Interfacial Electric Fields Drive Rearrangement of Adsorbed Cysteine and Electrolyte Ions on Au Electrodes

Electrode surfaces modified with
peptides or other biomolecules
are of great interest for applications in catalysis and separations.
At the electrochemical interface, the structure of biomolecular adsorbates
may be sensitive to the applied potential and the distribution of
solvent and ions near the electrode surface. Herein, periodic density
functional theory (DFT) calculations are used to describe changes
in the adsorption structure of the l-cysteine amino acid
on Au(111) as a function of applied potential. This theoretical study
reveals the fundamental mechanisms of potential-dependent rearrangement
of cysteine on electrode surfaces. These systems are analyzed using
a hybrid quantum–classical computational approach that combines
constant-potential periodic DFT with a classical representation of
the liquid electrolyte. In agreement with experimental measurements,
grand canonical thermodynamic analyses suggest that the cysteine exists
primarily in its zwitterionic form over a wide range of applied potentials.
The structure of adsorbed zwitterionic cysteine is dictated by the
cationic ammonium and anionic carboxylate functional groups, where
these charged moieties experience competing Coulombic interactions
with the charged Au(111) surface and the electrolyte ions within the
electric double layer. These competing interactions drive the rearrangement
of cysteine with applied potential, which in turn determines the nature
of ion structuring at the interface. The potential-dependent free
energies of cysteine zwitterions are also significantly influenced
by the ionic strength of the electrolyte because of the interactions
between charged zwitterion functional groups and oppositely charged
electrolyte ions. Understanding the interplay between adsorption structure,
applied potential, and electrolyte ion structuring can guide the assembly
of structured biomolecules on solid surfaces. The impact of zwitterionic
amino acids and peptides on near-surface electrolyte composition may
be further exploited to tailor microenvironments for various applications
of interfacial electrochemistry.