Li N, Huang X, Shao H. Exploring the pH Sensitivity of Ion-Pair Interactions on a Self-Assembled Monolayer by Scanning Electrochemical Microscopy.
LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2023;
39:6529-6538. [PMID:
37116313 DOI:
10.1021/acs.langmuir.3c00469]
[Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/10/2023]
Abstract
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 (ΔGo) 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.
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