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Special Issue "Free Radicals and Radical Ions"

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A special issue of Molecules (ISSN 1420-3049). This special issue belongs to the section "Organic Synthesis".

Deadline for manuscript submissions: closed (31 August 2014)

Special Issue Editors

Guest Editor
Prof. Dr. John C. Walton (Website)

Research Professor of Chemistry, EaStCHEM School of Chemistry, University of St. Andrews, North Haugh, St. Andrews, Fife, KY16 9ST, UK
Interests: : free radical chemistry; EPR (ESR) spectroscopy; photoredox catalysis
Guest Editor
Prof. Dr. Francon Williams (Website)

Department of Chemistry, University of Tennessee, Knoxville, Tennessee, USA
Phone: 865-974-3468
Interests: matrix ESR spectroscopy of radical anions and radical cations; quantum tunneling in hydrogen-atom transfer reactions; vibronic coupling in molecular ions; radical cation rearrangements; radiation-induced cationic polymerization

Special Issue Information

Dear Colleagues,

Reactive free radicals and radical ions are some of the most important classes of reactive intermediates in the chemical, polymer, and biological sciences. Also, stable varieties of free radicals are hugely important in conducting and magnetic materials. New radical-based reagents, including those derived from peroxides, boron-based compounds, oxime derivatives, epoxides, metal hydrides, and others, are at the forefront of advances in synthetic chemistry. Novel radical-mediated procedures, including cyclizations, cascades, radical to nucleophile couplings and cyclizations, and radical-regulated catalytic systems with one or two electron donors, are all equally exciting. They all have contributed to a notable flowering of innovative synthetic methodologies.

The involvement of fresh radical-ordered procedures has deeply penetrated polymer science. Furthermore, novel radical-containing magnetic materials and conductors are expanding horizons in materials science. Although relatively unexplored, in comparison with their neutral molecule counterparts, radical ions participate in a broad swath of important processes by virtue of rapid electron-transfer reactions, which can be induced by both chemical catalysis and either photo- or radiation-chemical means. The use of radical ions in organic synthesis continues to develop with the radical-cation modification of the Cope reaction and similar rearrangements, as well as by their participation in the well-established Hofmann-Löffler, Würtz, and Birch reactions. Biochemical applications feature the interplay of simple oxygen–containing species, such as the oxygen radical anion O2─ and its protonated form, HO2. Also, the key role of radical ions in the important emerging technologies of alternative energy sources is illustrated by their involvement in the complementary fields of photovoltaics and light-emitting diodes.

Research articles covering all areas of free radical and radical ion chemistry, such as synthetic advances and applications, mechanistic insights, spectroscopic, structural, computational, and kinetic studies, as well as biochemical and electrochemical applications, are welcomed for inclusion in this Special Issue of Molecules.

Prof. Dr. John C. Walton
Prof. Dr. T. Ffrancon Williams
Guest Editors

Submission

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. Papers will be published continuously (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are refereed through a peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Molecules is an international peer-reviewed Open Access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 1800 CHF (Swiss Francs).

Keywords

  • reactive intermediates
  • synthetic methods
  • redox (electron transfer) processes
  • radical and radical cation cyclizations
  • organic and biochemical mechanisms
  • electron donors
  • characterization by electron spin and paramagnetic properties

Published Papers (17 papers)

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Editorial

Jump to: Research, Review

Open AccessEditorial The Games Radicals Play: Special Issue on Free Radicals and Radical Ions
Molecules 2015, 20(2), 2831-2834; doi:10.3390/molecules20022831
Received: 3 February 2015 / Accepted: 5 February 2015 / Published: 9 February 2015
PDF Full-text (625 KB) | HTML Full-text | XML Full-text
Abstract
Chemistry and Physics have aptly been described as “most excellent children of Intellect and Art” [1]. Both these “children” engage with many playthings, and molecules rank as one of their first favorites, especially radicals, which are amongst the most lively and exciting. [...] Read more.
Chemistry and Physics have aptly been described as “most excellent children of Intellect and Art” [1]. Both these “children” engage with many playthings, and molecules rank as one of their first favorites, especially radicals, which are amongst the most lively and exciting. Checking out radicals dancing to the music of entropy round their potential energy ballrooms is surely both entertaining and enlightening. Radicals’ old favorite convolutions are noteworthy, but the new styles, modes and arrangements appearing on the scene are even more interesting. Some of these are ephemeral and enjoy only a brief appearance, others are retro-types reappearing in new guises, still others are genuinely new and “go viral” in the scientific world. This Special Issue of Molecules contains the observations and reflections of a select group of chemists and physicists fascinated by this spectacle. It contains an eclectic mix reflecting on new modes and advances as well as on permutations and combinations that revive mature themes. [...] Full article
(This article belongs to the Special Issue Free Radicals and Radical Ions)

