Electrochemistry Investigation on Protein Protection by Alkanethiol Self-Assembled Monolayers against Urea Impact

The present investigation reports the electrochemical measurements of azurin (Az) adsorbed on a series of alkanethiol self-assembled monolayers (SAMs) under the influence of urea molecules. Theoretical fitting with the Marcus model obtains the electron-transfer rate constant, ket, and the reorganization energy, λ. When the underlying SAM is longer than 10 methylene units, ket shows an obvious chain-length dependence from which an electron-tunneling coefficient, β, of 1.09 per methylene is deduced. Combined with cyclic voltammetric results, variations of both ket and λ imply that urea impact does not penetrate into the ion core part of Az but instead influences the network of molecular hydrogen bonds. The mechanism of urea impact is further discussed by means of the pH dependence of the equilibrium potential.