Important role of the 3-mercaptopropionamide moiety in glutathione: promoting effect on decomposition of the adduct of glutathione with the oxoammonium ion of TEMPO

Fluorescence Quenching Effect of a Highly Active Nitroxyl Radical on 7-amino-4-methylcoumarin and Glutathione Sensing

Nitroxyl radical compounds, such as 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO), are stable radical compounds with a variety of unique characteristics, including fluorescence quenching. In this study, we investigated the fluorescence quenching effect of nortropine N-oxyl (NNO), which is a highly active nitroxyl radical that is more active than TEMPO in oxidation catalysis. The fluorescence intensity of 7-amino-4-methylcoumarin (AMC) was quenched by NNO and TEMPO to 5% and 95% of the initial fluorescence intensity, respectively, indicating highly efficient quenching by NNO. In addition, we used this reaction to measure glutathione concentration. The quenching effect of NNO was abrogated by the chemical reaction with glutathione, resulting in restoration of AMC fluorescence. This response was observed at glutathione concentrations from 10 µM to 1 mM, and good calibration curves were obtained from 10 to 250 µM.

Recent advancements in the use of Bobbitt's salt and 4-acetamidoTEMPO

Recent advances in synthetic methodologies for selective, oxidative transformations using Bobbitt's salt (4-acetamido-2,2,6,6-tetramethyl-1-oxopiperidinium tetrafluoroborate, 1) and its stable organic nitroxide counterpart ACT (4-acetamidoTEMPO, 4-acetamido-2,2,6,6-tetramethylpiperidine-1-oxyl, 2) have led to increased applications across a broad array of disciplines. Current applications and mechanistic understanding of these metal-free, environmentally benign, and easily accessible organic oxidants now span well-beyond the seminal use of 1 and 2 in selective alcohol oxidations. New synthetic methodologies for the oxidation of alcohols, ethers, amines, thiols, C-H bonds and other functional groups with 1 and 2 along with the field's current mechanistic understandings of these processes are presented alongside our contributions in this area. Exciting new areas harnessing the unique properties of these oxidants include: applications to drug discovery and natural product total synthesis, the development of new electrocatalytic methods for depolymerization of lignin and modification of other biopolymers, in vitro and in vivo nucleoside modifications, applications in supramolecular catalysis, the synthesis of new polymers and materials, enhancements in the design of organic redox flow batteries, uses in organic fuel cells, applications and advancements in energy storage, the development of electrochemical sensors, and the production of renewable fuels.

Organic Synthesis Using Nitroxides

Nitroxides, also known as nitroxyl radicals, are long-lived or stable radicals with the general structure R1R2N-O•. The spin distribution over the nitroxide N and O atoms contributes to the thermodynamic stability of these radicals. The presence of bulky N-substituents R1 and R2 prevents nitroxide radical dimerization, ensuring their kinetic stability. Despite their reactivity toward various transient C radicals, some nitroxides can be easily stored under air at room temperature. Furthermore, nitroxides can be oxidized to oxoammonium salts (R1R2N═O+) or reduced to anions (R1R2N-O-), enabling them to act as valuable oxidants or reductants depending on their oxidation state. Therefore, they exhibit interesting reactivity across all three oxidation states. Due to these fascinating properties, nitroxides find extensive applications in diverse fields such as biochemistry, medicinal chemistry, materials science, and organic synthesis. This review focuses on the versatile applications of nitroxides in organic synthesis. For their use in other important fields, we will refer to several review articles. The introductory part provides a brief overview of the history of nitroxide chemistry. Subsequently, the key methods for preparing nitroxides are discussed, followed by an examination of their structural diversity and physical properties. The main portion of this review is dedicated to oxidation reactions, wherein parent nitroxides or their corresponding oxoammonium salts serve as active species. It will be demonstrated that various functional groups (such as alcohols, amines, enolates, and alkanes among others) can be efficiently oxidized. These oxidations can be carried out using nitroxides as catalysts in combination with various stoichiometric terminal oxidants. By reducing nitroxides to their corresponding anions, they become effective reducing reagents with intriguing applications in organic synthesis. Nitroxides possess the ability to selectively react with transient radicals, making them useful for terminating radical cascade reactions by forming alkoxyamines. Depending on their structure, alkoxyamines exhibit weak C-O bonds, allowing for the thermal generation of C radicals through reversible C-O bond cleavage. Such thermally generated C radicals can participate in various radical transformations, as discussed toward the end of this review. Furthermore, the application of this strategy in natural product synthesis will be presented.

Chemoselective Oxidation of Thiols with Oxoammonium Cations

The oxidation of various aryl and aliphatic thiols with the commercially available and environmentally benign reagent Bobbitt's salt (1) has been investigated. The reaction affords the corresponding disulfide products in good to excellent yields (71-99%) and can be accomplished in water, methanol, or acetonitrile solvent. Moreover, the process is highly chemoselective, tolerating traditionally oxidation-labile groups such as free amines and alcohols. Combined experimental and computational studies reveal that the oxidation takes place via a polar two-electron process with concomitant and unexpected deoxygenation of the oxoammonium cation through homolysis of the weak N-O bond, differing from prototypical radical-based thiol couplings. This unusual consumption of the oxidant has significant implications for the development of new nitroxide-based radical traps for probing S-centered radicals, the advancement of new electrochemical or catalytic processes involving nitroxide/oxoammonium salt redox couples, and applications to biological systems.

Electrochemical reactions of highly active nitroxyl radicals with thiol compounds

Nitroxyl radicals are known to electrochemically oxidize thiols as well as alcohols and amines. In this study, a preliminary investigation of the electrochemical reaction of thiols with 9-azabicyclo[3.3.1]nonane N-oxyl (ABNO), 2-azaadamantane N-oxyl (AZADO), and nortropine N-oxyl (NNO), which are highly active due to their bicyclo structures, for use in electrochemical analysis was performed and the results were compared with those for a typical nitroxyl radical compound, 2,2,6,6-tetramethylpiperidine N-oxyl (TEMPO). Mercaptopropane sulfonic acid (MPS) was used as a model compound to investigate the electrochemical response in aqueous solution. In addition, electrochemical detection of glutathione, a biological thiol molecule, was performed.

Emerging therapeutic opportunities of novel thiol-amides, NAC-amide (AD4/NACA) and thioredoxin mimetics (TXM-Peptides) for neurodegenerative-related disorders

Understanding neurodegenerative diseases have challenged scientists for decades. It has become apparent that a decrease in life span is often correlated with the development of neurodegenerative disorders. Oxidative stress and the subsequent inflammatory damages appear to contribute to the different molecular and biochemical mechanisms associated with neurodegeneration. In this review, I examine the protective properties of novel amino acid based compounds, comprising the AD series (AD1-AD7) in particular N-acetylcysteine amide, AD4, also called NACA, and the series of thioredoxin mimetic (TXM) peptides, TXM-CB3-TXM-CB16. Designed to cross the blood-brain-barrier (BBB) and permeate the cell membrane, these antioxidant/anti-inflammatory compounds may enable effective treatment of neurodegenerative related disorders. The review addresses the molecular mechanism of cellular protection exhibited by these new reagents, focusing on the reversal of oxidative stress, mitochondrial stress, inflammatory damages, and prevention of premature cell death. In addition, it will cover the outlook of the clinical prospects of AD4/NACA and the thioredoxin-mimetic peptides, which are currently in development.