Research

Jump to: Editorial, Review

Open AccessArticle In Situ EPR Studies of Reaction Pathways in Titania Photocatalyst-Promoted Alkylation of Alkenes
Molecules 2015, 20(3), 4055-4070; doi:10.3390/molecules20034055
Received: 15 January 2015 / Revised: 17 February 2015 / Accepted: 26 February 2015 / Published: 3 March 2015
PDF Full-text (826 KB) | HTML Full-text | XML Full-text
Abstract
In situ EPR spectroscopy at cryogenic temperatures has been used to observe and identify paramagnetic species produced when titania is irradiated in the presence of reactants used in the photocatalytic alkylation of maleimide with t-butyl carboxylic acid or phenoxyacetic acid. It is [...] Read more.
In situ EPR spectroscopy at cryogenic temperatures has been used to observe and identify paramagnetic species produced when titania is irradiated in the presence of reactants used in the photocatalytic alkylation of maleimide with t-butyl carboxylic acid or phenoxyacetic acid. It is shown that maleimide acts as an acceptor of conduction band electrons. Valence band holes oxidise t-butyl carboxylic acid to the t-butyl radical and phenoxyacetic acid to the phenoxyacetic acid radical cation. In the presence of maleimide, the phenoxymethyl radical is formed from phenoxyacetic acid. The relevance of these observations to the mechanisms of titania photocatalyst-promoted alkylation of alkenes is discussed. Full article
(This article belongs to the Special Issue Free Radicals and Radical Ions)
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Open AccessCommunication Electron Transfer-Induced Coupling of Haloarenes to Styrenes and 1,1-Diphenylethenes Triggered by Diketopiperazines and Potassium tert-Butoxide
Molecules 2015, 20(2), 1755-1774; doi:10.3390/molecules20021755
Received: 22 December 2014 / Accepted: 6 January 2015 / Published: 22 January 2015
Cited by 5 | PDF Full-text (853 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
The coupling of haloarenes to styrenes and 1,1-diarylethenes has been achieved with potassium tert-butoxide in the presence of N,N'-dialkyldiketopiperazines. In contrast to previously reported reactions where phenanthroline has been used to mediate the reactions, the use of diketopiperazines can [...] Read more.
The coupling of haloarenes to styrenes and 1,1-diarylethenes has been achieved with potassium tert-butoxide in the presence of N,N'-dialkyldiketopiperazines. In contrast to previously reported reactions where phenanthroline has been used to mediate the reactions, the use of diketopiperazines can lead to either 1,1,2-triarylethenes or 1,1,2-triarylethanes, depending on the conditions used. Full article
(This article belongs to the Special Issue Free Radicals and Radical Ions)
Open AccessArticle Transfer Hydrogenation in Open-Shell Nucleotides — A Theoretical Survey
Molecules 2014, 19(12), 21489-21505; doi:10.3390/molecules191221489
Received: 13 November 2014 / Revised: 8 December 2014 / Accepted: 11 December 2014 / Published: 22 December 2014
PDF Full-text (1103 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
The potential of a larger number of sugar models to act as dihydrogen donors in transfer hydrogenation reactions has been quantified through the calculation of hydrogenation energies of the respective oxidized products. Comparison of the calculated energies to hydrogenation energies of nucleobases [...] Read more.
The potential of a larger number of sugar models to act as dihydrogen donors in transfer hydrogenation reactions has been quantified through the calculation of hydrogenation energies of the respective oxidized products. Comparison of the calculated energies to hydrogenation energies of nucleobases shows that many sugar fragment radicals can reduce pyrimidine bases such as uracil in a strongly exothermic fashion. The most potent reducing agent is the C3' ribosyl radical. The energetics of intramolecular transfer hydrogenation processes has also been calculated for a number of uridinyl radicals. The largest driving force for such a process is found for the uridin-C3'-yl radical, whose rearrangement to the C2'-oxidized derivative carrying a dihydrouracil is predicted to be exothermic by 61.1 kJ/mol in the gas phase. Full article
(This article belongs to the Special Issue Free Radicals and Radical Ions)
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Open AccessArticle Spectral and Kinetic Properties of Radicals Derived from Oxidation of Quinoxalin-2-One and Its Methyl Derivative
Molecules 2014, 19(11), 19152-19171; doi:10.3390/molecules191119152
Received: 9 October 2014 / Revised: 12 November 2014 / Accepted: 13 November 2014 / Published: 19 November 2014
Cited by 2 | PDF Full-text (1756 KB) | HTML Full-text | XML Full-text
