313 research outputs found
Melting of cooperative Jahn-Teller distortion in KMg0.2Cu0.8F3
No full textKCuF3 is a Mott-Hubbard insulator with a distorted perovskite structure (space group I4/mcm). The structural distortion is due to orbital ordering (OO) associated with cooperative Jahn-Teller effect (JT) [1]. and corresponds to an alternate occupation of Cu-3dy2-z2 and Cu-3dx2-z2 hole states on Cu(3d9) ion [2]. The orbital configuration results in quasi one-dimensional magnetic properties. Nearest-neighbour superexchange (NN-SE) interactions are strong and antiferromagnetic (AF) along the c axis and, for T>TN=38 K, weak and ferromagnetic in the ab plane. Debate is open in the literature on the actual driving force (i.e. either OO or JT) of the structural distortion and of the related electronic and magnetic properties. We have recently found experimental evidence of the ideal situation in which OO is melted while the JT distortion is still present:, in fact OO is expected to be very sensitive to slight changes in the electronic structure. Electron paramagnetic resonance investigations revealed melting of OO at room temperature in the KCu1-xMgxF3 system for x=0.1 [3]. We presents here a Synchrotron Radiation X-ray powder diffraction (XRPD) study in a sample with composition KCu0.8Mg0.2F3, which at room temperature is isostructural with KCuF3, a prototypical system for studying Orbital Order (OO). This sample can be considered a realisation of the ideal situation in which OO is melted while the cooperative JT distortion is still present. The melting of the cooperative JT distortion is observed in this system for T~600 K. This result is discussed in the framework of the different energy scales for OO and cooperative JT distortion.
[1] L.F. Feiner, A.M. Oleś, J. Zaanen, Phys. Rev. Lett., 1997, 78, 2799. [2] R. Caciuffo, L. Paolasini, A. Sollier, P. Ghigna, E. Pavarini, J. van den Brink, M. Altarelli Phys.Rev., 2002, B65, 174425
[3] C. Oliva, M. Scavini, S. Cappelli, C. Bottalo, C. Mazzoli, P. Ghigna, J.Phys.Chem., 2007, B111, 597
Observation of charge transfer cascade in α-Fe2O3/IrO2 photoanodes by in-operando X-rays absorption spectroscopy
No full textIn this work we show the direct observation, by means of spectro-photoelectrochemical experiments, of charge transfer between a semiconductor (-Fe2O3) and a metal oxide overlayer (hydrous IrOx) as a photoanode architecture in photoelectrochemical water splitting.1 The aim is to clarify the ambiguous role of oxygen evolving catalysts used as overlayers on top of photoanodes in photoelectrochemical water splitting cells. Previous literature suggested that the real benefit of covering hematite with overlayers like iridium or cobalt oxides is not due to an increase of reaction kinetics but the decrease of the electron density in the hematite2 or the storage of photogenerates holes.3 These effects are likely more important when hydrous overlayer, that can act as adapting catalysts,4 are considered. All these hypothesis can explain the observed improved hole lifetime and reduce recombination with electrons.
The present experimental approach is similar to the one that allowed our recent disclosure of the oxidation states assumed by hydrous IrOx as catalyst for water oxidation.5 In the present case, FEXRAV6 and XANES have been used to probe changes in the charge state of Ir while the hematite was illuminated with 410nm radiation. Thanks to this in-operando setup, we were able to observe an increase of the density of empty Ir 5d states during hematite illumination and in correspondence of water spitting in the photoelectrochemical cell. The main conclusion is that a charge (hole) transfer between hematite and iridium occurs only when the hematite is illuminated. Hydrous iridium oxide is therefore capable of withdrawing holes from the semiconductor thus increasing the probability of interface reaction rather than charge recombination.
