Non-thermal plasmas were used as radical initiators in three radical chain reactions: dehalogenation, 5-exo-trig cyclisation and trifluoromethylation with the Togni II reagent. The use of non-volatile solvents (or solvent-free conditions) gave high yields in all reactions under optimised conditions. Short reaction times and mild conditions make this method an attractive alternative to conventional radical initiators.

•The roles of oxidants in the mechanism of hair melanin bleaching are proposed.•Both hydroxyl radical and perhydroxyl anion contribute to melanin bleaching.•Hydroxyl radicals pre-oxidise melanin facilitating reaction with perhydroxyl anion.•Ammonia plays no role in bleaching beyond melanin solubilisation and pH control.This work aims to determine the roles of reactive oxygen species HO∙ and HO2- in the bleaching of melanins by alkaline hydrogen peroxide. Experiments using melanosomes isolated from human hair indicated that the HO∙ radical generated in the outside solution does not contribute significantly to bleaching. However, studies using soluble Sepia melanin demonstrated that both HO2- and HO∙ will individually bleach melanin. Additionally, when both oxidants are present, bleaching is increased dramatically in both rate and extent. Careful experimental design enabled the separation of the roles and effects of these key reactive species, HO∙ and HO2-. Rationalisation of the results presented, and review of previous literature, allowed the postulation of a simplified general scheme whereby the strong oxidant HO∙ is able to pre-oxidise melanin units to o-quinones enabling more facile ring opening by the more nucleophilic HO2-. In this manner the efficiency of the roles of both species is maximised.Download high-res image (136KB)Download full-size image

The formation of alginate gel from low molecular weight alginate and very low molecular weight alginate in the presence of divalent cations was investigated using Electron Paramagnetic Resonance (EPR) spectroscopy. The transition from sol to gel in the presence of divalent cations was monitored by the changes in the dynamics of spin labelled alginate. The immobilisation of the spin labelled alginate in the gel reflects the strength of interaction between the cation and alginate chain. Diffusion experiments showed that both the cation and alginate polyanion in the gel fibres can exchange with molecules in solution. In particular, we showed that dissolved alginate polyanions can replace alginates in the gel fibres, which can hence diffuse through the bulk of the gel. This illustrates the surprisingly highly dynamic nature of these gels and opens up the possibility of preparing multicomponent alginate gels via polyanion exchange process.

The properties of a gel consisting of a covalent network formed by the reaction of isocyanate end-capped polyethylene glycol (PEG) with β-cyclodextrin, were investigated by EPR spectroscopy. Spin-labelled cyclodextrin was incorporated into the cross-link points of the gel and at the chain ends. The dynamics of the gel fibres as reported by the spin label, was found to be sensitive to the H-bonding ability of the solvent, density of cross-links and temperature. Addition of spin probes (e.g., TEMPO and adamantane-TEMPO) to the unlabelled gel made it possible to characterise the solvent pools in the gel. While TEMPO was uniformly distributed throughout the solvent pools, the adamantane derivative was located at the gel fibre-solvent pool interface; these two probes thus reported on the different locations in the solvent pools. At low temperature, the gels were shown to prevent ice crystallisation in the solvent pools resulting in the formation of supercooled water. Both probes showed that the water froze at ca. 250 K, thus suggesting that the properties of the supercooled water are uniform across the solvent pools.

Cu(II)-catalysed decomposition of hydrogen peroxide at alkaline pH in the presence of etidronic acid (HEDP) showed a sigmoid kinetic profile typical of autocatalytic reactions. However, the reaction abruptly stopped well before all hydrogen peroxide had decomposed, and further addition of Cu(II) and HEDP did not restart the reaction. Results of a mechanistic study suggest that the reaction involves the formation of an active catalyst which decomposes hydrogen peroxide and oxidizes HEDP. Once all HEDP has been consumed, the active complex triggers Cu(II) aggregation to form remarkably stable but catalytically inactive nanoparticles. The nanoparticles were found to be basic Cu(II) phosphate/carbonate. They exhibit self-poisoning behaviour in the hydrogen peroxide decomposition and undergo seed-mediated growth upon addition of further Cu(II).

