We investigate the elasticity of two types of single-stranded synthetic DNA homopolydeoxynucletides, poly(dA) and poly(dT), by AFM-based single-molecule force spectroscopy. We find that poly(dT) exhibits the expected entropic elasticity behavior, while poly(dA) unexpectedly displays two overstretching transitions in the force-extension relationship. We suggest that these transitions, which occur at approximately 23 pN and approximately 113 pN, directly capture, for the first time, the mechanical signature of base-stacking interactions among adenines in DNA, in the absence of base pairing.

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A previously proposed two-terminal carbon-nanotube-based device with closed-loop feedback is demonstrated through in situ scanning electron microscopy (SEM) experiments. The pull-in/pull-out tests were carried out using a multi-walled carbon nanotube (MWCNT) welded to a conductive probe attached to a nanomanipulator. The MWCNTs were cantilevered over a gold electrode and electrostatically actuated. The measured current-voltage curves exhibited the theoretically predicted hysteretic loop between the pull-in and pull-out processes. Both experiments and theoretical modeling demonstrated the bistability of the device confirming its utility in applications such as memory elements, NEMS switches, and logic devices. Failure mechanisms observed during the pull-in/pull-out event are also reported and discussed.

There is increasing evidence that UVA radiation, which makes up approximately 95% of the solar UV light reaching the Earth's surface and is also commonly used for cosmetic purposes, is genotoxic. However, in contrast to UVC and UVB, the mechanisms by which UVA produces various DNA lesions are still unclear. In addition, the relative amounts of various types of UVA lesions and their mutagenic significance are also a subject of debate. Here, we exploit atomic force microscopy (AFM) imaging of individual DNA molecules, alone and in complexes with a suite of DNA repair enzymes and antibodies, to directly quantify UVA damage and reexamine its basic mechanisms at a single-molecule level. By combining the activity of endonuclease IV and T4 endonuclease V on highly purified and UVA-irradiated pUC18 plasmids, we show by direct AFM imaging that UVA produces a significant amount of abasic sites and cyclobutane pyrimidine dimers (CPDs). However, we find that only approximately 60% of the T4 endonuclease V-sensitive sites, which are commonly counted as CPDs, are true CPDs; the other 40% are abasic sites. Most importantly, our results obtained by AFM imaging of highly purified native and synthetic DNA using T4 endonuclease V, photolyase, and anti-CPD antibodies strongly suggest that CPDs are produced by UVA directly. Thus, our observations contradict the predominant view that as-yet-unidentified photosensitizers are required to transfer the energy of UVA to DNA to produce CPDs. Our results may help to resolve the long-standing controversy about the origin of UVA-produced CPDs in DNA.