Quantum and classical dynamics in biochemical reactions

The classic experiment of deVault and Chance touched off a long series of theoretical and experimental studies of the interplay between quantum and classical dynamics in photosynthetic electron transfer. More recently these issues have also been addressed in experiments on ligand binding reactions in heme proteins and through the study of kinetic isotope effects in enzymatic proton transfer. Theoretical effort has focused on a class of relatively simple models which display a surprisingly rich spectrum of dynamical behavior. Much less attention has been paid to a very important issue: Why are we allowed to use such simple models to describe such obviously complex molecules? Here we provide some tentative answers to this question, contrasting the cases of electron and proton transfer. We suggest that ideas based on simple models can inspire novel strategies for 'realistic' simulations, and that we can begin to think about the general problems of enzymatic catalysis in terms of dynamical pictures that previously have been applied only to the simpler case of electron transfer.