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A PEG-Tyr block copolymer forms a kinetically stable colloidal solution in water at room temperature which undergoes an irreversible conversion to a gel phase upon heating. A micellar solution and a gel can therefore be studied under identical experimental conditions. This made it possible to compare physical properties and chemical reactivity of micelles and gels in identical chemical environments and under identical conditions. EPR spectra of the spin-labelled copolymer showed that tyrosine mobility in gels was slightly reduced compared to micelles. Chemical reactivity was studied using photochemical degradation of tyrosine and tyrosine dimerization, in the absence and in the presence of an Fe(iii) salt. The reactivity trends were explained by reduced tyrosine mobility in the gel environment. The largest reactivity difference in gels and micelles was observed for bimolecular dityrosine formation which was also attributed to the reduction in molecular mobility.

EPR spectroscopy was used to monitor formation of free radicals in human hair upon UV irradiation. While the EPR spectra of brown hair were dominated by melanin signal, those of white hair were keratin-derived. The decay of UV induced keratin radicals was enhanced at increased ambient humidity. We argue that at higher humidity the swollen hair provides a more liquid-like environment, and higher molecular mobility in this environment leads to faster radical reactions. This interpretation is consistent with the increased UV-triggered protein damage in hair at high humidity as demonstrated by the protein loss, MALDI-TOF and FT-IR data.

A dinuclear, butadiene-bridged complex, trans-μ2:η2,η2-1,3-butadiene-bis(trichloroplatinate(ii)) (1) was unexpectedly obtained on photolysis of acetone solutions of (NBu4)2[PtCl6].

Intramolecular spin exchange in four flexible PEG-based bis-nitroxides was studied by X-band electron paramagnetic resonance (EPR) spectroscopy as a function of temperature and solvent viscosity. A series of biradicals with different lengths of the ethylene glycol bridge connecting the two nitroxide groups was investigated in aqueous and mixed 1:1 i-propanol:water solutions. Conformational transitions in liquid solutions of the biradicals were analyzed, and thermodynamic parameters of the conformational reorganization were calculated from the EPR spectra. Intramolecular distances r between paramagnetic >N–O• fragments in biradicals in frozen solutions were calculated and compared with dynamic properties of the biradicals.

Monoclinic bismuth vanadate nanoparticles (nan-BiVO4) have been used for the selective photo-oxidation of benzyl alcohols (13 examples) to benzaldehydes under visible light irradiation using dioxygen as oxidant. Illumination with a blue LED (λmax = 470 nm) gave yields more than 30 times greater than bulk BiVO4 with >99% selectivity for several cases. Photo-oxidation of PhCH2OH/PhCD2OH isotopomers gave a kinetic isotope effect of 1.3 indicating that C–H bond activation is not the rate determining step in contrast to other reported photocatalysts including TiO2 and carbon nitride. Collectively, structural characterisation, spectroscopic, and reactivity data are correlated with the greater surface area of nan-BiVO4 compared to bulk BiVO4. Furthermore, conversion is shown to be limited as reaction progresses. Control experiments indicate that photo-oxidation is retarded by product aldehyde and that trace amounts (

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.

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.

Electron paramagnetic resonance (EPR) spectroscopy and spin trapping were used to explore the mechanism of alcohol oxidation over gold catalysts. Reaction of secondary alcohols with supported and unsupported gold catalysts (e.g., Au/CeO(2), polymer-incarcerated Au nanoparticles, PPh(3)-protected Au nanoparticles) in the presence of spin traps led to the formation of a hydrogen spin adduct. Using isotope labeling, we confirmed that the hydrogen in the spin adduct originates from the cleavage of the C-H bond in the alcohol molecule. The formation of the hydrogen spin adduct most likely results from the abstraction of hydrogen from the Au surface by a spin trap. These results thus strongly suggest intermediate formation of Au-H species during alcohol oxidation. The role of oxygen in this mechanism is to restore the catalytic activity rather than oxidize alcohol. This was further confirmed by carrying out gold-catalyzed alcohol oxidation in the absence of oxygen, with nitroxides as hydrogen abstractors. The support (e.g., metal oxides) can activate oxygen and act as an H abstractor from the gold surface and hence lead to a faster recovery of the activity. Peroxyl radicals were also observed during alcohol oxidation, consistent with a free-radical autoxidation mechanism. However, this mechanism is likely to be a minor side reaction, which does not lead to the formation of an appreciable amount of oxidation products.