Abstract
The kinetics and spectral characteristics of the transients formed in the reactions of OH and N3 with quinoxalin-2(1H)-one (Q), its methyl derivative, 3-methylquinoxalin-2(1H)-one (3-MeQ) and pyrazin-2-one (Pyr) were studied by pulse radiolysis in aqueous solutions [...] Read more.
The kinetics and spectral characteristics of the transients formed in the reactions of OH and N3 with quinoxalin-2(1H)-one (Q), its methyl derivative, 3-methylquinoxalin-2(1H)-one (3-MeQ) and pyrazin-2-one (Pyr) were studied by pulse radiolysis in aqueous solutions at pH 7. The transient absorption spectra recorded in the reactions of OH with Q and 3-MeQ consisted of an absorption band with λmax = 470 nm assigned to the OH-adducts on the benzene ring, and a second band with λmax = 390 nm (for Q) and 370 nm (for 3-MeQ) assigned, inter alia, to the N-centered radicals on a pyrazin-2-one ring. The rate constants of the reactions of OH with Q and 3-MeQ were found to be in the interval (5.9–9.7) × 109 M–1·s–1 and were assigned to their addition to benzene and pyrazin-2-one rings and H-abstraction from the pyrazin-2-one nitrogen. In turn, the transient absorption spectrum observed in the reaction of N3 exhibits an absorption band with λmax = 350 nm. This absorption was assigned to the N-centered radical on the Pyr ring formed after deprotonation of the respective radical cation resulting from one-electron oxidation of 3-MeQ. The rate constant of the reaction of N3 with 3 MeQ was found to be (6.0 ± 0.5) × 109 M–1·s–1. Oxidation of 3-MeQ by N3 and Pyr by OH and N3 confirms earlier spectral assignments. With the rate constant of the OH radical with Pyr (k = 9.2 ± 0.2) × 109 M–1·s‒1, a primary distribution of the OH attack was estimated nearly equal between benzene and pyrazin-2-one rings. Full article
(This article belongs to the Special Issue Free Radicals and Radical Ions)
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Open AccessArticle Thermal Reactivity of Neutral and Oxidized Ferrocenyl-Substituted Enediynes
Molecules 2014, 19(11), 18399-18413; doi:10.3390/molecules191118399
Received: 18 September 2014 / Revised: 1 November 2014 / Accepted: 3 November 2014 / Published: 12 November 2014
Cited by 1 | PDF Full-text (1559 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
The coupling of two equivalents of ethynylferrocene (2) with one equivalent of 1,2-diiodocyclohexene (1) and 1,2-diiodobenzene (4) using Sonogashira cross-coupling conditions led to 1,2-bis(ferrocenylethynyl)cyclohexene (3) and 1,2-bis(ferrocenylethy­nyl)benzene (5), respectively. At high temperatures [...] Read more.
The coupling of two equivalents of ethynylferrocene (2) with one equivalent of 1,2-diiodocyclohexene (1) and 1,2-diiodobenzene (4) using Sonogashira cross-coupling conditions led to 1,2-bis(ferrocenylethynyl)cyclohexene (3) and 1,2-bis(ferrocenylethy­nyl)benzene (5), respectively. At high temperatures enediynes 3 and 5 showed exothermic signals in differential scanning calorimetry (DSC) measurements, suggestive of intramolecular diradicaloid ring formation (Bergman (C1−C6) or Schreiner-Pascal (C1−C5) cyclizations). The oxidation of 3 and 5 to the mono-oxidized enediynes 3+ and 5+ decreased the onset temperatures drastically. Equally, 1-ferrocenylethynyl-2-(p-nitro-phenyl)ethynylbenzene (8) displayed a significant decrease in the onset temperature after oxidation to 8+. Because the insoluble nature of the polymeric material formed in the thermolysis of the oxidized enediynes prevented characterization, the origin of this drastic effect was studied by DFT. Contrary to expectations, one-electron oxidation does not lower the barrier for intramolecular cyclization. Rather, the computations suggest that the polymerization is initiated by a bimolecular process. Full article
(This article belongs to the Special Issue Free Radicals and Radical Ions)
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Open AccessArticle The Dynamical Behavior of the s-Trioxane Radical Cation—A Low-Temperature EPR and Theoretical Study
Molecules 2014, 19(11), 17305-17313; doi:10.3390/molecules191117305
Received: 1 September 2014 / Revised: 15 October 2014 / Accepted: 21 October 2014 / Published: 28 October 2014
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Abstract
The radical cation of s-trioxane, radiolytically generated in a freon (CF3CCl3) matrix, was studied in the 10–140 K temperature region. Reversible changes of the EPR spectra were observed, arising from both ring puckering and ring inversion through the [...] Read more.
