Xiaolin Ren

Synthesis and Characterization of Nanometric Iron and Iron-Titanium Oxides by Mechanical Milling: Electrochemical Properties as Anodic Materials in Lithium Cells

Nanometric mixed iron-titanium oxides were prepared by mechanical milling with a view to determining their ability to act as anodic materials in lithium cells. At a TiO2/Fe2O3 mole ratio of 0.4, a solid-state reaction occurs that leads to the formation of Fe5TiO8, which possesses a spinel-like structure; at lower ratios, however, the structure retains the hematite framework. Li/g-Fe2O3 cells exhibit poor electrochemical reversibility; by contrast, Ti-containing electrodes possess improved cycling properties. Changes in the electrodes upon cycling were examined by X-ray photoelectron spectroscopy (XPS). XPS data confirm the participation of electrolyte in the electrochemical reaction and the different type of electrochemical reversibility exhibited by samples. Both processes were influenced by the presence of titanium. Titanium dioxide, in the presence of iron oxides, seems to be inactive to the electrochemical process. Based on the step potential electrochemical spectroscopy (SPES) curves and photoelectron spectra obtained, the presence of Ti increases the reversibility of the redox reactions undergone by the electrolyte during discharge/charge processes. The increased active-material/electrolyte/inactive-material interaction which is reported here offers new perspectives for the use of well-known transition oxides as anode materials in Li-ion batteries.

Reactive oxygen-mediated damage to a human DNA replication and repair protein

Ultraviolet A (UVA) makes up more than 90% of incident terrestrial ultraviolet radiation. Unlike shorter wavelength UVB, which damages DNA directly, UVA is absorbed poorly by DNA and is therefore considered to be less hazardous. Organ transplant patients treated with the immunosuppressant azathioprine frequently develop skin cancer. Their DNA contains 6‐thioguanine—a base analogue that generates DNA‐damaging singlet oxygen (1O2) when exposed to UVA. Here, we show that this 1O2 damages proliferating cell nuclear antigen (PCNA), the homotrimeric DNA polymerase sliding clamp. It causes covalent oxidative crosslinking between the PCNA subunits through a histidine residue in the intersubunit domain. Crosslinking also occurs after treatment with higher—although still moderate—doses of UVA alone or with chemical oxidants. Chronic accumulation of oxidized proteins is linked to neurodegenerative disorders and ageing. Our findings identify oxidative damage to an important DNA replication and repair protein as a previously unrecognized hazard of acute oxidative stress.

Guanine sulphinate is a major stable product of photochemical oxidation of DNA 6-thioguanine by UVA irradiation

The DNA of patients taking the immunosuppressant and anticancer drugs azathioprine or 6-mercaptopurine contains 6-thioguanine (6-TG). The skin of these patients is selectively sensitive to ultraviolet A radiation (UVA) and they suffer an extremely high incidence of sunlight-induced skin cancer with long-term treatment. DNA 6-TG interacts with UVA to generate reactive oxygen species, which oxidize 6-TG to guanine sulphonate (GSO3). We suggested that GSO3 is formed via the reactive electrophilic intermediates, guanine sulphenate (GSO) and guanine sulphinate (GSO2). Here, GSO2 is identified as a significant and stable UVA photoproduct of free 6-TG, its 2′-deoxyribonucleoside, and DNA 6-TG. Mild chemical oxidation converts 6-TG into GSO2, which can be further oxidized to GSO3—a stable product that resists further reaction. In contrast, GSO2 is converted back to 6-TG under mild conditions. This suggests that cellular antioxidant defences might counteract the UVA-mediated photooxidation of DNA 6-TG at this intermediate step and ameliorate its biological effects. In agreement with this possibility, the antioxidant ascorbate protected DNA 6-TG against UVA oxidation and prevented the formation of GSO3.

Identification of potentially cytotoxic lesions induced by UVA photoactivation of DNA 4-thiothymidine in human cells

Photochemotherapy—in which a photosensitizing drug is combined with ultraviolet or visible radiation—has proven therapeutic effectiveness. Existing approaches have drawbacks, however, and there is a clinical need to develop alternatives offering improved target cell selectivity. DNA substitution by 4-thiothymidine (S4TdR) sensitizes cells to killing by ultraviolet A (UVA) radiation. Here, we demonstrate that UVA photoactivation of DNA S4TdR does not generate reactive oxygen or cause direct DNA breakage and is only minimally mutagenic. In an organotypic human skin model, UVA penetration is sufficiently robust to kill S4TdR-photosensitized epidermal cells. We have investigated the DNA lesions responsible for toxicity. Although thymidine is the predominant UVA photoproduct of S4TdR in dilute solution, more complex lesions are formed when S4TdR-containing oligonucleotides are irradiated. One of these, a thietane/S5-(6-4)T:T, is structurally related to the (6-4) pyrimidine:pyrimidone [(6-4) Py:Py] photoproducts induced by UVB/C radiation. These lesions are detectable in DNA from S4TdR/UVA-treated cells and are excised from DNA more efficiently by keratinocytes than by leukaemia cells. UVA irradiation also induces DNA interstrand crosslinking of S4TdR-containing duplex oligonucleotides. Cells defective in repairing (6-4) Py:Py DNA adducts or processing DNA crosslinks are extremely sensitive to S4TdR/UVA indicating that these lesions contribute significantly to S4TdR/UVA cytotoxicity.

