Proton-coupled electron transfer (PCET) is an important mechanism for charge transfer in a wide variety of systems including biology- and materials-oriented venues. We review several areas where the transfer of an electron and proton is tightly coupled and discuss model systems that can provide an experimental basis for a test of PCET theory. In a PCET reaction, the electron and proton may transfer consecutively (ET/PT) or concertedly (ETPT). The distinction between these processes is formulated, and rate-constant expressions for the two reaction channels are presented. Methods for the evaluation of these rate constants are discussed that are based on dielectric continuum theory. Electron donor hydrogen-bonded-interface electron acceptor systems displaying PCET reactivity are presented, and the rate-constant expressions corresponding to the ETPT and ET/PT channels for several model reaction complexes are evaluated.

Proton-coupled electron transfer.

The dynamics and energetics of an excess proton in bulk phase water are examined computationally with a special emphasis on a quantum-dynamical treatment of the nuclear motion. The potential model used, the recently developed multistate empirical valence bond (MS-EVB) approach [U. W. Schmitt and G. A. Voth, J. Phys. Chem. B 102, 5547 (1998)], is also further refined and described in more detail. The MS-EVB model takes into account the interaction of an exchange charge distribution of the charge-transfer complex with the polar solvent, which qualitatively changes the nature of the solvated complex. Classical and quantum molecular dynamics simulations of the excess proton in bulk phase water reveal that quantization of the nuclear degrees of freedom results in an increased stabilization of the solvated H5O2+ (Zundel) cation relative to the H9O4+ (Eigen) cation, though the latter is still more stable, and that a species intermediate between the two also exists. The quantum proton transport rate, which is eva...

The computer simulation of proton transport in water

Chance and design--proton transfer in water, channels and bioenergetic proteins.

Proton-conducting materials play a central role in many renewable energy and bioelectronics technologies, including fuel cells, batteries and sensors. Thus, much research effort has been expended to develop improved proton-conducting materials, such as ceramic oxides, solid acids, polymers and metal-organic frameworks. Within this context, bulk proton conductors from naturally occurring proteins have received somewhat less attention than other materials, which is surprising given the potential modularity, tunability and processability of protein-based materials. Here, we report proton conductivity for thin films composed of reflectin, a cephalopod structural protein. Bulk reflectin has a proton conductivity of ~2.6 × 10(-3) S cm(-1) at 65 °C, a proton transport activation energy of ~0.2 eV and a proton mobility of ~7 × 10(-3) cm(2) V(-1) s(-1). These figures of merit are similar to those reported for state-of-the-art artificial proton conductors and make it possible to use reflectin in protein-based protonic transistors. Our findings may hold implications for the next generation of biocompatible proton-conducting materials and protonic devices.

Bulk protonic conductivity in a cephalopod structural protein

For twenty years the photosynthetic reaction center (RC) has been the premier testing ground for theoretical understanding of electron transfer in aperiodic systems, with special, but not unique, reference to long distance biological electron transport. In addition to the known structure, many of the attributes that make RCs so well suited to studying electron transfer function equally well for any charge movement, including protons. These include the presence of intrinsic reporter groups (electrochromically active pigments), high time resolution through light activation, and a large number and variety of distinct reactions, ranging from loosely coupled responses of the protein dielectric to specific, long distance proton transfers in and out of active sites, and bond making in terminal chemical transformations. A wide variety of biophysical methods have been coupled with site directed mutagenesis to reveal mechanisms of proton uptake, transfer and chemistry in the RC. This review summarizes our progress to date, which suggests that the RC can serve as a paradigm, not only for many energy coupled, membrane proteins, but for the electrostatic and dielectric properties of proteins that are critical to their general function.

Proton and electron transfer in the acceptor quinone complex of photosynthetic reaction centers from Rhodobacter sphaeroides.

Mechanisms of Electron Transfer (R.D. Cannon). Electron Transfer Effect in Chemical Compounds (B. Jezowska-Trzebiatowska and W. Wojciechowski). Light Induced Electron Transfer of Metal Complexes (A. Vogler and H. Kunkely). Coordinative Aspects of Single Electron Exchange between Main Group or Transition Metal Compounds and Unsaturated Organic Substrates (W. Kaim). Electron Hopping and Delocalization in Mixed-Valence Metal-Oxygen Clusters (H. So and M.T. Pope). Electron Transfer in Semiconducting Colloids and Membranes, Applications in Artificial Photosynthesis (M. Gratzel). Electron Transfer in Photosynthetic Reaction Centers (Ch.C. Moser et al.). Exchange Interaction in Electron Transfer Proteins and their Model Compounds (W. Haase and S. Gehring). Multi-Electron Transfer Processes in Nitrogen Fixation and other Natural Systems (D.J. Lowe). Electron Transfer in Anaerobic Microorganisms (A. Kroger et al.). The Importance of Inhibitors as an Analytic Tool for the Study of the Quinol Oxidation Centre and the Quinol Oxidase Reaction (G. von Jagow and U. Brandt). Electron and Proton Transfer through the Mitochondrial Respiratory Chain (T.A. Link). Coupled Proton and Electron Transfer Pathways in the Acceptor Quinone Complex of Reaction Centers from Rhodobacter Sphaeroides (E. Takahashi et al.). The Water Oxidizing Enzyme - An Alternative Model (E.K. Pistorius). Proton Pumps, Proton Flow and Proton ATP Synthases in Photosynthesis of Green Plants (W. Junge et al.). Diffusion of Proton in Microscopic Space: Effect of Geometric Constraints and Dielectric Discontinuities (M. Gutman et al.). Protonation of the Schiff Base Chromophore in Rhodopsins (C. Sandorfy). Proton Transfer along the Hydrogen Bridge in Some Hydrogen-Bonded Molecular Complexes (H. Ratajczak). Hydrogen-Bonded Systems with Large Proton Polarizability due to Collective Proton Motion as Pathways of Protons in Biological Systems (G. Zundel). NMR Studies of Multiple Proton and Deuteron Transfers in Liquids, Crystals and Organic Glasses (H.-H. Limbach). Proton Transfer Reactions in Solutions: A Molecular Approach (D. Borgis). Recent Developments in Solitonic Model of Proton Transfer in Quasi-One-Dimensional Infinite Hydrogen-Bonded Systems (E. Kryachko). Index of Contributing Authors. Subject Index.

E. Diemann

Papers

Electron and Proton Transfer in Chemistry and Biology