Potential, electrostatic redox

RB Yelle, N-S Park, T Ichiye. Molecular dynamics simulations of rubredoxm from Clostridium pasteurianum Changes m structure and electrostatic potential during redox reactions. Proteins 22 154-167, 1995. 

Due to the presence of interactions, the apparent redox potential of a redox couple inside a polyelectrolyte film can differ from that of the isolated redox couple in solution (i.e. the standard formal redox potential) [121]. In other words, the free energy required to oxidize a mole of redox sites in the film differs from that needed in solution. One particular case is when these interations have an origin in the presence of immobile electrostatically charged groups in the polymer phase. Under such conditions, there is a potential difference between this phase and the solution (reference electrode in the electrolyte), knovm as the Donnan or membrane potential that contributes to the apparent potential of the redox couple. The presence of the Donnan potential in redox polyelectrolyte systems was demonstrated for the first time by Anson [24, 122]. Considering only this contribution to peak position, we can vwite ... 

A very pronounced double-layer effect is also observed in the voltammetry of adsorbed outer-sphere reactants." For instance, the voltammetry of self-assembled monolayers (SAMs) of alkane-thiols containing a terminal redox group (e.g., ferrocene) is strongly influenced by the electric field across the SAM, and this is manifested in peak broadening and a shift in the half-wave potential. Because redox-active SAMs are frequently geometrically very well defined, and their dielectric properties can be measured, the electric field across these layers can be readily computed from electrostatics to obtain the electric potential at the redox center. This in turn can be used to compute the influence of the electric potential distribution on voltammetric response, which can then be quantitatively compared to experiment. 

PD Swartz, BW Beck, T Ichiye. Stiaictural origins of redox potential m iron-sulfur proteins Electrostatic potentials of crystal structures. Biophys 1 71 2958-2969, 1996. 

Upon further contact with a redox reagent or at higher redox potentials, additional electrons can be transferred. After a two-electron transfer, each redox unit can accept one charge with formation of a singlet or triplet dianion, or (less favourably from an electrostatic point of view) both charges can enter one redox unit. Here again, an intramolecular electron-exchange process is possible. 

Fig. 16 (a) Scheme of the interface of a two Hg-drops electrochemical junction where Ru(NH3)63+ complexes red circles) are electrostatically trapped in between the electrodes, (b) I—V curves obtained by keeping constant the potential of the collector and sweeping the potential of the generator (potentials are measured against an Ag/AgCl). (c) Scheme of the operating redox cycling mechanism... 

Since the energy of electrons in a material is specified by the Fermi level, ep, the flow of electrons across an interface must likewise depend on the relative Fermi levels of the materials in contact. Redox properties are therefore predicted to be a function of the Fermi energy and one anticipates a simple relationship between the Fermi level and redox potential. In fact, the Fermi level is the same as the chemical potential of an electron. Clearly when dealing with charged particles, the local energy levels e are increased by qV, where q is the charge on the particle and V is the local electrostatic potential. The e, should therefore be replaced by e,- + qV and so... 

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