Tin-, titanium-, and magnesium-doped α-Cr2O3: Characterisation and rationalisation of the structures

Tin- and titanium-doped α-Cr2O3 have been prepared by the calcination of precipitates. Rietveld structure refinement of the X-ray powder diffraction patterns shows that the dopant metals occupy interstitial sites and partially substitute on octahedral chromium sites in the corundum-related α-Cr2O3 structure. Interatomic potential calculations show that such defects are energetically more favourable than the exclusive substitution of chromium or the occupation by the dopant of interstitial sites in α-Cr2O3 with charge balance being achieved by an appropriate number of cation vacancies. However, the attempted incorporation of magnesium within α-Cr2O3 results in the formation of spinel-related MgCr2O4. The observation is rationalised in terms of the greater thermodynamic stability of the spinel-related structure.

Photo-oxidation of 6-thioguanine by UVA: the formation of addition products with low molecular weight thiol compounds.

The thiopurine, 6‐thioguanine (6‐TG) is present in the DNA of patients treated with the immunosuppressant and anticancer drugs azathioprine or mercaptopurine. The skin of these patients is selectively sensitive to UVA radiation—which comprises >90% of the UV light in incident sunlight—and they suffer high rates of skin cancer. UVA irradiation of DNA 6‐TG produces DNA lesions that may contribute to the development of cancer. Antioxidants can protect 6‐TG against UVA but 6‐TG oxidation products may undergo further reactions. We characterize some of these reactions and show that addition products are formed between UVA‐irradiated 6‐TG and N‐acetylcysteine and other low molecular weight thiol compounds including β‐mercaptoethanol, cysteine and the cysteine‐containing tripeptide glutathione (GSH). GSH is also adducted to 6‐TG‐containing oligodeoxynucleotides in an oxygen‐ and UVA‐dependent nucleophilic displacement reaction that involves an intermediate oxidized 6‐TG, guanine sulfonate (GSO3). These photochemical reactions of 6‐TG, particularly the formation of a covalent oligodeoxynucleotide–GSH complex, suggest that crosslinking of proteins or low molecular weight thiol compounds to DNA may be a previously unrecognized hazard in sunlight‐exposed cells of thiopurine‐treated patients.

The Magnetic hyperfine field in tin-doped Fe3O4: variations during oxidation and subsequent phase transformations

Tin K-edge EXAFS recorded in situ at 498 K from tin-doped Fe3O4 show that the octahedral coordination of tin in the inverse spinel-related structure remains essentially unchanged as it is oxidized to structurally-related γ-Fe2O3. Changes in the magnetic hyperfine field distributions in the tin-119 Mossbauer spectra recorded in situ between 285 and 500 K show that at the higher temperatures some of the tin-doped Fe3O4 is converted to structurally-related tin-doped γ-Fe2O3. Comparison with data recorded from similar materials by 57Fe Mossbauer spectroscopy show that the phase transition is sensitive to the partial pressure of oxygen and the period of time at the elevated temperatures. The changes in the magnetic hyperfine field distributions show that conversion of tin-doped Fe3O4 to tin-doped γ-Fe2O3 is largely completed at 600 K and occurs without the segregation of tin. At 700 K some tin segregates to form tin dioxide as the conversion of tin-doped Fe3O4 to tin-doped γ-Fe2O3 is completed. At higher temperatures the inverse spinel-related γ-Fe2O3 phase is converted to the corundum-related α-Fe2O3 structure.

57Fe Mössbauer spectroscopy study of LaFe1-xCoxO3 (x = 0 and 0.5) formed by mechanical milling

Perovskite-related phases of the type LaFe1−x Co x O3 (x = 0 and 0.5) have been synthesised by milling techniques. The materials are of smaller particle size and more susceptible to reduction in a flowing hydrogen/nitrogen gas mixture than their counterparts made by conventional solid state calcination methods. The presence of cobalt enhances the reducibility of iron in the perovskite-related structure and, in contrast to LaFeO3 where the oxide structure is at least partially retained even after treatment in the reducing atmosphere at 1150°C, treatment of LaFe0.5Co0.5O3 in a reducing atmosphere leads to reduction of both Fe3+ and Co3+, their segregation from the oxide matrix, and the formation of an iron-cobalt alloy.

57Fe Mössbauer Spectroscopy Investigation of Cobalt-Substituted Lanthanum Orthoferrites

The products derived from the temperature programmed reduction of cobalt substituted lanthanum ortoferrites have been examined by 57Fe Mossbauer spectroscopy. The results show that LaFeO3 is partially reduced in 10% hydrogen — 90% nitrogen to metallic iron. The material resisted complete reduction even at 1200°C. In contrast, iron and cobalt in the material LaFe0.5Co0.5O3 segregated under treatment in hydrogen to form an iron-cobalt alloy and metallic iron.

57Fe‐ and 119Sn‐Mössbauer Studies of Tin Doped Chromium Iron Oxides of Composition α‐Cr2−xFexO3

Tin‐doped iron chromium oxides of composition α‐Cr2−xFexO3 prepared by the calcination of precipitates adopt the corundum‐related structure. 57Fe Mossbauer spectroscopy shows the materials to be composed of small superparamagnetic particles and no evidence for a Morin transition was observed above 80K. The supertransferred hyperfine magnetic field at the tin site is shown by 119Sn Mossbauer spectroscopy to be less than that experienced in tin‐doped α‐Fe2O3.