Electrochemical intercalator binding to single- and double-strand DNA- and LNA-based molecules on Au(111)-electrode surfaces

We have explored and compared interfacial electrochemical electron transfer (ET) of three organic aromatic redox molecules, methylene blue (MB), resorufin (RSF), and anthraquinone monosulfonate (AQMS), non-covalently bound to single- and double-strand (ss and ds) DNA- and LNA-based ("locked nucleic acids") oligonucleotides (ONs). The redox probes were chosen for their equilibrium potentials and particularly for their different electrostatic charges. The ONs were linked to Au(111)-surfaces via AuS bonds, diluted by mercaptohexanol (MCH) which blocks direct redox probe electrochemical electron transfer and ensures that space is left for in situ ds formation. The study combines nucleic acid synthesis, electrochemistry at single-crystal electrode surfaces, and scanning tunnelling microscopy under electrochemical control (in situ STM). The three redox molecules bind differently to the ONs. Electrostatic binding dominates for positively charged MB. ET through the MB-marked LNA ONs appears but no ss/ds LNA ON difference could be distinguished. Electrostatically neutral RSF binds in p-p stacking or other non-electrostatic, non-covalent modes onto both DNA and LNA ONs. RSF voltammetric signals on MCH-diluted ON adlayers could be associated with "long-range" ET. There is no electrochemical RSF ss/ds difference on LNA ONs, but clear ss/ds distinction on RSF binding to DNA ONs with a strong RSF signal for ds and no signal for ss. Negatively charged AQMS also binds in intercalating, non-electrostatic modes with clear ds voltammetric signals of both DNA and LNA but no ss signal. (C) 2020 Elsevier B.V. All rights reserved.