Low-spin cyanide complexes of 3-mercaptopropionic acid dioxygenase (MDO) reveal impact of outer sphere SHY motif residues

We recently demonstrated that cyanide can be used as a spectroscopic probe to determine the orientation of Tyr159 H-bond donation to the axial Fe-position within the substrate-bound ferric MDO active site [referred to as (3MPA/CN)-bound Fe(III)-MDO]. In this report, a comparison of WT and H157N variant reveal diagnostic shifts in low-spin ferric g-values observed by EPR. Characteristic shifts are also observed in the UV-visible absorption features attributed to the Fe(III)-S LMCT band. Significantly, DFT computational models, CASSCF/NEVP2 calculated g-values, and TD-DFT predicted electronic spectra faithfully reproduce all spectroscopic features assuming reorientation of Tyr159 H-bond to favor donation to the d1-O-atom of the Asn-amide (H157N) rather than the Fe-bound cyanide. Of note, the Fe-S LMCT is blue-shifted by 560 cm-1 (6.7 kJ‧mol-1) in the absence of Tyr159 H-bond donation. This value closely matches the enthalpy of activation (∆Hǂ = 7 kJ‧mol-1) obtained from Eyring analysis of WT and H157N variant. Spectroscopic characterization of the (CYS/CN)-bound Fe(III)-CDO was reported previously [Biochemistry 2013 52 (51): 9104-9119]. In this work, we demonstrated that the formation of the Cys93-Tyr157 cross-link altered the rhombic low-spin ferric g-values relative to the nascent, unmodified protein. These studies highlight the utility of cyanide as a spectroscopic probe to measure the directionality of H-bond donation for both CDO and MDO.

A full description of this work is reported in Inorganic Chem. 2021, 60, 24, 18639–18651.

Cooperative redox and spin activity from three redox congeners of sulfur-bridged iron nitrosyl and nickel dithiolene complexes

This project is a multi-investigator collaboration led by Prof. Marcetta Darensbourg at Texas A&M University (Department of Chemistry). Inspired by the electron-transfer pathways of metalloenzymes which use multiple earth-abundant transition metals in close proximity, the overarching goal for this project is to explore mechanisms of electron-exchange across “noninnocent” bridging molecular scaffolds. For our part in this project, we provide analytical EPR characterization of synthetic multi-spin complexes using both transverse and parallel microwave field (B1) polarization. For the [Fe2Ni2][BArF]2 complex shown above, multimodal CW EPR was used to monitor the temperature dependent population of the non-Krammer’s excited triplet (S = 1) state formed by antiferromagnetic exchange of iron-nitrosyl sites across ~9 Å. By fitting the temperature-normalized signal intensity of the excited state |±1> spin-manifold, the Heisenberg–Dirac–van Vleck exchange (J) term (-40 ± 7 cm-1) could be determined. This value closely matches what was obtained from SQUID magnetometry J (-36.5 cm-1). In addition, EPR spectroscopic simulations (dashed lines) were produced to obtain zero field splitting terms (D and E/D) for the triplet state.

More details on this work can be found in Proc Natl Acad Sci U S A. 2022 119, 25 and Chemical Science 2023, 14, 9167-9174.