Spectroscopic, electrochemical, and kinetic trends in Fe(III)–thiolate disproportionation near physiologic pH

In addition to its primary oxygen-atom-transfer function, cysteamine dioxygenase (ADO) exhibits a relatively understudied anaerobic disproportionation reaction (ADO-Fe(III)-SR → ADO-Fe(II) + ½ RSSR) with its native substrates. Inspired by ADO disproportionation reactivity, we employ [Fe(tacn)Cl3] (tacn = 1,4,7-triazacyclononane) as a precursor for generating Fe(III)–thiolate model complexes in buffered aqueous media. A series of Fe(III)–thiolate model complexes are generated in situ using aqueous [Fe(tacn)Cl3] and thiol-containing ligands cysteamine, penicillamine, mercaptopropionate, cysteine, cysteine methyl ester, N-acetylcysteine, and N-acetylcysteine methyl ester. We observe trends in UV–Vis and electron paramagnetic resonance (EPR) spectra, disproportionation rate constants, and cathodic peak potentials as a function of thiol ligand. These trends will be useful in rationalizing substrate-dependent Fe(III)–thiolate disproportionation reactions in metalloenzymes. Graphical abstract Supplementary Information The online version contains supplementary material available at 10.1007/s00775-024-02051-3.


Figure S1 .
Figure S1.Representative UV-vis spectra of 1(aq) + cysteine methyl ester (37 °C, BES pH 7.5) demonstrating a blue shift concomitant with decay of the S→Fe charge transfer absorption of the complex.The blue shift is consistent with hydrolysis of cysteine methyl ester generating cysteine.

Figure S2 .
Figure S2.Representative absorbance and reciprocal absorbance as a function of time plots for 1(aq) after mixing with (top to bottom) N-acetylcysteine, mercaptopropionate, cysteamine, cysteine, and penicillamine.Insets show time intervals with high linearity in second order decay except for the reaction with cysteine which exhibited highly linear second-order decay for the entire experiment duration.

Figure S3 .
Figure S3.Representative absorbance as a function of time plots for 1(aq) after mixing with cysteine (left) and penicillamine (right) at pH 7.5 and pH 7.1.A pH dependence is observed with respect to complex formation, but the decay by disproportionation appears insensitive to pH.

Figure
Figure S4. 1 H-NMR spectra (D 2 O, 400 MHz) of cysteine (blue), cystine (green), and the reaction product of 1(aq) + cysteine (dark red) isolated as a precipitate.Cystine and reaction product were dissolved through the dropwise addition of DCl (20% w/w solution in D 2 O) until all solids dissolved (~1-3 drops).

Figure S9 .
Figure S9.EPR spectrum of 2 (7 mM) in acetonitrile.Acquisition parameters include a sample temperature of 77 K, microwave frequency of 9.440 GHz, modulation amplitude of 10 G, microwave power of 0.63 mW, and 10 scans.

Figure S10 .
Figure S10. 1 H-NMR spectra (CD 3 CN, 400 MHz) of 2 at 17 °C (red) and -20 °C (gray) scaled to the highest intensity signals in the diamagnetic region (top) and to low-intensity signals in the paramagnetic region (bottom).Broad, paramagnetic resonances exhibited relatively large (~5 ppm) shifts upon temperature change.The diamagnetic region contains proton resonances from tetraphenylborate counterion near 7.0 ppm (chemical shift unperturbed by temperature change and no observable coupling) and residual water near 2.2 ppm exhibiting a small shift (0.2-0.3 ppm) by temperature change.