Proton-Coupled Electron Transfer of Tyrosine Oxidation: Buffer Dependence and Parallel Mechanisms

摘要

Proton-coupled electron transfer (PCET) in chemical reactions is of great interest as an elementary reaction step that is frequently encountered in biological systems. The proton and the electron need not transfer as a hydrogen atom to be coupled.2 An example of this type of mechanism, in which the proton and electron are transferred between one donor and separate acceptors, has been termed MS-EPT or bidirectional PCET.This reaction is exemplified by Y_z in photosystem II, in which tyrosine oxidation is coupled to proton transfer to a hydrogen-bonded HI90 residue. Y_z oxidation could proceed by either a stepwise (ETPT or PTET; pathway 1 and 2, respectively, in Scheme 1) or conceited electron— proton transfer (CEP) mechanism, the latter defined as occurring with a single transition state. Our previous studies on systems with Y appended to Ru(bpy)_3~(2+) (RuY), Ru(bpy-4,4'-COOEt)_2(bpy)~(2+) (Ru_(ester)Y), and Re(phen)(CO)_3(PPh_3)+ (Re(P-Y)) (Chart 1) showed a pH-dependent rate constant for the PCET oxidation of Y. The nature of this pH-dependence has recently been of great debate.