Dynamics electron transfer across

Part C. Dynamics of Electron Transfer Across Polypeptides by Stephan S. Isied (Rutgers University)... 

Despite the fact that at present many important regularities of PET processes across the membranes seem to be understood at a qualitative level, this turns out to be insufficient for PET across membranes to be controlled purposefully. For this, more detailed quantitative mechanistic studies are needed. In particular, more data are needed concerning the location of the photosensitizer and electron carriers inside the membrane, dynamics and spatial limits of their diffusion in the membrane, the actual role of electron tunneling in providing electron transfer across the hydrophobic core of the membrane. 

A new kind of dynamic process is added to the picture compared to reactions in homogeneous solution. The electrochemical experiment allows one to control the rate of electron transfer across the interface via adjustment of the potential across the interface. This potential difference creates an intense electric field whose magnitude is of the order of 1 V nm-1 or about 109 V m-1. 

To facilitate a self-contained description, we will start with well-established aspects related to the semiconductor energy band model and the electrostatics at semiconductor electrolyte interfaces in the dark . We shall then examine the processes of light absorption, electron-hole generation and charge separation at these interfaces. Finally, the steady-state and dynamic (i.e., transient or periodic) aspects of charge transfer will be considered. Nanocrystalline semiconductor films and size quantization are briefly discussed, as are issues related to electron transfer across chemically modified semiconductor electrolyte interfaces. 

Recendy, Sneddon and Brooks (39), on the basis of their CHARMM simulations of conformational dynamics of Pro peptides in aqueous solution, have postulated involvement in electron transfer across the -(Pro) -bridge of P — a transitions at the i i angle, as the latter occur more rapidly and bring the donor-acceptor distance to a shorter range than the trans cis interconversion... 

R 466 J. G. Kempf and J. P. Loria, Theory and Applications of Protein Dynamics from Solution NMR , Cell Biochem. Biophys., 2002,37,187 R467 E. Kennett and P. Kuchel, Redox Reactions and Electron Transfer Across the Red Cell Membrane , lUBMB Life, 2003,55, 375 R 468 R. G. Khalifah, Reflections on Edsall s Carbonic Anhydrase Paradoxes of an Ultra Fast Enzyme , Biophys. Chem., 2003,100,159 R 469 A. A. Khrapitchev and P. T. Callaghan, Spatial Dependence of Dispersion , Magn. Reson. Imaging, 2003, 21, 373 R 470 I. V. Khudyakov, N. Arsu, S. Jockusch and N. J. Turro, Magnetic and Spin Effects in the Photoinitiation of Polymerization , Des. Monomers Polym., 2003, 6, 91... 

A profound understanding of those factors that influence the dynamics of electron transfer across electrode/solution and homogeneous chemical reactions has been achieved. However, a level of experimental and theoretical insight into coupled chemical and electron transfer reactions, for example, coupled proton and electron transfer, remains elusive. It is probable that new approaches and models will emerge in this key area over the next five years. 

Among the dynamical properties the ones most frequently studied are the lateral diffusion coefficient for water motion parallel to the interface, re-orientational motion near the interface, and the residence time of water molecules near the interface. Occasionally the single particle dynamics is further analyzed on the basis of the spectral densities of motion. Benjamin studied the dynamics of ion transfer across liquid/liquid interfaces and calculated the parameters of a kinetic model for these processes [10]. Reaction rate constants for electron transfer reactions were also derived for electron transfer reactions [11-19]. More recently, systematic studies were performed concerning water and ion transport through cylindrical pores [20-24] and water mobility in disordered polymers [25,26]. 

Early studies of ET dynamics at externally biased interfaces were based on conventional cyclic voltammetry employing four-electrode potentiostats [62,67 70,79]. The formal pseudo-first-order electron-transfer rate constants [ket(cms )] were measured on the basis of the Nicholson method [99] and convolution potential sweep voltammetry [79,100] in the presence of an excess of one of the reactant species. The constant composition approximation allows expression of the ET rate constant with the same units as in heterogeneous reaction on solid electrodes. However, any comparison with the expression described in Section II.B requires the transformation to bimolecular units, i.e., M cms . Values of of the order of 1-2 x lO cms (0.05 to O.IM cms ) were reported for Fe(CN)g in the aqueous phase and the redox species Lu(PC)2, Sn(PC)2, TCNQ, and RuTPP(Py)2 in DCE [62,70]. Despite the fact that large potential perturbations across the interface introduce interferences in kinetic analysis [101], these early estimations allowed some preliminary comparisons to established ET models in heterogeneous media. 

Following the early studies on the pure interface, chemical and electrochemical processes at the interface between two immiscible liquids have been studied using the molecular dynamics method. The most important processes for electrochemical research involve charge transfer reactions. Molecular dynamics computer simulations have been used to study the rate and the mechanism of ion transfer across the water/1,2-dichloroethane interface and of ion transfer across a simple model of a liquid/liquid interface, where a direct comparison of the rate with the prediction of simple diffusion models has been made. ° ° Charge transfer of several types has also been studied, including the calculations of free energy curves for electron transfer reactions at a model liquid/liquid... 

Dynamic electroanalytical measurements at a solid electrode involve heterogeneous electron transfer. Electrons are transferred across the solution electrode interface during the electrode reaction. In fact, the term electrode reaction implies that such an electron-transfer process occurs. 

While such a device has yet to be constructed, Debreczeny and co-workers have synthesized and studied a linear D-A, -A2 triad suitable for implementation in such a device.11641 In this system, compound 6, a 4-aminonaphthalene monoimide (AN I) electron donor is excited selectively with 400 nm laser pulses. Electron transfer from the excited state of ANI to Ai, naphthalene-1,8 4,5-diimide (NI), occurs across a 2,5-dimethylphenyl bridge with x = 420 ps and a quantum yield of 0.95. The dynamics of charge separation and recombination in these systems have been well characterized.11651 Spontaneous charge shift to A2, pyromellitimide (PI), is thermodynamically uphill and does not occur. The mechanism for switching makes use of the large absorption cross-section of the NI- anion radical at 480 nm, (e = 28,300). A second laser pulse at 480 nm can selectively excite this chromophore and provide the necessary energy to move the electron from NI- to PI. These systems do not rely on electrochemical oxidation-reduction reactions at an electrode. Thus, switching occurs on a subpicosecond time scale. 

A general mathematical formulation and a detailed analysis of the dynamic behavior of this mass-transport induced N-NDR oscillations were given by Koper and Sluyters [8, 65]. The concentration of the electroactive species at the electrode decreases owing to the electron-transfer reaction and increases due to diffusion. For the mathematical description of diffusion, Koper and Sluyters [65] invoke a linear diffusion layer approximation, that is, it is assumed that there is a diffusion layer of constant thickness, and the concentration profile across the diffusion layer adjusts instantaneously to a linear profile. Thus, they arrive at the following dimensionless set of equations for the double layer potential, [Pg.117]

R. J. Forster, Hopping Across Interfaces Heterogeneous Electron Transfer Dynamics, Interface 9(4) 24, 2000. 

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