers to make optimal use of biomolecular components, but can also help medical technology to cure disease on the molecular level.