The radical cation of s-trioxane, radiolytically generated in a freon (CF3CCl3) matrix, was studied in the 10–140 K temperature region. Reversible changes of the EPR spectra were observed, arising from both ring puckering and ring inversion through the molecular plane. The ESREXN program based on the Liouville density matrix equation, allowing the treatment of dynamical exchange, has been used to analyze the experimental results. Two limiting conformer structures of the s-trioxane radical cation were taken into account, namely “rigid” half-boat and averaged planar ones, differing strongly in their electron distribution. The spectrum due to the “rigid” half-boat conformer can be observed only at very low (<60 K) temperatures, when the exchange of conformers is very slow. Two transition states for interconversion by puckering and ring-inversion were identified, close in activation energy (2.3 and 3.0 kJ/mol calculated). Since the energy difference is very small, both processes set on at a comparable temperature. In the case of nearly complete equilibration (fast exchange) between six energetically equivalent structures at T > 120 K in CF3CCl3, a septet due to six equivalent protons (hfs splitting constant 5.9 mT) is observed, characteristic of the dynamically averaged planar geometry of the radical cation. DFT quantum chemical calculations and spectral simulation including intramolecular dynamical exchange support the interpretation. Full article
(This article belongs to the Special Issue Free Radicals and Radical Ions)
Open AccessArticle Photochemical Aryl Radical Cyclizations to Give (E)-3-Ylideneoxindoles
Molecules 2014, 19(10), 15891-15899; doi:10.3390/molecules191015891
Received: 2 September 2014 / Revised: 19 September 2014 / Accepted: 24 September 2014 / Published: 30 September 2014
Cited by 2 | PDF Full-text (815 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract (E)-3-Ylideneoxindoles are prepared in methanol in reasonable to good yields, as adducts of photochemical 5-exo-trig of aryl radicals, in contrast to previously reported analogous radical cyclizations initiated by tris(trimethylsilyl)silane and azo-initiators that gave reduced oxindole adducts. Full article
(This article belongs to the Special Issue Free Radicals and Radical Ions)
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Open AccessArticle Hydrogenations without Hydrogen: Titania Photocatalyzed Reductions of Maleimides and Aldehydes
Molecules 2014, 19(9), 15324-15338; doi:10.3390/molecules190915324
Received: 20 August 2014 / Revised: 16 September 2014 / Accepted: 17 September 2014 / Published: 24 September 2014
Cited by 2 | PDF Full-text (825 KB) | HTML Full-text | XML Full-text
Abstract
A mild procedure for the reduction of electron-deficient alkenes and carbonyl compounds is described. UVA irradiations of substituted maleimides with dispersions of titania (Aeroxide P25) in methanol/acetonitrile (1:9) solvent under dry anoxic conditions led to hydrogenation and production of the corresponding succinimides. [...] Read more.
A mild procedure for the reduction of electron-deficient alkenes and carbonyl compounds is described. UVA irradiations of substituted maleimides with dispersions of titania (Aeroxide P25) in methanol/acetonitrile (1:9) solvent under dry anoxic conditions led to hydrogenation and production of the corresponding succinimides. Aromatic and heteroaromatic aldehydes were reduced to primary alcohols in similar titania photocatalyzed reactions. A mechanism is proposed which involves two proton-coupled electron transfers to the substrates at the titania surface. Full article
(This article belongs to the Special Issue Free Radicals and Radical Ions)
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Open AccessArticle Photochemistry and Radical Chemistry under Low Intensity Visible Light Sources: Application to Photopolymerization Reactions
Molecules 2014, 19(9), 15026-15041; doi:10.3390/molecules190915026
Received: 28 July 2014 / Revised: 9 September 2014 / Accepted: 12 September 2014 / Published: 18 September 2014
Cited by 3 | PDF Full-text (1382 KB) | HTML Full-text | XML Full-text
Abstract
The search for radical initiators able to work under soft conditions is a great challenge, driven by the fact that the use of safe and cheap light sources is very attractive. In the present paper, a review of some recently reported photoinitiating [...] Read more.
The search for radical initiators able to work under soft conditions is a great challenge, driven by the fact that the use of safe and cheap light sources is very attractive. In the present paper, a review of some recently reported photoinitiating systems for polymerization under soft conditions is provided. Different approaches based on multi-component systems (e.g., photoredox catalysis) or light harvesting photoinitiators are described and discussed. The chemical mechanisms associated with the production of free radicals usable as initiating species or mediators of cations are reported. Full article
