Evidence for a Near-Resonant Charge Transfer Mechanism for Double-Stranded Peptide Nucleic Acid

Abstract

We present evidence for a near-resonant mechanism of charge transfer in short peptide nucleic acid (PNA) duplexes obtained through electrochemical, STM break junction (STM-BJ), and computational studies. A seven base pair (7-bp) PNA duplex with the sequence (TA)(3)-(XY)-(TA)(3) was studied, in which XY is a complementary nucleobase pair. The experiments showed that the heterogeneous charge transfer rate constant (k(0)) and the single-molecule conductance (σ) correlate with the oxidation potential of the purine base in the XY base pair. The electrochemical measurements showed that the enhancement of k(0) is independent, within experimental error, of which of the two PNA strands contains the purine base of the XY base pair. 7-bp PNA duplexes with one or two GC base pairs had similar measured k(0) and conductance values. While a simple superexchange model, previously used to rationalize charge transfer in single stranded PNA (Paul et al. J. Am. Chem. Soc. 2009, 131, 6498-6507), describes some of the experimental observations, the model does not explain the absence of an enhancement in the experimental k(0) and σ upon increasing the G content in the duplexes from one to two. Moreover, the superexchange model is not consistent with other studies (Paul et al. J. Phys. Chem. B 2010, 114, 14140), that showed a hopping charge transport mechanism is likely important for PNA duplexes longer than seven base pairs. A quantitative computational analysis shows that a near-resonant charge transfer regime, wherein a mix of superexchange and hopping mechanisms are expected to coexist, can rationalize all of the experimental results.

Intermediate tunnelling–hopping regime in DNA charge transport

“…The delocalization of holes over multiple bases suggests that the coherency of holes does not become fully washed out over a short distance, and the sequential hopping model must be modified to include this feature. This conclusion is consistent with the theory of Renaud and colleagues 18 for poly(A)-poly(T) DNA hairpins, and the theoretical analysis by Venkatramani and co-workers 40 for peptide nucleic acids and de Pablo et al 14 for λ-DNA. We also note that coherent superexchange is considered in the variable range hopping model of charge transfer in DNA developed by Yu and colleagues 41 and Renger and colleagues 42 (see Supplementary Fig.…”

Electron Transfer Proteins as Electronic Conductors: Significance of the Metal and Its Binding Site in the Blue Cu Protein, Azurin

“…In a similar fashion, in our case, the presence of Cu(II) may support an electronic state which is energetically close to the Fermi level of the electrodes (for a schematic 1D energy diagram see Figure S3, Supporting Information). The temperature independence of this process is further supported by results from a theoretical investigation by Segal et al,21 who showed that in the case of small barrier between the electrode and the bridge (in this case due to the presence of Cu(II) ion), an activationless process should be seen.…”

Conductance in a bis-terpyridine based single molecular breadboard circuit

“…This computational framework has been described in detail previously and shown to be effective in several previous studies of charge transport properties of small organic molecules. 40–42 We outline the procedure for studying TP1 below. A more detailed description is provided in Sections S.3–S.5 of ESI †…”