TAML activators enable unprecedented, rapid, ultradilute oxidation catalysis where substrate inhibitions might seem improbable. Nevertheless, while TAML/HO rapidly degrades the drug propranolol, a micropollutant (MP) of broad concern, propranolol is shown to inhibit its own destruction under concentration conditions amenable to kinetics studies ([propranolol] = 50 μM). Substrate inhibition manifests as a decrease in the second-order rate constant k for HO oxidation of the resting Fe-TAML (RC) to the activated catalyst (AC), while the second-order rate constant k for attack of AC on propranolol is unaffected. This kinetics signature has been utilized to develop a general approach for quantifying substrate inhibitions. Fragile adducts [propranolol, TAML] have been isolated and subjected to ESI-MS, florescence, UV-vis, FTIR, H NMR, and IC examination and DFT calculations. Propranolol binds to Fe-TAMLs via combinations of noncovalent hydrophobic, coordinative, hydrogen bonding, and Coulombic interactions. Across four studied TAMLs under like conditions, propranolol reduced k 4-32-fold (pH 7, 25 °C) indicating that substrate inhibition is controllable by TAML design. However, based on the measured k and calculated equilibrium constant K for propranolol-TAML binding, it is possible to project the impact on k of reducing [propranolol] from 50 μM to the ultradilute regime typical of MP contaminated waters (≤2 ppb, ≤7 nM for propranolol) where inhibition nearly vanishes. Projecting from 50 μM to higher concentrations, propranolol completely inhibits its own oxidation before reaching mM concentrations. This study is consistent with prior experimental findings that substrate inhibition does not impede TAML/HO destruction of propranolol in London wastewater while giving a substrate inhibition assessment tool for use in the new field of ultradilute oxidation catalysis.
Ac yclic voltammetry study of as eries of iron(III) TAML activators of peroxides of severalg enerationsi na cetonitrile as solvent reveals reversible or quasireversibleF e III/IV and Fe IV/V anodic transitions, the formal reduction potentials (E8')f or which are observed in the ranges 0.4-1.2 and 1.4-1.6 V, respectively,v ersusA g/AgCl. The slope of 0.33 for a linear E8'(IV/V) against E8'(III/IV) plot suggests that the TAML ligand system plays ab iggerr ole in the Fe III/IV transition, whereas the second electron transfer is to al arger extenta n iron-centered phenomenon. The reduction potentials appear to be ac onvenient tool for analysis of variousp roperties of iron TAML activators in terms of linear free energy relationships (LFERs). The values of E8'(III/IV) and E8'(IV V À1)c orrelate 1) with the pK a values of the axial aqua ligand of iron(III) TAMLs with slopes of 0.28 and 0.06 V, respectively;2)with the Stern-Volmer constants K SV for the quenching of fluorescence of propranolol, am icropollutant of broad concern; 3) with the calculated ionization potentials of Fe III and Fe IV TAMLs;a nd 4) with rate constants k I and k II for the oxidation of the restingi ron(III) TAML state by H 2 O 2 and reactions of the active forms of TAMLs formed with donors of electrons S, respectively.I nterestingly,s lopes of log k II versus E8'(III/IV) plots are lower for fast-to-oxidize St han for slow-to-oxidize S. The log k I versus E8'(III/IV) plot suggeststhat the manmade TAML catalyst can never be as reactive toward H 2 O 2 as a horseradish peroxidase enzyme.
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