(This article belongs to the Special Issue Free Radicals and Radical Ions)
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Open AccessArticle Presolvated Electron Reactions with Methyl Acetoacetate: Electron Localization, Proton-Deuteron Exchange, and H-Atom Abstraction
Molecules 2014, 19(9), 13486-13497; doi:10.3390/molecules190913486
Received: 24 July 2014 / Revised: 22 August 2014 / Accepted: 25 August 2014 / Published: 1 September 2014
Cited by 3 | PDF Full-text (1656 KB) | HTML Full-text | XML Full-text
Abstract
Radiation-produced electrons initiate various reaction processes that are important to radiation damage to biomolecules. In this work, the site of attachment of the prehydrated electrons with methyl acetoacetate (MAA, CH3-CO-CH2-COOCH3) at 77 K and subsequent reactions [...] Read more.
Radiation-produced electrons initiate various reaction processes that are important to radiation damage to biomolecules. In this work, the site of attachment of the prehydrated electrons with methyl acetoacetate (MAA, CH3-CO-CH2-COOCH3) at 77 K and subsequent reactions of the anion radical (CH3-CO•-CH2-COOCH3) in the 77 to ca. 170 K temperature range have been investigated in homogeneous H2O and D2O aqueous glasses by electron spin resonance (ESR) spectroscopy. At 77 K, the prehydrated electron attaches to MAA forming the anion radical in which the electron is delocalized over the two carbonyl groups. This species readily protonates to produce the protonated electron adduct radical CH3-C(•)OH-CH2-COOCH3. The ESR spectrum of CH3-C(•)OH-CH2-COOCH3 in H2O shows line components due to proton hyperfine couplings of the methyl and methylene groups. Whereas, the ESR spectrum of CH3-C(•)OH-CH2-COOCH3 in D2O glass shows only the line components due to proton hyperfine couplings of CH3 group. This is expected since the methylene protons in MAA are readily exchangeable in D2O. On stepwise annealing to higher temperatures (ca. 150 to 170 K), CH3-C(•)OH-CH2-COOCH3 undergoes bimolecular H-atom abstraction from MAA to form the more stable radical, CH3-CO-CH•-COOCH3. Theoretical calculations using density functional theory (DFT) support the radical assignments. Full article
(This article belongs to the Special Issue Free Radicals and Radical Ions)
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Open AccessArticle Dissociative Electron Transfer to Diphenyl-Substituted Bicyclic Endoperoxides: The Effect of Molecular Structure on the Reactivity of Distonic Radical Anions and Determination of Thermochemical Parameters
Molecules 2014, 19(8), 11999-12010; doi:10.3390/molecules190811999
Received: 24 June 2014 / Revised: 29 July 2014 / Accepted: 1 August 2014 / Published: 11 August 2014
Cited by 2 | PDF Full-text (728 KB) | HTML Full-text | XML Full-text
Abstract
The heterogeneous electron transfer reduction of the bicyclic endoperoxide 1,4-diphenyl-2,3-dioxabicyclo[2.2.1]hept-5-ene (4) was investigated in N,N-dimethylformamide  at a glassy carbon electrode. The endoperoxide reacts by a concerted dissociative ET mechanism resulting in reduction of the O-O bond with [...] Read more.
The heterogeneous electron transfer reduction of the bicyclic endoperoxide 1,4-diphenyl-2,3-dioxabicyclo[2.2.1]hept-5-ene (4) was investigated in N,N-dimethylformamide  at a glassy carbon electrode. The endoperoxide reacts by a concerted dissociative ET mechanism resulting in reduction of the O-O bond with an observed peak potential of −1.4 V at 0.2 V s−1. The major product (90% yield) resulting from the heterogeneous bulk electrolysis of 4 at −1.4 V with a rotating disk glassy carbon electrode is 1,4-diphenyl-cyclopent-2-ene-cis-1,3-diol with a consumption of 1.73 electrons per mole. In contrast, 1,4-diphenyl-2,3-dioxabicyclo[2.2.2]oct-5-ene (1), undergoes a two-electron reduction mechanism in quantitative yield. This difference in product yield between 1 and 4 is suggestive of a radical-anion mechanism, as observed with 1,4-diphenyl-2,3-dioxabicyclo-[2.2.2] octane (2) and 1,4-diphenyl-2,3-dioxabicyclo[2.2.1]heptane (3). Convolution potential sweep voltammetry is used to determine unknown thermochemical parameters of 4, including the O-O bond dissociation energy and the standard reduction potential and a comparison is made to the previously studied bicyclic endoperoxides 13 with respect to the effect of molecular structure on the reactivity of distonic radical anions. Full article
(This article belongs to the Special Issue Free Radicals and Radical Ions)
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Review

