communications www.MaterialsViews.com 432 www.small-journal.com © 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Long-Range Charge Transport in Adenine-Stacked RNA:DNA Hybrids Yuanhui Li, Juan M. Artés, and Joshua Hihath* hopping mechanism. These site-to-site charge transfer results are consistent with previous photochemical measurements on A:T stacks of dsDNA, but contradict direct conduct- ance measurements of short A:T-stacked dsDNA that have resulted in exponential length dependencies (suggesting a tunneling mechanism). [17] In this work, we directly study the charge transport prop- erties of RNA:DNA hybrid duplexes at the single-molecule level to understand the transport mechanisms in these A:T rich, A-form duplexes. To obtain conductance values for these molecules we use the scanning tunneling microscope (STM)- break junction technique [18] in a sodium phosphate buffer solution (250 × 10 -3 m). This method has been used to obtain reproducible conductance values for many biomolecules ranging from dsDNA to proteins. [19] It allows thousands of conductance measurements to be performed in a short time, thus enabling statistical determination of the most probable conductance of a single-molecule junction. [20] Here we study the conductance of adenine-stacked RNA:DNA sequences over lengths ranging from 11 to 21 base pairs (bp). The length-dependent conductance data is fit using a coherence- corrected hopping model, resulting in a coherence length of ≈5 bp. This finding is supported by ab initio electronic struc- ture calculations which show that the HOMO (highest occu- pied molecular orbital) level is distributed over several bp. These results indicate that neither simple hopping nor direct tunneling dominates transport in these A:T rich RNA:DNA hybrids. Thus, these results provide new insights into the transport mechanisms of these important systems, indicate that RNA:DNA hybrids may act as an efficient molecular wire, and open the door to transport-based biomedical studies using RNA-centered systems. To examine the charge transport properties of adenine- stacked RNA:DNA hybrids, amine endgroups are added to the 3′ and 5′ termini of the DNA strand of the hybrids in order to obtain a reproducible binding to the gold elec- trodes. [21] Figure 1a illustrates the experimental approach; a diamine functionalized 11 bp duplex with an RNA sequence GGG–A 5 –GGG (and DNA complement) is linked between the gold tip and substrate. During the STM-break junction measurements, the STM tip first approaches the sub- strate surface until the current amplifier is saturated, and is then retracted (≈80 nm s -1 ) while the current is recorded. If molecules bind between the tip and the substrate during the retraction process, steps appear in the conductance versus distance traces as seen in Figure 1b (blue traces); otherwise, a pure exponential decay appears (Figure 1b, gray traces). DOI: 10.1002/smll.201502399 Molecular Electronics Y. Li, Dr. J. M. Artés, Prof. J. Hihath Department of Electrical and Computer Engineering University of California Davis Davis, CA 95616, USA E-mail: jhihath@ucdavis.edu Charge transport in oligonucleotides has been suggested to play important roles in a variety of biological processes including long-range communication, [1] signaling, [2] damage detection, [3] and repair. [4] In particular, charge transport in double-stranded (ds) DNA has drawn significant interest in recent decades, [5,6] and it is well known that the π-stack in these systems allows long-range charge transport under certain conditions. [7,8] However, little is known about charge transport in other oligonucleotides, such as RNA. RNA is an extremely versatile biological molecule, and RNA:DNA duplexes are involved in many important cellular functions including DNA replication, [9] transcription, [10] and reverse transcription. [11] Moreover, RNA has recently become an attractive target for diagnostic applications because i) the transcription process naturally amplifies RNA within the cell; ii) RNA provides direct information about the gene expression; and iii) RNA is the primary carrier of genetic information for many pathogens. [12] Therefore, developing a thorough understanding of the charge-transport properties of RNA:DNA duplexes may open the door to the develop- ment of novel electronic diagnostic or sensing platforms that do not rely on enzymatic amplification for detection or iden- tification. Nevertheless, despite both the biological impor- tance and potential technological significance of RNA:DNA hybrids, the charge transport properties of these systems remain relatively unexplored. Although direct contact charge transport measure- ments have not been performed on these systems, several recent experimental approaches have been employed to examine single-electron (or hole) charge transfer processes in RNA:DNA hybrids using either electrochemical [13] or photochemical [14] measurements. These measurements indicate that despite the fact that RNA:DNA hybrids have a different helical and chemical structure than dsDNA, charge transfer in these systems is still possible. [15] Furthermore, despite expectations that A-form oligonucleotides should be poor conductors, [7] the charge transfer rate was observed to be weakly length dependent, [16] which is indicative of a small 2016, 12, No. 4, 432–437