Universal Readers Based on Hydrogen Bonding or π–π Stacking for Identification of DNA Nucleotides in Electron Tunnel Junctions

A reader molecule, which recognizes all the naturally occurring nucleobases in an electron tunnel junction, is required for sequencing DNA by a recognition tunneling (RT) technique, referred to as a universal reader. In the present study, we have designed a series of heterocyclic carboxamides based on hydrogen bonding and a large-sized pyrene ring based on a π–π stacking interaction as universal reader candidates. Each of these compounds was synthesized to bear a thiolated linker for attachment to metal electrodes and examined for their interactions with naturally occurring DNA nucleosides and nucleotides by 1 H NMR, ESI-MS, computational calculations, and surface plasmon resonance. RT measurements were carried out in a scanning tunnel microscope. All of these molecules generated electrical signals with DNA nucleotides in tunneling junctions under physiological conditions (phosphate buffered aqueous solution, pH 7.4). Using a support vector machine as a tool for data analysis, we found that these candidates distinguished among naturally occurring DNA nucleotides with the accuracy of pyrene (by π–π stacking interactions) > azole carboxamides (by hydrogen-bonding interactions). In addition, the pyrene reader operated efficiently in a larger tunnel junction. However, the azole carboxamide could read abasic (AP) monophosphate, a product from spontaneous base hydrolysis or an intermediate of base excision repair. Thus, we envision that sequencing DNA using both π–π stacking and hydrogen-bonding-based universal readers in parallel should generate more comprehensive genome sequences than sequencing based on either reader molecule alone.