The polymerisation inhibitor, 2-nitrophenol reacts with radicals generated in self-initiated thermal styrene polymerisation to form the unexpected benzoxazine 1. We propose the mechanism for the formation of 1 and show that it is produced by a radical mediated C–H activation.

We describe the synthesis of spin-labeled bis-ureas which coassemble with bis-urea gelators and report on self-assembly as detected using electron paramagnetic resonance spectroscopy (EPR). Specifically, EPR detects the gel–sol transition and allows us to quantify how much spin-label is immobilized within the gel fibers and how much is present in mobile solvent pools—as controlled by temperature, gelator structure, and thermal history. EPR is also able to report on the initial self-assembly processes below the gelation threshold which are not macroscopically visible and appears to be more sensitive than NMR to intermediate-sized nongelating oligomeric species. By studying dilute solutions of gelator molecules and using either single or double spin-labels, EPR allows quantification of the initial steps of the hierarchical self-assembly process in terms of cooperativity and association constant. Finally, EPR enables us to estimate the degree of gel-fiber solvation by probing the distances between spin-labels. Comparison of experimental data against the predicted distances assuming the nanofibers are only composed of gelator molecules indicates a significant difference, which can be assigned to the presence of a quantifiable number of explicit solvent molecules. In summary, EPR provides unique data and yields powerful insight into how molecular-scale mobility and solvation impact on assembly of supramolecular gels.

A series of gold nanoparticles functionalized with TEMPO-modified disulfide 2 have been prepared and studied by electron paramagnetic resonance (EPR) spectroscopy, UV–vis, transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDX), and thermogravimetric analysis (TGA). In order to increase the packing of spin labels on the particle surface, heat-induced size evolution and ligand exchange reactions were used. The optimized synthesis included a one-pot reaction at room temperature that led to gold nanoparticles with a controlled large size (ca. 7 nm) and high coverage of radicals. These nanoparticles showed a |Δms| = 2 transition at half-field, which gives direct evidence of the presence of a high-spin state and permits an EPR study of the nature of the magnetic coupling between the spins. The results showed dominant antiferromagnetic interactions between radicals, but at lower temperatures, a ferromagnetic contribution was observed.

Inorganic nanoparticles can be embedded within gels by selectively preloading them with suitable molecular precursors followed by reduction or another suitable reaction. Here, we exploit the selective sorption properties of cross-linked β-cyclodextrin/poly(ethylene glycol) hydrogels, in analogy with polyurethane foams, to preconcentrate metal salts (HAuCl4 and K2[Co(SCN)4]) and subsequently generate gel-embedded metal nanoparticles (10–50 nm). The nanoparticles are shown to be immobilized within the gel network as a consequence of their large dimensions in comparison to the gel network pore size. We suggest this is a useful approach for the generalized synthesis of hybrid soft–hard nanomaterials.

Nanosized gold particles were functionalised with two types of paramagnetic surface tags, one having a nitroxide radical and the other one carrying a DTPA complex loaded with Gd3+. Selective measurements of nitroxide–nitroxide, Gd3+–nitroxide and Gd3+–Gd3+ distances were performed on this system and information on the distance distribution in the three types of spin pairs was obtained. A numerical analysis of the dipolar frequency distributions is presented for Gd3+ centres with moderate magnitudes of zero-field splitting, in the range of detection frequencies and resonance fields where the high-field approximation is only roughly valid. The dipolar frequency analysis confirms the applicability of DEER for distance measurements in such complexes and gives an estimate for the magnitudes of possible systematic errors due to the non-ideality of the measurement of the dipole–dipole interaction.

Disproportionation of TEMPO in acids leads to the formation of an N-oxoammonium salt, which can further decompose under thermal conditions, yielding the corresponding hydroxylamine, N2O, CO2 and a series of dimerisation products. Overall, acid-catalysed thermal decay of TEMPO leads to ca. 80% yield of hydroxylamine.

