Controllable Biped Walking Device Constructed from DNA
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RESEARCH/RESEARCHERS Controllable Biped Walking Device Constructed from DNA Recent literature contains many accounts of the fabrication and isolation of molecular machines. Some, like synthetic DNA devices, even display a certain degree of controllable motion, although it is essentially intramolecular and therefore limited. The precise control of a range of motion in fabricated molecular devices remains a challenge. Recently, however, W.B. Sherman and N.C. Seeman at the Department of Chemistry, New York University, achieved controlled molecular motion relative to an external substrate. As reported in Nano Letters (Web release date, April 22), Sherman and Seeman synthesized and demonstrated a molecular walking motor built from DNA. The nanodevice consists of two components that are connected only by labile hydrogen bonds. The researchers term the first component the “footpath”—a rigid structure composed of a DNA motif known as a triple crossover, in which three DNA double helices are linked to each other, twice each, in an essentially coplanar arrangement. The second component is the “biped,” which is composed of two double helices linked
MRS BULLETIN/JULY 2004
Figure 1. Schematic depicting a biped system taking a full step. Matching colors represent complementary sequences between strands. The red section at the end of each foot represents psoralen, and the blue circle represents the biotin group responsible for removing the unset complex.
by three short, flexible, single strands of DNA that are not complementary to any other single-stranded DNA in the system. In addition, each biped helix contains a single strand of DNA termed a “foot” by
the researchers. Similarly, each of the footpath’s three domains contains singlestranded DNA “footholds.” Sherman and Seeman selected the sequences of feet and footholds to minimize complementarity between them (i.e., to prevent them from hydrogen-bonding to each other directly). Observation was made possible with the use of psoralen molecules that were attached to the ends of the feet, linking the foot strands covalently to their respective set and foothold strands. Sherman and Seeman used a previously developed system of “fuel strands” to power their device, that is, to make the biped walk. “Set strands” are singlestranded DNA molecules with a sequence region complementary to the foot and another sequence region complementary to the foothold. The addition of a set strand attaches a foot to a foothold whereas the addition of an “unset strand,” which has a higher affinity for the set strand than either the foot or the foothold, detaches a foot from a foothold. Figure 1 depicts the biped taking a full step, which involves five states ( Figure 1a, 1c–1f). In the initial state (Figure 1a), the left foot and right foot are attached to the left and middle footholds, respectively, by set strands. The addition
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RESEARCH/RESEARCHERS
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