Redox Properties of CeIVDOTA in Carbonated Aqueous Solutions. A Radiolytic and an Electrochemical Study.

The redox chemistry of CeIIIDOTA in cage in carbonate solutions was studied using electrochemistry and radiolysis techniques (continuous radiolysis and pulse radiolysis). Spectroscopic measurements point out that the species present in the solutions at high bicarbonate concentrations are [CeIIIDOTA(CO3)]3- (or less plausible [CeIIIDOTA(HCO3)]2-) with the carbonate (bicarbonate) anion as the ninth ligand versus [CeIIIDOTA(H2O)]- present in the absence of bicarbonate. Electrochemical results show a relatively low increase in the thermodynamic stabilization of the redox couple CeIV/III in the presence of carbonate versus its aqueous analogue. [CeIVDOTA(CO3)]2- and [CeIVDOTA(H2O)], prepared electrochemically, decompose photolytically. However, kept in the dark, both are relatively long lived; [CeIVDOTA(H2O)], though, is orders of magnitude kinetically more stable (a considerably longer half-life). Thus, one concludes that the carbonate species have a different mechanism of decomposition depending also on the presence of dioxygen after its preparation (in deaerated/aerated solutions). The [CeIVDOTA(CO3)]2- species is produced radiolytically by oxidation of the trivalent species by CO3•- with a rate constant, measured using pulse radiolysis, of 3.3 × 105 M-1 s-1. This rate constant is at least 1 order of magnitude smaller than most of the rate constants so far reported for the reaction of CO3•- with transition metal/lanthanide (cerium)/actinide complexes. This result together with the bulkiness of the reactants might suggest an outer-sphere electron transfer rather than the inner-sphere one so far proposed. The lifetime of the tetravalent cerium species obtained radiolytically in the presence of carbonate is shorter than the electrochemical one, suggesting a different conformer involved.

New insights into HER catalysis: the effect of nano-silica support on catalysis by silver nanoparticles.

Supported metal catalysts have recently attracted considerable attention in the field of catalysis. The effect of surface chemical groups (SiO-/SiOH2+) on SiO2-Ag0-NPs along with the average negative charge induced by (CH3)2COH˙ radicals on the catalytic reduction of H2O/H3O+ towards the hydrogen evolution reaction (HER) is reported. The results indicate that similar effects are observed both above and below the point of zero charge (PZC) of silica. More importantly, it is shown that a high concentration of this catalyst does not necessarily contribute to boosting the hydrogen formation, but instead, the density of charge on its surface is a decisive factor. A mechanistic explanation of the observed effect is given.

H/D kinetic isotope effect as a tool to elucidate the reaction mechanism of methyl radicals with glycine in aqueous solutions.

The H/D kinetic isotope effect (KIE) for the reaction of methyl radicals with glycine in aqueous solutions at pH 10.6 equals 16 ± 3. This result proves that the methyl radical abstracts a hydrogen atom from the methylene group of glycine and not an electron from the unpaired couple on the nitrogen atom. The rate constant of the reaction of methyl radicals with glycine at pH 7.0 is orders of magnitude smaller than that at pH 10.6.

On the lifetime of the transients (NP)-(CH3)n (NP = Ag0, Au0, TiO2 nanoparticles) formed in the reactions between methyl radicals and nanoparticles suspended in aqueous solutions.

Methyl radicals react in fast reactions, with rate constants k>1×10(8) M(-1) s(-1), with Au(0), Ag(0) and TiO(2) nanoparticles (NPs) dispersed in aqueous solutions to form intermediates, (NP)-(CH(3))(n), in which the methyl groups are covalently bound to the NPs. These intermediates decompose to form ethane. As n≥2 is required for the formation of C(2)H(6), the minimal lifetime (τ) of the methyls bound to the NPs, (NP)-CH(3), can be estimated from the rate of production of the CH(3)(·) radicals and the NPs concentration. The results obtained in this study, using a very low dose rate γ-source for NP = Ag(0), Au(0), and TiO(2) point out that τ of these intermediates is surprisingly long, for example, ≥8 and ≥188 sec for silver and gold, respectively. These data point out that the NP-C bond dissociation energies are ≥70 kJ mol(-1). Under low rates of production of CH(3)(·), that is, when the rate of formation of ethane is very low, other reactions may occur, consequently the mechanism proposed is "broken". This is observed in the present study only for TiO(2) NPs. These results have to be considered whenever alkyl radicals are formed near surfaces. Furthermore, the results point out that the rate of reaction of methyl radicals with (NP)-(CH(3))(n) depends on n, that is, the number of methyl radicals bound to the NPs affect the properties of the NPs.

On the reactions of Cu(II/I)ATP complexes with methyl radicals.

The CuI/IIATP react with methyl radicals to form methane and methanol, where CuIATP reacts with •CH3 in a process that is surprisingly slow. The low-rate constant of this process is attributed to the significant rearrangement of the chelating ligand required for the transient's formation. These results were corroborated by DFT calculations of the relevant compounds.

Reactions of methyl, hydroxyl and peroxyl radicals with the DOTA chelating agent used in medical imaging.

The mechanism of reaction of DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) with ·CH3, CH3O2· and ·OH radicals were studied. The radicals were formed in situ radiolytically. The methyl radicals react orders of magnitude slower with DOTA and with MIII(DOTA)- than the hydroxyl radicals. The various final products were identified and mechanisms for their formation are proposed. CH3O2· radicals do not react, or react too slowly to be observed, with DOTA and with MIII(DOTA)- as long as the central cation is not oxidized by the peroxyl radical. The results imply that synthesis of the MIII(DOTA)-(MIII = radioisotope) complexes in a water-organic solvent (ethanol or 2-propanol or acetonitrile) mixture is not only kinetically desired but the so formed complex also decreases the radiolytic decomposition of DOTA.

