FOREWORD Beyond the Historical Perspective on Hydrogen and Electron Transfers

A brief overview of proton and electron transfer history is given, and various features influencing enzymatic catalysis are discussed. Examples of generic behavior are considered, together with questions that can be addressed for both experimental and computational results. Examples of high and low pre-exponential factors A of the intrinsic rate constant kH ranging from B10 17 s 1 to B10 s 1 and normal (B10) are noted with significant error bars and discussed. This series of chapters covers almost every aspect of reactions in enzyme catalysis from many leading participants in the field. They range from pedagogic descriptions of the relevant quantum theory and quantum/classical theoretical methodology to the description of experimental results. The theoretical interpretation of these large systems includes both quantum-mechanical and statistical-mechanical computations, as well as simple more approximate models. Most of the chapters focus on enzymatic catalysis of hydride, proton and H transfer, an example of the latter being proton-coupled electron transfer. There is also a chapter on electron transfer in proteins, timely since the theoretical framework evolved some fifty years ago for treating electron transfers has been adapted to H-transfers and electron transfers in proteins. It is perhaps therefore of some interest to recall briefly some of the early history in the protonand electron-transfer fields, briefly since the history covers some 85 or so years. v D ow nl oa de d on 1 6/ 03 /2 01 5 18 :2 8: 44 . Pu bl is he d on 2 7 M ar ch 2 00 9 on h ttp :// pu bs .r sc .o rg | do i:1 0. 10 39 /9 78 18 47 55 99 75 -F P0 05 Brönsted’s treatment of acid–base catalysis originated in the 1920s and involved in part the transfer of a proton from one reactant to another. It focused on linear kinetic-thermodynamic plots such as the logarithm of the reaction rate vs. some thermodynamic measure of the effect of the driving force of the reaction, for example, the logarithm of an acid or base strength (dissociation constant). These linear free-energy plots were subsequently applied to many other types of reaction rates in solution. A deviation from linearity was found by Eigen in the 1950s in his studies of very fast proton-transfer reactions. The deviation occurred at a high driving force. Ultimately, the reaction rate was limited by the rate of diffusion of the reactants toward each other. Many conferences were held on the theme of linear free-energy relations in chemical reaction rates. In the late 1940s and in the 1950s, experiments on electron-transfer reactions between ions in solution differing only in their valence state were initiated using isotopes as radioactive tracers. These reactions form the simplest class of reactions in all of chemistry, no chemical bonds being broken or formed in some cases and there being zero chemical ‘‘driving force’’ – zero standard free energy of reaction. Such experimental studies provided information thereby on other factors that influence the reaction rate. Based on the results of such studies, Bill Libby in 1952, citing a suggestion of James Franck, introduced the notion of the Franck–Condon (FC) principle controlling the rate of electron transfer. Stimulated by Libby’s work, I formulated in 1956 an electron-transfer theory. The task was to satisfy the FC principle without violating (as had previously been done) the law of conservation of energy during the electron transfer. A ‘‘reorganisation’’ of the system had to occur prior to and following the electron transfer in order to satisfy both criteria. For simplicity, the solvent was treated as a dielectric continuum and a nonequilibrium dielectric polarisation of the solvent at every point was determined by converting the problem to one of thermodynamics of a system with a nonequilibrium dielectric polarisation in the transition state. One distinguished here between the fast (electronic) and slow (nuclear) polarisation of the solvent. In 1960 this work was extended using statistical mechanics instead of the dielectric continuum theory, and now included changes in nuclear configurations of the reactants (e.g., bond lengths and angles). To this end a global reaction coordinate was needed and was introduced to treat the system of some 10 coordinates. The coordinate used was the energy of the products/solvent in their nuclear environment minus that of the reactants/solvent with the same values of the nuclear solvent/vibrational coordinates (the vertical energy difference of the two 10 or so dimensional potential-energy surfaces). It was possible in this way to reduce the description to that of a one-coordinate plot of the free energy of the reactants/solvent (a parabola) and that of the products/solvent along the reaction coordinate (a parabola) and calculate the free energy of activation, the transition state occurring at the intersection of the two parabolas. The outcome of the theory were many predictions of relations between various types of rate constants, including the effect of driving force, and a hitherto unsuspected effect termed in this 1960 paper the ‘‘inverted effect.’’ It vi Beyond the Historical Perspective on Hydrogen and Electron Transfers D ow nl oa de d on 1 6/ 03 /2 01 5 18 :2 8: 44 . Pu bl is he d on 2 7 M ar ch 2 00 9 on h ttp :// pu bs .r sc .o rg | do i:1 0. 10 39 /9 78 18 47 55 99 75 -F P0 05 View Online