Cross-linking of ethylene propylene diene monomer (EPDM) rubbers containing different amounts of dicyclopentadiene (DCPD), 5-ethylidene-2-norbornene (ENB) and 5-vinyl-2-norbornene (VNB) dienes was examined by EPR spectroscopy. The cross-linking was initiated by thermal decomposition of dicumyl peroxide at 440 K. The concentration of free radicals increased towards the end of the cross-linking process before reaching a maximum and decaying to zero. This is explained by the spatial confinement of the radicals in the cross-linked rubber, which leads to increased life time and, hence, higher radical concentration at a time when most peroxide has decomposed. The EPR spectra showed the presence of two components: a well-resolved spectrum overlapping a broad line. Both components are assigned to allyl radicals possessing different mobility. The more mobile component is assigned to allyl radicals along the EPDM chains, whereas the immobilised allyl radicals are formed in the cross-links. The stability of the allyl radicals decreases in the order DCPD > ENB > VNB. EPDMs containing two dienes show more persistent radicals than their single-diene counterparts. The most persistent radicals are observed for highly cross-linked (e.g., 28% ENB) or mixed diene EPDMs (e.g., 2.2% DCPD–4.4% ENB); the EPR spectra of free radicals in these systems can be observed for several hours.

The mechanism of a place-exchange reaction of ligand-protected Au nanoparticles was investigated using diradical disulfide spin labels. Analysis of reaction mixtures using a combination of GPC and EPR allowed us to determine concentration profile and propose a kinetic model for the reaction. In this model, only one branch of the disulfide ligand is adsorbed on the Au surface during exchange; the other branch forms mixed disulfide with the outgoing ligand. The two branches of the disulfide ligand therefore do not adsorb in adjacent positions on the surface of Au nanoparticles; this was ultimately proven by the powder EPR spectra of frozen exchange reaction mixtures. Our data also suggest the presence of different binding sites with different reactivity in the exchange reaction. The most-active sites are likely to be nanoparticle surface defects.

The cooperative effect in reactions of multifunctional compounds has been probed by monitoring the kinetics of the Michael addition of a dendritic polyamine to acrylate-terminated Au nanoparticles. Comparison of the kinetic data for this reaction with those for model monofunctional compounds showed the absence of any cooperativity. This was tentatively attributed to the high flexibility of the dendrimer and the inability of the system to bring mutually reactive functional groups sufficiently close to each other to achieve the rate enhancement effect. The subsequent reaction, methanolysis of the Michael adduct, was found to follow different mechanistic pathways in model monofunctional compounds and Au nanoparticles: while the model reaction undergoes intramolecular general base catalysis, no anchimeric assistance was observed for the reaction of Au nanoparticles. This difference in reactivity, which was attributed to steric effects, resulted in substantially different product distribution patterns for rea...

EPR spectroscopy and spin-trapping experiments showed that polymer-encapsulated Au nanoparticles promote aldehyde oxidation via a radical pathway by initiating formation of acyl radicals.

Self-assembled monolayers (SAMs) are excellent models for studying interfacial reactions. Here monolayer chemistry is reviewed, focusing on the features that have no analogues in solution chemistry. The growth of surface-attached polymers, intrafilm reactions, chemistry, photochemistry and reactivity issues are all discussed.

Monolayers containing a reactive p-nitrophenyl ester group at different levels with respect to the monolayer interface have been self-assembled on a gold surface. Analysis of grazing angle IR spectra, surface plasmon resonance (SPR), and wettability measurements suggests disordered organization of the alkane chains in the monolayers. Kinetics of monolayer reactions with external reagents (alkylamines) have been studied and compared with those of the same process in bulk medium. Burying of a reaction center under the surface and other structural changes of monolayers were shown to have only a minor effect on the rates of reaction, implying that these monolayers could be easily penetrated by guest molecules. The higher reaction rates with monolayers than in bulk solution are possibly due to a weak binding of the external reagent to the monolayer prior to reaction.

