The Rate Ladder of Proton-Coupled Tyrosine Oxidation in Water: A Systematic Dependence on Hydrogen Bonds and Protonation State

摘要

Proton-coupled electron transfer (PCET) is currently much debated as it is an elementary reaction step involved in, for example, radical enzymes and solar fuel production schemes. One example is the electron transfer (ET) from Y_z, in photosystem II (PSII), where the phenolic proton simultaneously transfers to a nearby histidine via a hydrogen bond (H-bond). It is an example of bidirectional PCET where the electron and proton are transferred in different directions. Our previous studies on tyrosine (Y) oxidation in water solution, with Y attached to Ru(bpy)_3~(2+) (RuY, Chart 1), showed unexpectedly a rate-dependence on pH that could not be explained by simple first-order dependencies on [OH~-] or buffer species. Instead we showed that the mechanism of PCET in RuY at pH < 10 is concerted (CEP) when Y is deprotonated directly to water. Recent studies on bimolecular oxidation of phenols with internal H-bonding bases showed instead a pH-independent rate of CEP when the internal base is the primary proton acceptor. Mechanistic studies on H-bonded phenols have also been made by others, using electrochemical and/or transient methods. These are, however, for heterogeneous oxidation and/or in nonaqueous solvents.