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Open AccessReview Strategies for the Synthesis of Yardsticks and Abaci for Nanometre Distance Measurements by Pulsed EPR
Molecules 2014, 19(12), 20227-20256; doi:10.3390/molecules191220227
Received: 12 September 2014 / Revised: 19 November 2014 / Accepted: 27 November 2014 / Published: 3 December 2014
Cited by 2 | PDF Full-text (1061 KB) | HTML Full-text | XML Full-text
Abstract
Pulsed electron paramagnetic resonance (EPR) techniques have been found to be efficient tools for the elucidation of structure in complex biological systems as they give access to distances in the nanometre range. These measurements can provide additional structural information such as relative [...] Read more.
Pulsed electron paramagnetic resonance (EPR) techniques have been found to be efficient tools for the elucidation of structure in complex biological systems as they give access to distances in the nanometre range. These measurements can provide additional structural information such as relative orientations, structural flexibility or aggregation states. A wide variety of model systems for calibration and optimisation of pulsed experiments has been synthesised. Their design is based on mimicking biological systems or materials in specific properties such as the distances themselves and the distance distributions. Here, we review selected approaches to the synthesis of chemical systems bearing two or more spin centres, such as nitroxide or trityl radicals, metal ions or combinations thereof and outline their application in pulsed EPR distance measurements. Full article
(This article belongs to the Special Issue Free Radicals and Radical Ions)
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Open AccessReview Synthetic Applications of Intramolecular Thiol-Ene “Click” Reactions
Molecules 2014, 19(11), 19137-19151; doi:10.3390/molecules191119137
Received: 17 October 2014 / Revised: 13 November 2014 / Accepted: 14 November 2014 / Published: 19 November 2014
Cited by 7 | PDF Full-text (864 KB) | HTML Full-text | XML Full-text
Abstract
The intermolecular thiol-ene reaction is emerging as a highly efficient; free-radical mediated “click” process with diverse applications in biofunctionalisation and materials science. The related intramolecular thiol-ene reactions offer significant potential for the preparation of a wide range of sulphur containing heterocycles including [...] Read more.
The intermolecular thiol-ene reaction is emerging as a highly efficient; free-radical mediated “click” process with diverse applications in biofunctionalisation and materials science. The related intramolecular thiol-ene reactions offer significant potential for the preparation of a wide range of sulphur containing heterocycles including synthetic therapeutics such as cyclic peptides and thiosugars. Herein, we review recent advances in intramolecular thiyl-radical mediated reactions and their applications for synthetic and medicinal chemistry. Full article
(This article belongs to the Special Issue Free Radicals and Radical Ions)
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Open AccessReview Antioxidant Property of Coffee Components: Assessment of Methods that Define Mechanisms of Action
Molecules 2014, 19(11), 19180-19208; doi:10.3390/molecules191119180
Received: 6 September 2014 / Revised: 4 November 2014 / Accepted: 4 November 2014 / Published: 19 November 2014
Cited by 10 | PDF Full-text (677 KB) | HTML Full-text | XML Full-text
Abstract
Coffee is a rich source of dietary antioxidants, and this property, coupled with the fact that coffee is one of the world’s most popular beverages, has led to the understanding that coffee is a major contributor to dietary antioxidant intake. Brewed coffee [...] Read more.