Low temperature (‘cold’) atmospheric pressure plasmas have gained much attention in recent years due to their biomedical effects achieved through the interactions of plasma induced species with the biological substrate. Monitoring of the radical species in an aqueous biological milieu is usually performed via electron paramagnetic resonance (EPR) spectroscopy using various nitrone spin traps, which form persistent radical adducts with the short-lived radicals. However, the stability of these nitroxide radical adducts in the plasma-specific environment is not well known. In this work, chemical transformations of nitroxide radicals in aqueous solutions using a model nitroxide 4-oxo-TEMPO were studied using EPR and LC-MS. The kinetics of the nitroxide decay when the solution was exposed to plasma were assessed, and the reactive pathways proposed. The use of different scavengers enabled identification of the types of reactive species which cause the decay, indicating the predominant nitroxide group reduction in oxygen-free plasmas. The 2H adduct of the PBN spin trap (PBN-D) was shown to decay similarly to the model molecule 4-oxo-TEMPO. The decay of the spin adducts in plasma-treated solutions must be considered to avoid rendering the spin trapping results unreliable. In particular, the selectivity of the decay indicated the limitations of the PTIO/PTI nitroxide system in the detection of nitric oxide.

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.

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 Gd(3+) 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.

A catalytic reactor for the trapping of free radicals originating from gas phase catalytic reactions is described and discussed. Radical trapping and identification were initially carried out using a known radical generator such as dicumyl peroxide. The trapping of radicals was further demonstrated by investigating genuine radical oxidation processes, e.g., benzaldehyde oxidation over manganese and cobalt salts. The efficiency of the reactor was finally proven by the partial oxidation of cyclohexane over MoO(3), Cr(2)O(3), and WO(3), which allowed the identification of all the radical intermediates responsible for the formation of the products cyclohexanol and cyclohexanone. Assignment of the trapped radicals was carried out using spin trapping technique and X-band electron paramagnetic resonance spectroscopy.

We present a molecular probe technique for accessing interfacial surface electrostatics of ligand-protected gold nanoparticles. A series of ligands with variable length of the hydrocarbon bridge between the anchoring sulfur and the reporting pH-sensitive nitroxide is described. The protonation state of this probe is directly observed by EPR spectroscopy. For tiopronin-protected Au nanoparticles, we observed an increase in pKa of up to ca. 1.1 pH units that was affected by the position of the reporter moiety with respect to the monolayer interface.

Complexation of beta-cyclodextrin with flexible nitroxide biradicals linked by a polyethylene glycol chain was monitored by EPR spectroscopy. The EPR spectra of the uncomplexed biradicals show an exchange interaction due to the flexibility of the polyethylene glycol chain. Complexation with cyclodextrin leads to the disappearance of the exchange interaction in the EPR spectra. The complexation can be reversed by the addition of competing guests (e.g., adamantane derivatives). At high concentration, the inclusion complexes precipitate, and differential scanning calorimetry (DSC) of the precipitates proved the formation of complexes. Elemental analysis data revealed that the complexes contain several cyclodextrin units per biradical but that the composition was not stoichiometric.

Spin-trapping experiments show the formation of sulfur-centered radicals during a ligand-exchange reaction between Ph3P-protected Au nanoparticles and alkanethiols in air (see picture). Oxidation of the alkanethiols by molecular O2 adsorbed on the nanoparticles is proposed as the key step. The feasibility of such a process is demonstrated by the Au-nanoparticle-catalyzed oxidation of BH4- and tBuOOH with air.

The effect of additives on the photostability of semiconductor nanoparticles was studied using CdSe, CdSe/ZnSe and CdSe/ZnS particles of various sizes and composition. The additives included phosphine oxides, amines, and antioxidants. Strong initial enhancement of band edge emission by hexadecylamine was observed for the uncoated particles but stability over a week period in light and air was shown to be poor. Inorganic coatings rendered the nanoparticles initially insensitive to additive but provided little extra stability in terms of photoluminescence. Antioxidant was shown to be effective at reducing the rate of photooxidative degradation for all particles in chloroform but not in toluene.

