Exploring the pH Sensitivity of Ion-Pair Interactions on a Self-Assembled Monolayer by Scanning Electrochemical Microscopy

Insights into the chemical essence of weak interactions on the surface of biomacromolecules may help to regulate biological processes. In this work, the pH sensitivity of ion-pair interactions occurring on a cysteine self-assembled monolayer (Cys SAM) that simulates the local surface of a protein was probed by scanning electrochemical microscopy (SECM). Cys SAM and the ion-pair interactions subsequently formed with the introduced aspartic acid (Asp) were both pH-sensitive, as confirmed by the tip current changes in the feedback mode. After continuous pH measurements, the most significant negative feedback was observed at pH 5.50, indicating the most robust ion-pair interactions, which were simultaneously identified by voltammetry. In this case, the extra addition of the inorganic cation (i.e., Ca²⁺) did not disrupt the existing ion-pair interactions, and the binding constant (K) and Gibbs free energy (ΔG^o) of the ion pair were finally determined to be 6.44 × 10⁵ M^-1 and −33.14 kJ mol^-1, respectively. Overall, the pH sensitivity of ion-pair interactions was found to be mainly attributable to pH-induced changes in the deprotonated/protonated states of the α-amino acid moieties, which may provide insights into the artificial manipulation of complex binding events at the molecular level on the biological surface.