1 Minguzzi A., Lugaresi O., Achilli E., D'Acapito F., Naldoni A., Malara F., Locatelli C., Vertova A., Rondinini S., Ghigna P., In preparation
2 Badia-Bou L., Mas-Marza E., Rodenas P., M. Barea E., Fabregat-Santiago F., Gimenez S., Peris E., Bisquert J., J. Phys. Chem. C, 2013, 117, 3826−3833
3 Lin F., Boettcher S.W. Nature Materials, 2014, 13, 81-86
4 Barroso M., Mesa C.A., Pendlebury S.R. , Cowana A.J., Hisatomi T., Sivula K., Grätzel M., Klug D.R., Durrant J.R. PNAS, 2012, 109, 15640–15645
5 Minguzzi A., Lugaresi O., Achilli E., Locatelli C., Vertova A., Ghigna P., Rondinini S., Chem. Sci., 2014, 5, 3591-3597
6 Minguzzi, A.; Lugaresi, O.; Locatelli, C.; Rondinini S.; d'Acapito, F.; Achilli, E.; Ghigna, P. Anal. Chem. 2013, 85, 7009-7013
Alternative Splicing: Role in Cancer Development and Progression
No full textAlternative splicing of precursor messenger RNAs (premRNAs)
is a fundamental step in the regulation of gene
expression. This processing step of the nascent messenger
amplifies the coding potential of eukaryotic genomes by
allowing the production of multiple protein isoforms with
distinct structural and functional properties. The advent
of high-throughput sequencing techniques has recently
revealed that alternative splicing of exons and introns represents
a major source of proteomic diversity in complex organisms
characterized by a limited number of protein-coding
genes. Nevertheless, the evolutionary advantage provided by
alternative splicing can also turn into a source of deleterious
problems for the organism. Indeed, the extreme flexibility
of its regulation, which relies on the combinatorial action
of multiple non stringent factors, is subject to errors and
the aberrant splicing of key genes can result in the onset of
many human genetic and sporadic diseases. In this regard,
mounting evidence illustrates how changes in alternative
splicing patterns of specific genes is an important tool used
by cancer cells to produce protein isoforms involved in all
areas of cancer cell biology, including numerous aspects of
tumor establishment, progression, and resistance to therapeutic
treatments. Importantly, cancer-specific splice variants
have the potential to become suitable therapeutic targets for
human cancer, as novel tools to correct splicing defects are
being developed and, in some cases, have entered clinical
trials for other human diseases, such as spinal muscular
atrophy. Nevertheless, these findings are likely to represent
just the tip of the iceberg and important questions regarding
the role of alternative splicing in cancer still remain to be
addressed.
The main focus of this special issue is to emphasize key
mechanisms involved in oncogenic splicing changes, their
connection with other steps of gene expression, and the
therapeutic potential of cancer-associated alternative splicing
isoforms.
More specifically, M. Ladomery discusses alternative
splicing in the context of the so-called hallmarks of cancer,
originally proposed by Hanahan and Weinberg in 2000.
The list of hallmarks was originally six; recently it was
augmented to ten. M. Ladomery proposes that a comprehensive
dysregulation of alternative splicing could, in itself,
be considered yet another hallmark of cancer. The idea is
that the aberrant expression and activity of key oncogenic
splicing factors and/or their regulatory kinases could lead
to a systematic change in gene expression by favouring the
concurrent production of several oncogenic splice variants of
genes involved in critical biological aspects of tumour cells.
S. C. Lenzken et al. review our current knowledge of the
role of alternative splicing in the multiple and various aspects
of the DNA damage response (DDR) and the control of
genome stability. This review illustrates several mechanisms
through which pre-mRNA splicing and genomic stability can
influence each other and contribute to tumorigenesis.
M. Romano and colleagues draw attention to the function
that pseudoexons and pseudointrons can play directly in
cancer pathology. These sequences can be found in genes
that have well-established roles in cancer, including BRCA1,
2 International Journal of Cell Biology
BRCA2, NF-1, and ATM. They describe the mechanisms
through which pseudoexons and pseudointrons can be activated
or repressed. In addition, they discuss their potential
use as tumour biomarkers to provide a more detailed staging
and grading of cancer.
C. Naro and C. Sette discuss the key role that reversible
phosphorylation plays in the regulation of alternative splicing.
Both splice factors and core components of the spliceosome
are affected by phosphorylation. The review focuses
on the role of protein kinases and phosphatases whose
activity has specifically been linked to aberrant alternative
splicing associated with neoplastic transformation. Moreover,
it illustrates the fact that signal transduction routes that are
frequently altered in cancer cells, such as the RAS/ERK and
the PI3K/AKT pathways, can modulate alternative splicing
events through the direct or indirect phosphorylation of
splicing regulatory proteins. Thus, protein kinases and phosphatases
involved in this step of gene expression regulation
may provide exciting opportunities for novel drug design.
