Structural Changes in Adsorbed Cytochrome c upon Applied Potential Characterized by Neutron Reflectometry.

The structural behavior of an electron-transfer protein, cytochrome c, at the 316L stainless steel electrode/aqueous interface was investigated over a range of applied potentials using neutron reflectometry supported by solution depletion isotherms, X-ray reflectometry, and quartz crystal microbalance measurements. A custom-made electrochemical cell allowed in situ observation of the adsorbed protein across a range of applied potentials; models fitted to the NR data showed a compact inner protein layer at the metal/electrolyte interface and a further thicker but highly diffuse layer that could be removed by rinsing. The overall amount adsorbed was found to be strongly dependent on the applied potential and buffer pH. Subtle but significant changes in the structure of the adsorbed protein layer were seen as the potential was swept between ±0.40 V, reflecting changing attractive/repulsive interactions between the protein's charged side groups and the surface. At greater applied potentials, irreversible changes in the stainless steel film structure were also observed and attributed to deuterium absorption into the metal.

A Neutron Diffraction Study of the Electrochemical Double Layer Capacitor Electrolyte Tetrapropylammonium Bromide in Acetonitrile.

Neutron diffraction with isotopic substitution has been used to characterize the bulk liquid structure of the technologically relevant electrolyte solution, 1 M tetrapropylammonium bromide (TPA Br) in acetonitrile (acn), and of pure deuterated acetonitrile. Empirical potential structure refinement modeling procedures have been used to extract detailed structural information about solvent-solvent, solvent-ion, and ion-ion correlations. Analysis of the refined data shows the expected local dipolar conformation of acn in the pure solvent. This short-range dipolar ordering is also present within the solutions of TPA Br in acn, and it affects how the solvent orders itself around the ions. The solvation numbers of the TPA cations and the bromide anions are deduced, 8 and 5, respectively, as are the orientations of the solvent molecules that surround the ions. Evidence for ion association is also presented, with nearly two-thirds of the ions in the system being in associated pairs or clusters.