The role of externally-modulated electrostatic interactions in amplifying charge transport across lysine-doped peptide junctions

Many evolved biomolecular functions such as ion pumping or redox catalysis rely on controlled charge transport through the polypeptide matrix, which can be regulated by shifts in molecular protonation states and dependent supramolecular packing modes in response to environmental cues. However, the exact roles of such dynamic, non-covalent interactions in peptide charge transport have remained elusive. To tackle this challenge, here we report the modulation of charge transport in a series of lysine (Lys)-substituted hepta-glycine (Gly) peptide self-assembled monolayers (SAMs) on template-striped gold (Au^TS) bottom electrodes with eutectic gallium-indium (EGaIn) liquid metal top electrodes. We demonstrate systematic modulation of hydrogen bonding and more general electrostatic interactions by shifting the position of the charged Lys-residue and creating different protonation patterns by changing the environmental pH in the Au^TS/peptide//GaO_x/EGaIn junctions. The effective modulation is evidenced by current density–voltage (J-V) measurements combined with SAM characterization using ultraviolet photoelectron spectroscopy (UPS) and angle-resolved X-ray photoelectron spectroscopy (ARXPS), polarization modulation–infrared reflection-absorption spectroscopy (PM-IRRAS), and molecular dynamics (MD) simulations. Decreasing the hydrogen bonding inside the peptide SAMs and increasing the electrostatic interactions by environmental counterions amplifies the charge transport differently with Lys-position, which means that the sensitive electrical response of peptide SAMs can be tuned by the peptide sequence. Our results provide insights into the relationship between molecular design and in situ modulation of charge transport properties for the development of bionanoelectronics.