Solutions of alkanethiol-protected Au nanoparticles undergo a slow aging process over a period of several days, which dramatically reduces their reactivity in a place-exchange reaction with disulfides. This behavior was tentatively attributed to the reorganization of the most reactive binding sites (e.g., defect sites) on the nanoparticle surface.

An EPR study of a place-exchange reaction of a diradical disulfide with butanethiol-protected Au nanoparticles showed that the two branches of the disulfide molecule do not adsorb adjacent to each other on the Au surface. The most likely mechanism includes adsorption of only one branch of the disulfide molecule in the exchange process. The exchange reaction was found to be zeroth-order with respect to the diradical, indicative of a dissociative "SN1"-type mechanism.

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, N(2)O, CO(2) and a series of dimerisation products. Overall, acid-catalysed thermal decay of TEMPO leads to ca. 80% yield of hydroxylamine.

Formation of organogels from covalently linked polyethylene glycols and cyclodextrins and their encapsulation/release properties were monitored with EPR spectroscopy using spin labels and spin probes; changes in molecular tumbling rates provided information on spin probe/label location inside the gel network, and molecular properties of the gel.

TEMPO-based spin probes and beta-cyclodextrin form 1 ratio 2 guest-host complexes in viscous aqueous glycerol solutions at low temperatures. The spin probe mobility in these complexes is much higher than in the bulk glassy solution. The similar tumbling rates of the spin probe in viscous host-guest complexes and in solvent-free complexes suggest that the complexation effectively shields the nitroxide unit from the solvent molecules. This is consistent with the low activation barrier for molecular tumbling of immobilised label.

The formation of radicals in the gas phase oxidation of cyclohexane over metal oxides was studied using spin trapping and EPR spectroscopy; the results confirmed that during the catalytic initiation, the oxygen responsible for the radicals formation originates from the lattice of the metal oxide.

Reactions of chloroform over triphenylphosphine-protected Au nanoparticles have been studied using electron paramagnetic resonance (EPR) spectroscopy and a spin trapping technique. Two competing reactions, abstraction of hydrogen and halogen atoms, were identified. The hydrogen abstraction reaction showed an inverse kinetic isotope effect. Treatment of nanoparticles with oxidizing or reducing reagents made it possible to tune the selectivity of radical formation from halogen to hydrogen (deuterium) abstraction. Treatment with PbO(2) promoted the deuterium abstraction reaction followed by the loss of nanoparticle activity, whereas treatment with NaBH(4) regenerated the nanoparticle activity towards Cl atom abstraction. X-ray photoelectron spectroscopy showed an increased Au:P ratio upon treatment with oxidizing reagents. This is likely due to the oxidation of some phosphine ligands to phosphine oxides which then desorb from the nanoparticle surface.

N-Heterocyclic carbene coated Au and Pd nanoparticles have been prepared by a ligand exchange reaction; although carbenes quantitatively displaced the thioether and phosphine ligands from the nanoparticle surface, the resultant nanoparticles spontaneously leached metal complexes and aggregated in solution.

We report on the use of EPR spectroscopy and spin trapping technique to detect free radical intermediates formed in the presence of gold nanoparticles. Phosphine- and amine-protected gold nanoparticles were found to initiate air oxidation of organic substrates containing active hydrogen atoms, such as amines and phosphine oxides. Nanoparticles protected by stronger bound ligands (e.g., thiols) were inactive in these reactions. We also found that gold nanoparticles are able to abstract a halogen atom from the halogenated compounds, presumably due to the high affinity of gold metal for halogens. Reaction of Au nanoparticles with chloroform showed an unusual inverse isotope effect. The trichloromethyl spin adduct was observed when Au nanoparticles were mixed with CDCl(3) but not with CHCl(3). This unexpected behaviour suggests that C-H bond breaking is not the rate-determining step in Au-initiated hydrogen abstraction.