On the reactions of methyl radicals with TiO2 nanoparticles and granular powders immersed in aqueous solutions.

Methyl radicals react with TiO(2) nanoparticles (NPs) immersed in aqueous solutions to form transients in which the methyls are covalently bound to the particles. The rate constant for this reaction approaches the diffusion-controlled limit and increases somewhat with the number of methyls bound to the particle. The transients decompose to yield ethane. Thus, formally the particles "catalyse" the dimerization of the radicals, a reaction that is diffusion-controlled. Rutile powders behave similarly to the TiO(2) NPs whereas the mechanism for the decomposition of the transients formed in the analogous reaction of the radicals with anatase powders differs. These results are of importance as alkyl radicals are formed near the surface of TiO(2) in a variety of important photocatalytic processes. The results imply that the reactions of alkyl radicals with TiO(2) have to be considered in these processes.

Radicals in 'biologically relevant' concentrations behave differently: Uncovering new radical reactions following the reaction of hydroxyl radicals with DMSO.

Methyl radicals play key roles in various chemical and biological processes. Mechanistic studies of methyl radicals with their precursor, Dimethyl Sulfoxide (DMSO), were extensively studied. Though the involved mechanisms seemed to be clarified, essentially none of the studies have been performed at conditions relevant to both biological and catalytic systems, i.e. low steady state radical concentrations. A chain-like reaction, as an inverse function of the radicals concentrations ([•CH3]ss), increases the methyl radical yields. The nature of the additional products obtained differs from those commonly observed. Furthermore it is shown that methyl radicals abstract a methyl group from DMSO to yield ethane. Herein we report a novel mechanism relevant mainly at low steady state radical concentrations, which may change the understanding of certain reaction routes present in both biological systems and catalytic chemical systems. Thus the results point out that mechanistic studies have to be carried out at dose rates forming radicals at analogous concentrations to those present in the process of interest.

Role of lycopene in preventing lipid peroxidation products, in commercial infant milk formula.

OBJECTIVES: The aim of the study was to investigate whether lycopene from natural sources (tomato extract) is able to protect newborns milk formula, against oxidative damage caused by exposure to hydroxyl radicals, and is there a difference between milk substitutes from various sources. METHODS: Four commercial brands of infant milk formula: two of the formulas were dairy milk (A-d and B-d) and two were based on soy bean vegan milk (A-s and B-s), were exposed to ionizing radiation radical (·OH). Lipid peroxidation was determined by measuring malondialdehyde (MDA) using thiobarbituric acid reactive substance test (TBARS). RESULTS: When suspensions containing the four brands of formula were subjected to oxidizing media produced by ionizing radiation (hydroxyl radicals), lipid peroxidation increased linearly as a function of the irradiation dose (R = 0.99). It was found that lycopene in a concentration of 0.6 mM, reduced the radiation damage only in the soy-based formula; decrease of ∼40% of the damage achieved in B-s, and ∼20% reduction in the damage caused to A-s, significantly p < 0.01. CONCLUSIONS: Lycopene in dairy milk did not protect against hydroxyl radicals; however, lycopene found to protect against hydroxyl radicals in soy milk. This result suggests different mechanisms of radical production that arises from high iron levels present in the soy milk and involving the high-valent iron peroxo species.

Crystal structure of (acetato-κO)(ethanol-κO)[(9S,17S,21S,29S)-9,17,21,29-tetra-hydroxy-18,30-dioxa-octa-cyclo-[18.10.0.0(2,7).0(8,19).0(9,17).0(11,16).0(21,29).0(23,28)]triaconta-1,3,5,7,11(16),12,14,19,23(28),24,26-undeca-ene-10,22-dione-κ(3) O (18),O (21),O (22)]caesium ethanol monosolvate.

The title compound, [Cs(CH3COO)(C28H16O8)(C2H5OH)]·C2H5OH, is the product of the complexation between one vasarene analogue [1], bis ninhydrin naphthalene-1,3-diol and CsF, where the F(-) ion has reacted with residual acetic acid (AcOH), to form a [1]·CsOAc complex. The inter-molecular inter-actions with the multiple oxygen-containing functional groups of the ligand, as well as O-H⋯O hydrogen bonds involving the ethanol solvent mol-ecules, stabilize the complex, forming a chain along [100]. Additional parallel-displaced π-π stacking, with an inter-planar distance of 3.669 (1) Å, connect several unit cells in a three-dimensional supra-molecular structure, though, the larger size of AcO(-) (1.60 Å) compared to F(-) (1.33 Å) prevents the tight packing that was once achieved with other vasarene complexes of CsF.

Role of radicals in the lipid peroxidation products of commercial infant milk formula.

Four commercial brands of infant milk formula were investigated, and the amount of shelf lipid peroxidation was determined by measuring malondialdehyde (MDA) or, more exactly, thiobarbituric acid reactive substance (TBARS) levels, which are the end products of lipid peroxidation. TBARS level, indicated by MDA concentration, was higher in the vegetarian brands. The vegetarian brands are, as expected, more prone to oxidation than dairy brands, as they contain more unsaturated fats. The introduction of formate anion diminishes the peroxide amounts initially present in the milk samples as well as those induced by radiation-induced oxidation.