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Redox-Active Guanidines in Proton-Coupled Electron-Transfer Reactions: Real Alternatives to Benzoquinones?

ABSTRACT: Guanidino-functionalized aromatics (GFAs) are readily available, stable organic redox-active compounds. In this work we apply one particular GFA compound, 1,2,4,5-tetrakis(tetramethylguanidino)benzene, in its oxidized form in a variety of oxidation/oxidative coupling reactions to demonstrate the scope of its proton-coupled electron transfer (PCET) reactivity. Addition of an excess of acid boosts its oxidation power, enabling the oxidative coupling of substrates with redox potentials of at least +0.77?V vs. Fc⁺ /Fc. The green recyclability by catalytic re-oxidation with dioxygen is also shown. Finally, a direct comparison indicates that GFAs are real alternatives to toxic halo- or cyano-substituted benzoquinones.

SUBMITTER: Wild U

PROVIDER: S-EPMC7065378 | biostudies-literature | 2019 Sep

REPOSITORIES: biostudies-literature

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Redox-Active Guanidines in Proton-Coupled Electron-Transfer Reactions: Real Alternatives to Benzoquinones?

Wild Ute U Hübner Olaf O Himmel Hans-Jörg HJ

Chemistry (Weinheim an der Bergstrasse, Germany) 20191023 70

Guanidino-functionalized aromatics (GFAs) are readily available, stable organic redox-active compounds. In this work we apply one particular GFA compound, 1,2,4,5-tetrakis(tetramethylguanidino)benzene, in its oxidized form in a variety of oxidation/oxidative coupling reactions to demonstrate the scope of its proton-coupled electron transfer (PCET) reactivity. Addition of an excess of acid boosts its oxidation power, enabling the oxidative coupling of substrates with redox potentials of at least ...[more]

PMID: 31535741

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Project description:Tyrosyl radicals play essential roles in biological proton-coupled electron transfer (PCET) reactions. Ribonucleotide reductase (RNR) catalyzes the reduction of ribonucleotides and is vital in DNA replication in all organisms. Class Ia RNRs consist of ?2 and ?2 homodimeric subunits. In class Ia RNR, such as the E. coli enzyme, an essential tyrosyl radical (Y122O(•))-diferric cofactor is located in ?2. Although Y122O(•) is extremely stable in free ?2, Y122O(•) is highly reactive in the quaternary substrate-?2?2 complex and serves as a radical initiator in catalytic PCET between ?2 and ?2. In this report, we investigate the structural interactions that control the reactivity of Y122O(•) in a model system, isolated E. coli ?2. Y122O(•) was reduced with hydroxyurea (HU), a radical scavenger that quenches the radical in a clinically relevant reaction. In the difference FT-IR spectrum, associated with this PCET reaction, amide I (CO) and amide II (CN/NH) bands were observed. Specific (13)C-labeling of the tyrosine C1 carbon assigned a component of these bands to the Y122-T123 amide bond. Comparison to density functional calculations on a model dipeptide, tyrosine-threonine, and structural modeling demonstrated that PCET is associated with a Y122 rotation and a 7.2 Å translation of the Y122 phenolic oxygen. To test for the functional consequences of this structural change, a proton inventory defined the origin of the large solvent isotope effect (SIE = 16.7 ± 1.0 at 25 °C) on this reaction. These data suggest that the one-electron, HU-mediated reduction of Y122O(•) is associated with two, rate-limiting (full or partial) proton transfer reactions. One is attributable to HU oxidation (SIE = 11.9, net H atom transfer), and the other is attributable to coupled, hydrogen-bonding changes in the Y122O(•)-diferric cofactor (SIE = 1.4). These results illustrate the importance of redox-linked changes to backbone and ring dihedral angles in high potential PCET and provide evidence for rate-limiting, redox-linked hydrogen-bonding interactions between Y122O(•) and the iron cluster.

Dataset Information

Redox-Active Guanidines in Proton-Coupled Electron-Transfer Reactions: Real Alternatives to Benzoquinones?

Publications

Redox-Active Guanidines in Proton-Coupled Electron-Transfer Reactions: Real Alternatives to Benzoquinones?

Similar Datasets

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