Conductance through DNA Figure 3d represents the case of two electrodes bridged by a large molecule containing several functional groups. Of the four junction types illustrated in figure 3, this is the most general and interesting. Consider a protein that spans cell membrane and shuttles information across that membrane. The protein self-assembles and self orientsin the membrane; it recognizes and binds to other very specific proteins; it also might switch between two forms, only one of which will transmit the chemical signal. Proteins are large molecules, and, indeed, a certain molecular size is required to achieve such a rich combination of properties. The rotaxane molecule shown in figure 3dmight appear large and complex, but it is actually small and efficiently designed, given the set of mechanical, chemical, and electronic properties that have been built into it. DNA oligomers represent perhaps the best-studied experimental example of this category. In addition to its biological importance and its use as a synthetic component of molecular nanostructures, the DNA molecule is of interests a charge transfer species.4 here the relationship between intermolecular electron transfer rates in solution and solid-state molecular junction transport becomes crucial to our understanding of transport processes. Connections to electrodes are also of great importance. Intramolecular electron transfer rates in DNA have been extensively investigated in solution *ok*, and it is now becoming clear that the underlying processes exhibit large mechanistic diversity. In general, for very short distance motion (over a few base pairs), coherent tunneling can occur. For transfer over more than six or seven base pairs, inelastic hopping has been strongly suggested. The fundamental motion of electrons or holes from one site to another very broadly follows the standard model developed by Rudy Marcus, Noel Hush, and Joshua Jortner for charge transfer rates. The model assumes that for a charge tunneling from a donor to an acceptor site, the tunneling height and width of the tunnel modulated by interactions with a bath of harmonic oscillators that account for the chemical environment. Exponential decay in the conductance with increasing distance has been seen directly in DNA molecules folded into hairpin shapes, and the transition to incoherent hopping has been seen in measurements of the efficiency with which holes are transferred along the molecule. The rich mechanistic palette observed for DNA charge transfer is becoming well understood due to elegant experiments and theory (see, for instance, figure 5).4,10 Indeed, the early suggestions that, because of its broad range of
Conductance through DNA Figure 3d represents the case of two electrodes bridged by a large molecule containing several functional groups. Of the four junction types illustrated in figure 3, this is the most general and interesting. Consider a protein that spans cell membrane and shuttles information across that membrane. The protein self-assembles and self orientsin the membrane; it recognizes and binds to other very specific proteins; it also might switch between two forms, only one of which will transmit the chemical signal. Proteins are large molecules, and, indeed, a certain molecular size is required to achieve such a rich combination of properties. The rotaxane molecule shown in figure 3dmight appear large and complex, but it is actually small and efficiently designed, given the set of mechanical, chemical, and electronic properties that have been built into it. DNA oligomers represent perhaps the best-studied experimental example of this category. In addition to its biological importance and its use as a synthetic component of molecular nanostructures, the DNA molecule is of interests a charge transfer species.4 here the relationship between intermolecular electron transfer rates in solution and solid-state molecular junction transport becomes crucial to our understanding of transport processes. Connections to electrodes are also of great importance. Intramolecular electron transfer rates in DNA have been extensively investigated in solution *ok*, and it is now becoming clear that the underlying processes exhibit large mechanistic diversity. In general, for very short distance motion (over a few base pairs), coherent tunneling can occur. For transfer over more than six or seven base pairs, inelastic hopping has been strongly suggested. The fundamental motion of electrons or holes from one site to another very broadly follows the standard model developed by Rudy Marcus, Noel Hush, and Joshua Jortner for charge transfer rates. The model assumes that for a charge tunneling from a donor to an acceptor site, the tunneling height and width of the tunnel modulated by interactions with a bath of harmonic oscillators that account for the chemical environment. Exponential decay in the conductance with increasing distance has been seen directly in DNA molecules folded into hairpin shapes, and the transition to incoherent hopping has been seen in measurements of the efficiency with which holes are transferred along the molecule. The rich mechanistic palette observed for DNA charge transfer is becoming well understood due to elegant experiments and theory (see, for instance, figure 5).4,10 Indeed, the early suggestions that, because of its broad range of
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