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

Abstract

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, k et , and the reorganization energy, λ. When the underlying SAM is longer than 10 methylene units, k et 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 k et 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.

Fundamental signatures of short- and long-range electron transfer for the blue copper protein azurin at Au/SAM junctions

“…Previous studies that exploited SAM-coated electrodes revealed two kinds of phenomenological behavior (17)(18)(19)(20)(21)(22)(24)(25)(26)(27)(28)(29)(30)(31). For the case of thicker SAMs (long-range ET), these studies showed an exponential decay of k et with the SAM thickness (that is a part of the ET distance, R e ); in accordance with the nonadiabatic ET (tunneling) theory (1, 3, 17-22, 24-32, 46-50):…”

“…According to earlier work (19,20,25,26,30,31), Az can be immobilized by the interaction of its hydrophobic patch with the hydrophobic CH 3 -terminal groups of the SAM of the general type:…”

“…61), was applied to obtain unimolecular standard rate constants and reorganization free energies through a fitting algorithm for values of the cathodic and anodic peak potentials as a function of the scan rate (see also refs. 23,24,29,31,57). Furthermore, we have generalized the fitting algorithm to include the impact of temperature over the range studied in this work (0-60°C) and applied a global fitting procedure to increase the accuracy for the determination of λ o .…”

“…The use of bioelectrochemical tunneling junctions, such as self-assembled monolayer (SAM) films of variable composition and thickness on metal electrodes, with redox proteins immobilized at the solution interface (or freely diffusing to the SAM terminal groups) have been shown to provide an assembly with well-defined and variable control parameters. As such, these assemblies are well suited for fundamental studies (15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32) and offer promise for versatile nanotechnology applications (32,33). On the basis of earlier fundamental efforts, this work studies ET between a Au electrode that is coated with a SAM alkanethiol film of variable thickness and a "model" biomolecular target, the blue copper protein, Az, from Pseudomonas aeruginosa that is immobilized through hydrophobic interactions onto the SAM.…”

“…1 Å −1 for alkanethiol SAMs (17)(18)(19)(20)(21)(22)(24)(25)(26)(27)(28)(29)(30)(31)(32). Because of this well-recognized signature, a common consensus exists for the nonadiabatic nature of longrange ET through alkanethiol or other insulating SAMs, independent of the type of redox probe and its interaction mode with the SAM (1, 3, 17-22, 24-32, 46-50).…”