Coffee is a rich source of dietary antioxidants, and this property, coupled with the fact that coffee is one of the world’s most popular beverages, has led to the understanding that coffee is a major contributor to dietary antioxidant intake. Brewed coffee is a complex food matrix with numerous phytochemical components that have antioxidant activity capable of scavenging free radicals, donating hydrogen and electrons, providing reducing activity and also acting as metal ion pro-oxidant chelators. More recent studies have shown that coffee components can trigger tissue antioxidant gene expression and protect against gastrointestinal oxidative stress. This paper will describe different in vitro, cell-free and cell-based assays that both characterize and compare the antioxidant capacity and mechanism of action of coffee and its bioactive constituents. Moreover, evidence of cellular antioxidant activity and correlated specific genomic events induced by coffee components, which are relevant to antioxidant function in both animal and human studies, will be discussed. Full article
(This article belongs to the Special Issue Free Radicals and Radical Ions)
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Open AccessReview New Developments in Spin Labels for Pulsed Dipolar EPR
Molecules 2014, 19(10), 16998-17025; doi:10.3390/molecules191016998
Received: 1 September 2014 / Revised: 7 October 2014 / Accepted: 13 October 2014 / Published: 23 October 2014
Cited by 6 | PDF Full-text (414 KB) | HTML Full-text | XML Full-text
Abstract
Spin labelling is a chemical technique that enables the integration of a molecule containing an unpaired electron into another framework for study. Given the need to understand the structure, dynamics, and conformational changes of biomacromolecules, spin labelling provides a relatively non-intrusive technique [...] Read more.
Spin labelling is a chemical technique that enables the integration of a molecule containing an unpaired electron into another framework for study. Given the need to understand the structure, dynamics, and conformational changes of biomacromolecules, spin labelling provides a relatively non-intrusive technique and has certain advantages over X-ray crystallography; which requires high quality crystals. The technique relies on the design of binding probes that target a functional group, for example, the thiol group of a cysteine residue within a protein. The unpaired electron is typically supplied through a nitroxide radical and sterically shielded to preserve stability. Pulsed electron paramagnetic resonance (EPR) techniques allow small magnetic couplings to be measured (e.g., <50 MHz) providing information on single label probes or the dipolar coupling between multiple labels. In particular, distances between spin labels pairs can be derived which has led to many protein/enzymes and nucleotides being studied. Here, we summarise recent examples of spin labels used for pulse EPR that serve to illustrate the contribution of chemistry to advancing discoveries in this field. Full article
(This article belongs to the Special Issue Free Radicals and Radical Ions)
Open AccessReview Radical Addition to Iminium Ions and Cationic Heterocycles
Molecules 2014, 19(10), 16190-16222; doi:10.3390/molecules191016190
Received: 28 August 2014 / Revised: 22 September 2014 / Accepted: 22 September 2014 / Published: 10 October 2014
Cited by 12 | PDF Full-text (1705 KB) | HTML Full-text | XML Full-text
Abstract
Carbon-centered radicals represent highly useful reactive intermediates in organic synthesis. Their nucleophilic character is reflected by fast additions to electron deficient C=X double bonds as present in iminium ions or cationic heterocycles. This review covers diverse reactions of preformed or in situ [...] Read more.
Carbon-centered radicals represent highly useful reactive intermediates in organic synthesis. Their nucleophilic character is reflected by fast additions to electron deficient C=X double bonds as present in iminium ions or cationic heterocycles. This review covers diverse reactions of preformed or in situ-generated cationic substrates with various types of C-radicals, including alkyl, alkoxyalkyl, trifluoromethyl, aryl, acyl, carbamoyl, and alkoxycarbonyl species. Despite its high reactivity, the strong interaction of the radical’s SOMO with the LUMO of the cation frequently results in a high regioselectivity. Intra- and intermolecular processes such as the Minisci reaction, the Porta reaction, and the Knabe rearrangement will be discussed along with transition metal and photoredox catalysis or electrochemical methods to generate the odd-electron species. Full article
(This article belongs to the Special Issue Free Radicals and Radical Ions)
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