A. Best et al. describe the emerging role of Tra2, an
SR-related protein, in human cancer. The gene encoding this
splicing factor is amplified in various types of cancer and
the increased expression of Tra2 is associated with cancer
cell survival. Interestingly, the Tra2 gene is a transcriptional
target of the proto-oncogene ETS-1, whereas known Tra2
splicing targets play key roles in cancer cells, where they affect
metastasis, proliferation, and cell survival. These observations
point to regulation of Tra2 expression in cancer cells
as an important step in tumorigenesis.
The review by Z. Siegfried et al. gives a series of specific
examples that cover misregulated alternative splicing events
affecting both the Ras-MAPK and PI3K-mTOR signalling
pathways during carcinogenesis.These pathways show extensive
crosstalk and are commonly altered in many cancers by
genetic and epigenetic aberrations. This article also addresses
how these signalling pathways play key roles in the transmission
of extracellular signals to the splicing machinery and to
specific RNA-binding proteins that ultimately regulate exon
definition events.
C. Jackson et al. give an overview on a topic of significant
clinical interest: the roles (often opposed) of the HER2 splice
isoforms in breast cancer progression and drug resistance.
M. P. Paronetto describes the function of the Ewing
Sarcoma protein (EWS) in cancer biology. EWS is best
known for its involvement in translocations associated with
sarcomas. Recent evidence has implicated EWS in the regulation
of DNA damage response (DDR) in cancer. EWS is a
multifunctional protein thought to help coordinate multiple
steps in the synthesis and processing of pre-mRNAs. This
review illustrates in detail the biochemical features and the
physiological roles of this RNA binding protein and provides
some hints on its possible contribution to human cancer.
Other two reviews give a series of specific examples of
cancer-associated splicing variants. C. Sette discusses the
growing evidence that dysregulated alternative splicing is a
major factor in the remarkable biological heterogeneity of
prostate cancer. Key genes, including the androgen receptor
itself, are alternatively spliced, thereby expressing isoforms
with opposing functions. The review also illustrates how
the regulation of alternative splicing is likely to present
novel opportunities in the diagnosis, prognosis, and treatment
of prostate cancer. S. Bonomi et al. deal with novel
information on how alternative splice variants of many
cancer-related genes can directly contribute to the oncogenic
phenotype, focusing on a number of processes involved in
cancer progression, such as response to hypoxia, migration,
invasion, and metastasis. Furthermore, they discuss
some significant examples of alternative splicing isoforms
selectively expressed by tumors and not by normal tissues,
which may not only represent diagnostic and prognostic
tumor biomarkers, but also provide potential targets for the
development of new therapeutic strategies.
In their article, L. Spraggon and L. Cartegni focus on the
role of U1 snRNP, an essential component of the splicing
machinery, in the regulation of alternative polyadenylation
and they strongly emphasize its implications in cancer pathogenesis.
Moreover, this review underlines the interesting
possibility of manipulating this U1 snRNP function for
anticancer therapeutic purposes.
Lastly, S. Barberan-Soler and J. M. Ragle give an overview
of the advantages of using the nematode Caenorhabditis
elegans to study splicing regulation in vivo. Importantly,
a large percentage of genes undergo alternative splicing
also in this simple and genetically useful organism. A big
proportion of these events are functional, conserved, and
under strict regulation across development, suggesting that
their investigation is likely to provide general mechanisms of
regulation that can be applied also to human genes. Notably,
the review illustrates several examples of alternatively spliced
genes that have human homologues implicated in cancer
biology.
