Phosphorylation-Dependent Charge Transport in Biomolecular Junctions of Major Histocompatibility Complex Phosphopeptides

This study investigates the hypothesis that phosphorylation states of major histocompatibility complex (MHC) proteins are correlated with their charge transport variations. We measured the rates of charge transport across self-assembled monolayers (SAMs) of MHC phosphopeptides containing phosphate groups at different sites supported on gold, with a Eutectic Gallium-Indium (EGaIn) top electrode. Measurements of the tunneling current densities across AuTS/MHC//Ga2O3/EGaIn molecular junctions show that the charge transport rates strongly depend on phosphorylation location. Surface characterization by AFM together with XPS and UPS confirms the formation of peptide SAMs and reveals variations in interfacial electronic structure. The presence of a phosphate group introduces localized dipoles that modify the tunneling barrier and interfacial energetics. Density functional theory (DFT) calculations suggest that phosphopeptides with increased charge transport rates have stronger dipole moments, which correspond to enhanced conductance due to a reduced energy barrier for electron transport. UPS measurements also indicate that phosphopeptides with increased charge transport have more negative HOMO energies. Our findings suggest the critical role of phosphorylation states in altering the fundamental charge transport characteristics across proteins.