Kinetic isotope effects for proton-coupled electron transfer reactions

The coupling between proton and electron transfer reactions is vital for a wide range of chemical and biological processes, including photosynthesis,1 - 7 respiration,8,9 and numerous enzyme reactions.10 In this chapter, reactions involving the simultaneous transfer of at least one proton and one electron are denoted proton-coupled electron transfer (PCET). Within the framework of this definition, hydrogen atom and hydride transfer reactions are viewed as specific types of PCET reactions in which the proton and electrons transfer in the same direction between the same centers. In general, however, the electron and proton could transfer between different centers and could transfer either in the same direction or in different directions. Nuclear quantum effects such as zeropoint motion and hydrogen tunneling play an important role in many types of PCET reactions. A powerful tool for probing the fundamental nature of nuclear quantum effects in PCET reactions is the investigation of the deuterium kinetic isotope effect (KIE), which is the ratio of the rate with hydrogen to the rate with deuterium.