Kinetics of proton transfer onto bridging oxo ligands in metalloenzyme model complexes

The protonation of bridging oxo ligands in metalloenzyme is a common reaction. However, due to the complexity intrinsic in dealing with enzymes there has been very little investigation into the basic reaction chemistry involved in the enzymes which contain bridging oxo ligands. This project was started to investigate proton transfer reaction in model compounds that contain bridging oxo ligands. Proton transfer to oxygen in "normal" organic compounds, such as diethyl ether, have very low kinetic barriers. Therefore the rates of these protonations are fast, diffusion controlled (10;UV-vis stopped-flow techniques have been used to measure the rate for the protonation of bridging oxo ligands in four compounds. In the case of the porphyrin dimer 26 the rate measured (k;The three other compounds which have given measurable rates of protonation are manganese complexes two of which are crude models for the active site of the oxygen evolving complex of PSII. The third compound is a functional mimic of the enzyme manganese catalase which decomposes hydrogen peroxide.;Structural evidence suggest that substantial rearrangement accompanies protonation of bridging oxo ligands in inorganic compounds (eq 1).(DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI)This large angle change at oxygen should cause a larger kinetic barrier to proton transfer in 26 than exists for proton transfer to the oxygen of diethyl ether.