Autoredox activity is observed in thiol dioxygenases where protein-bound Fe(III)-thiolates are converted to disulfides and protein-bound Fe(II). To better understand how thiol substrate binding mode affects autoredox rates, we employ tacn-chelated Fe(III) (tacn = 1,4,7-triazacyclononane) in aqueous solution near physiological pH. The use of tacn as a supporting ligand solubilizes Fe(III) near neutral pH and replicates the 3N coordination environment found in thiol dioxygenase active sites. We vary the labile Fe(III) coordination sites using N-acetylcysteine and mercaptopropionate as bidentate S-O binders, cysteamine and cysteine methyl ester as bidentate S-N binders, and cysteine and penicillamine as tridentate S-N-O binders. Within this series we observe clear trends in UV-vis absorption maxima, cathodic peak potentials, and autoredox rate constants as a function of thiol substrate binding mode. To characterize thermodynamics of Fe(III)-thiolate autoredox, we are measuring autoredox rate constants at variable temperatures to extract activation enthalpy and entropy using the Eyring equation.

Characterizing the kinetics and thermodynamics of Fe(III)-thiolate reactions near physiological pH

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