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Radical Reactivity: Nucleophilic Radicals01:16

Radical Reactivity: Nucleophilic Radicals

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Radicals adjacent to electron-donating groups are called nucleophilic radicals. These radicals readily react with electrophilic alkenes. The SOMO–LUMO interactions are the driving force for the reaction, where the high-energy SOMO of the electron-rich, nucleophilic radicals interacts with the low-energy LUMO of the electron-deficient, electrophilic alkenes. Such SOMO–LUMO interactions are the basis of reactive radical traps, affecting the selectivity in radical reactions. For...
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Electron Paramagnetic Resonance (EPR) Spectroscopy: Organic Radicals01:17

Electron Paramagnetic Resonance (EPR) Spectroscopy: Organic Radicals

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Ideally, an unpaired electron shows a single peak in the EPR spectrum due to the transition between the two spin energy states. However, coupling interactions can occur between the spins of the unpaired electron and any neighboring spin-active nuclei. This hyperfine coupling results in hyperfine splitting, where the EPR signal is split into multiplets. The signals split into 2nI + 1 peaks, where n is the number of equivalent nuclei and I is the nuclear spin. These splitting patterns provide...
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Radical Reactivity: Overview01:11

Radical Reactivity: Overview

2.2K
Radicals, the highly reactive species, gain stability by undergoing three different reactions. The first reaction involves a radical-radical coupling, in which a radical combines with another radical, forming a spin‐paired molecule. The second reaction is between a radical and a spin‐paired molecule, generating a new radical and a new spin‐paired molecule. The third reaction is radical decomposition in a unimolecular reaction, forming a new radical and a spin‐paired...
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Radical Reactivity: Electrophilic Radicals01:02

Radical Reactivity: Electrophilic Radicals

1.6K
Radicals adjacent to electron‐withdrawing groups are called electrophilic radicals. These radicals readily react with nucleophilic alkenes. For example, the malonate radical, in which the radical center is flanked by two electron‐withdrawing groups, reacts readily with butyl vinyl ether, which consists of an electron‐donating oxygen substituent. The reaction between electrophilic malonate radical and nucleophilic vinyl ether is favored because the radical has a...
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Radical Formation: Addition00:47

Radical Formation: Addition

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Radicals can be formed by adding a radical to a spin-paired molecule. This is typically observed with unsaturated species, where the addition of a radical across the π bond leads to the production of a new radical by dissolving the π bond. For example, the addition of a Br radical to an alkene yields a carbon-centered radical.
Similar to charge conservation in chemical reactions, spin conservation is implicit for radical reactions. Accordingly, the product formed must possess an...
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Radical Formation: Overview01:03

Radical Formation: Overview

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A bond can be broken either by heterolytic bond cleavage to form ions or homolytic bond cleavage to yield radicals. A fishhook arrow is used to represent the motion of a single electron in homolytic bond cleavage. There are two main sources from which radicals can be formed:
Radicals from spin-paired molecules:
Radicals can be obtained from spin-paired molecules either by homolysis or electron transfer. While two radicals are formed in the former, an electron is added in the...
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Paramagnetic intermediates generated by radical S-adenosylmethionine (SAM) enzymes.

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Radical SAM enzymes use a [4Fe-4S] cluster to cleave S-adenosylmethionine, initiating diverse chemistry. Electron paramagnetic resonance (EPR) spectroscopy reveals how these enzymes control radical intermediates, offering insights into mechanisms.

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Area of Science:

  • Biochemistry
  • Enzymology
  • Spectroscopy

Background:

  • Radical SAM enzymes catalyze diverse reactions initiated by S-adenosylmethionine cleavage.
  • The precise control mechanisms for radical generation and propagation remain largely unknown.
  • Electron paramagnetic resonance (EPR) spectroscopy is a key tool for studying radical intermediates.

Purpose of the Study:

  • To summarize the use of EPR spectroscopy in elucidating radical SAM enzyme mechanisms.
  • To highlight specific examples where EPR has provided critical mechanistic insights.
  • To underscore the importance of EPR in understanding enzyme-controlled radical chemistry.

Main Methods:

  • Utilizing Electron Paramagnetic Resonance (EPR) spectroscopy to trap and characterize radical intermediates.
  • Investigating four distinct radical SAM enzyme systems: lysine 2,3-aminomutase, biotin synthase, HydG, and Cfr.
  • Analyzing EPR data to propose and confirm mechanistic pathways for radical SAM enzymes.

Main Results:

  • EPR studies have identified numerous intermediates in lysine 2,3-aminomutase.
  • A paramagnetic intermediate in biotin synthase involves an auxiliary [FeS] cluster crucial for vitamin B7 biosynthesis.
  • EPR confirmed the mechanism for HydG's conversion of L-tyrosine and Cfr's methylation of ribosomal RNA.

Conclusions:

  • EPR spectroscopy is invaluable for dissecting the complex mechanisms of radical SAM enzymes.
  • Understanding these mechanisms provides insights into fundamental biochemical processes and potential therapeutic targets.
  • Continued EPR investigation promises to further unravel the diverse chemistry of radical SAM enzymes.