Trapping a cross-linked lysine–tryptophan radical in the catalytic cycle of the radical SAM enzyme SuiB

The radical S-adenosylmethionine (rSAM) enzyme SuiB catalyzes the formation of an unusual carbon–carbon bond between the sidechains of lysine (Lys) and tryptophan (Trp) in the biosynthesis of a ribosomal peptide natural product. Prior work on SuiB has suggested that the Lys–Trp cross-link is formed via radical electrophilic aromatic substitution (rEAS), in which an auxiliary [4Fe-4S] cluster (AuxI), bound in the SPASM domain of SuiB, carries out an essential oxidation reaction during turnover. Despite the prevalence of auxiliary clusters in over 165,000 rSAM enzymes, direct evidence for their catalytic role has not been reported. Here, we have used electron paramagnetic resonance (EPR) spectroscopy to dissect the SuiB mechanism. Our studies reveal substrate-dependent redox potential tuning of the AuxI cluster, constraining it to the oxidized [4Fe-4S]2+ state, which is active in catalysis. We further report the trapping and characterization of an unprecedented cross-linked Lys–Trp radical (Lys–Trp•) in addition to the organometallic Ω intermediate, providing compelling support for the proposed rEAS mechanism. Finally, we observe oxidation of the Lys–Trp• intermediate by the redox-tuned [4Fe-4S]2+ AuxI cluster by EPR spectroscopy. Our findings provide direct evidence for a role of a SPASM domain auxiliary cluster and consolidate rEAS as a mechanistic paradigm for rSAM enzyme-catalyzed carbon–carbon bond-forming reactions.

Redox Potential Tuning of s-Tetrazine by Substitution of Electron-Withdrawing/Donating Groups for Organic Electrode Materials

Herein, we tune the redox potential of 3,6-diphenyl-1,2,4,5-tetrazine (DPT) by introducing various electron-donating/withdrawing groups (methoxy, t-butyl, H, F, and trifluoromethyl) into its two peripheral benzene rings for use as electrode material in a Li-ion cell. By both the theoretical DFT calculations and the practical cyclic voltammetry (CV) measurements, it is shown that the redox potentials (E1/2) of the 1,2,4,5-tetrazines (s-tetrazines) have a strong correlation with the Hammett constant of the substituents. In Li-ion coin cells, the discharge voltages of the s-tetrazine electrodes are successfully tuned depending on the electron-donating/withdrawing capabilities of the substituents. Furthermore, it is found that the heterogeneous electron transfer rate (k0) of the s-tetrazine molecules and Li-ion diffusivity (DLi) in the s-tetrazine electrodes are much faster than conventional electrode active materials.

Redox potential tuning in bio-relevant heterocycles via (anti)aromaticity modulated H-bonding (AMHB)

Hydrogen bonds are arguably the most important non-covalent interactions in chemistry and biology, and their strength and directionality have been elegantly exploited in the rational design of complex structures. We recently noted that the variable responses of cyclic π-systems upon H-bond formation reciprocally lead to modulations of the H-bonds’ strengths, a phenomenon that we dubbed (anti)aromaticity-modulated hydrogen bonding (AMHB) [J. Am. Chem. Soc. 2016, 138, 3427–3432]. Species that switch from aromatic to antiaromatic or vice versa upon changing π-electron counts should be oppositely stabilized by the AMHB effects, so their redox potentials should be significantly “tuned” by H-bond formation. Herein, using quantum chemical simulations, we explore the effects of these H-bond induced π-electron polarizations on the redox potentials of (anti)aromatic heterocycles. The systems chosen for this study have embedded amide groups and amidine moieties capable of forming two-point H-bonds in their cyclic π-systems. Thus, as the 4-electron and 6-electron π-systems in redox-capable monocycles (e.g., quinones) can be differentially stabilized, their redox potentials can be modulated by H-bond formation by as much as 6 kcal/mol (258 mV for one electron transfer). In fused rings, the connectivity patterns are as important as the π-electron counts. Extending these ideas to flavin, a biologically relevant case, we find that H-bonding patterns like those found in its crystals can vary its redox potential by up to 1.3 kcal/mol.

Redox potential tuning by redox-inactive cations in nature's water oxidizing catalyst and synthetic analogues

Fundamental differences between synthetic manganese clusters and the biological water oxidizing catalyst are demonstrated in the modulation of their redox potential by redox-inactive cations.

Significant improvement of oxidase activity through the genetic incorporation of a redox-active unnatural amino acid

Incorporation of 3-methoxytyrosine boosts the oxidase activity of the myoglobin model of oxidase, stressing the importance of the redox potential tuning of tyrosine.