The factors limiting the relaxivity (r) of MRI contrast agents based on small (∼2.0 nm) gold nanoparticles functionalised with paramagnetic chelates were explored using EPR spectroscopy. The EPR analysis suggested that nanoparticle-attached chelates exhibit relatively high tumbling rates which restrict their relaxivity. Two different strategies were employed in order to test this hypothesis and hence improve the relaxivity of the nanoparticle-based contrast agents. In the first approach, the particle diameter was increased. This resulted in lower surface curvature and hence tighter ligand packing, which in turn led to increased relaxivity. In the second approach, the nanoparticles were overcoated with multilayers of oppositely charged polyelectrolytes. The restricted motion of Gd3+ chelates coated by 2–4 polymer layers led to increased relaxivity which was dramatically reduced for thicker layers, presumably due to restricted diffusion of water molecules.

Aging of thiolate protected gold nanoparticles (AuNPs) results in reduced reactivity in the disulfide exchange as monitored by electron paramagnetic resonance (EPR) spectroscopy with a bisnitroxide disulfide incoming ligand. Factors determining the reactivity of the aged particles were investigated. The presence of different binding sites on the surface of AuNPs and a surface reorganization process during aging can explain observed reactivity trends.

Disproportionation of TEMPO in acids leads to the formation of an N-oxoammonium salt, which can further decompose under thermal conditions, yielding the corresponding hydroxylamine, N2O, CO2 and a series of dimerisation products. Overall, acid-catalysed thermal decay of TEMPO leads to ca. 80% yield of hydroxylamine.

Non-thermal plasmas were used as radical initiators in three radical chain reactions: dehalogenation, 5-exo-trig cyclisation and trifluoromethylation with the Togni II reagent. The use of non-volatile solvents (or solvent-free conditions) gave high yields in all reactions under optimised conditions. Short reaction times and mild conditions make this method an attractive alternative to conventional radical initiators.

Cu(II)-catalysed decomposition of hydrogen peroxide at alkaline pH in the presence of etidronic acid (HEDP) showed a sigmoid kinetic profile typical of autocatalytic reactions. However, the reaction abruptly stopped well before all hydrogen peroxide had decomposed, and further addition of Cu(II) and HEDP did not restart the reaction. Results of a mechanistic study suggest that the reaction involves the formation of an active catalyst which decomposes hydrogen peroxide and oxidizes HEDP. Once all HEDP has been consumed, the active complex triggers Cu(II) aggregation to form remarkably stable but catalytically inactive nanoparticles. The nanoparticles were found to be basic Cu(II) phosphate/carbonate. They exhibit self-poisoning behaviour in the hydrogen peroxide decomposition and undergo seed-mediated growth upon addition of further Cu(II).

The factors limiting the relaxivity (r) of MRI contrast agents based on small (∼2.0 nm) gold nanoparticles functionalised with paramagnetic chelates were explored using EPR spectroscopy. The EPR analysis suggested that nanoparticle-attached chelates exhibit relatively high tumbling rates which restrict their relaxivity. Two different strategies were employed in order to test this hypothesis and hence improve the relaxivity of the nanoparticle-based contrast agents. In the first approach, the particle diameter was increased. This resulted in lower surface curvature and hence tighter ligand packing, which in turn led to increased relaxivity. In the second approach, the nanoparticles were overcoated with multilayers of oppositely charged polyelectrolytes. The restricted motion of Gd3+ chelates coated by 2–4 polymer layers led to increased relaxivity which was dramatically reduced for thicker layers, presumably due to restricted diffusion of water molecules.

Nanosized gold particles were functionalised with two types of paramagnetic surface tags, one having a nitroxide radical and the other one carrying a DTPA complex loaded with Gd3+. Selective measurements of nitroxide–nitroxide, Gd3+–nitroxide and Gd3+–Gd3+ distances were performed on this system and information on the distance distribution in the three types of spin pairs was obtained. A numerical analysis of the dipolar frequency distributions is presented for Gd3+ centres with moderate magnitudes of zero-field splitting, in the range of detection frequencies and resonance fields where the high-field approximation is only roughly valid. The dipolar frequency analysis confirms the applicability of DEER for distance measurements in such complexes and gives an estimate for the magnitudes of possible systematic errors due to the non-ideality of the measurement of the dipole–dipole interaction.