We hope that this special issue will attract the attention of
researchers on new progresses in the fields of alternative splicing
and cancer. In particular, the articles presented herein
might highlight how this posttranscriptional mechanism of
gene expression plays important roles in the generation
of oncogenes and tumor suppressors, describe its interplay
with signaling pathways, and suggest how our knowledge
of these processes is leading to a better comprehension
of malignant transformation, thus helping develop novel
therapeutic strategies for the treatment of cancers
Electro- and photo-electrochemical water splitting as studied by In-Operando X-Rays Absorption Spectroscopy
No full textIn this work we show our most recent results obtained by in-operando X-Ray absorption spectroscopy on hydrous/amorphous [1] and on crystalline/dry [2] iridium oxide electrodes as electrocatalysts for the oxygen evolution reaction (OER). In all cases, XAS evidenced the role of Ir active sites, and the relevant oxidation states assumed during the catalytic cycle. Moreover, the local structure is not significantly influenced by the applied potential, thus suggesting a negligible reorganization energy of the catalyst.On the bases of these results, we were able to directly observe, by means of spectro-photoelectrochemical experiments, the charge transfer between a semiconductor (α-Fe2O3) and hydrous IrOx, the latter used as overlayer for generating a high performance photoanode architecture in photoelectrochemical water splitting[3]. The aim is to clarify the ambiguous role of oxygen evolving catalysts used as overlayers on top of photoanodes in photoelectrochemical water splitting cells. Previous literature suggested that the real benefit of covering hematite with overlayers like iridium or cobalt oxides is not due to an increase of the reaction rate but to a decrease of the electron density in the hematite[4] or to the storage of photogenerates holes[5]. These effects are likely more important when hydrous overlayer, that can act as adapting catalysts[6], are considered. All these hypotheses can explain the observed improved hole lifetime and reduce recombination with electrons. The experimental approach is similar to the one adopted to study Ir oxide particles electrocatalysts[1,2]. In the present case, FEXRAV [7] and XANES have been used to probe changes in the charge state of Ir while the hematite was illuminated with a 410nm diode. Thanks to this setup, we were able to observe an increase of the density of empty Ir 5d states during hematite illumination and in correspondence of water spitting in the photoelectrochemical cell. The main conclusion is that a charge (hole) transfer between hematite and iridium occurs only when the hematite is illuminated. Hydrous iridium oxide is therefore capable of withdrawing holes from the semiconductor thus increasing the probability of interface reaction rather than charge recombination.
References
[1] A. Minguzzi, O. Lugaresi, E. Achilli, C. Locatelli, A. Vertova, P. Ghigna, Rondinini S., Chem. Sci., 2014, 5, 3591-3597
[2] A. Minguzzi, C. Locatelli, O. Lugaresi, E. Achilli, G. Cappelletti, M. Scavini, M. Coduri, P. Masala, B. Sacchi, A. Vertova, P. Ghigna, S. Rondinini, submitted
[3] A. Minguzzi, O. Lugaresi, E. Achilli, F. D'Acapito, A. Naldoni, F. Malara, C. Locatelli, A. Vertova, S. Rondinini, P. Ghigna, In preparation
[4] M. Barroso, C.A. Mesa, S.R. Pendlebury, A.J. Cowana, T. Hisatomi, K. Sivula, M. Grätzel, D.R. Klug, J.R. Durrant PNAS, 2012, 109, 15640–15645
[5] L. Badia-Bou, E. Mas-Marza, P. Rodenas, E M. Barea., F. Fabregat-Santiago, S. Gimenez, E. Peris, J. Bisquert, J. Phys. Chem. C, 2013, 117, 3826−3833
[6] F. Lin, S.W. Boettcher Nature Materials, 2014, 13, 81-86
[7] A. Minguzzi, O. Lugaresi, C. Locatelli, S. Rondinini, F. d'Acapito, E. Achilli, P. Ghigna, Anal. Chem. 2013, 85, 7009-7013
In-situ X-ray absorption spectroscopy on (photo-)electrocatalysts for water oxidation: towards new insights on the reaction mechanism
No full textHere we introduce the Fixed Energy X-Ray Absorption Voltammetry (1), a novel in-situ/in-operando X-Ray Absorption Spectroscopy (XAS) technique for fast and easy preliminary characterization of electrodes and photoelectrodes which consists in recording the absorption coefficient at a fixed energy while varying at will the electrode potential. The energy is chosen close to a core level absorption edge, in order to give the maximum contrast between different oxidation states of an element. It follows that any shift from the initial oxidation state determines a variation of the X-ray absorption coefficient. In this work we demonstrate that FEXRAV allows to quickly map the variation of the oxidation states of the element under consideration in a desired potential window. At this purpose, we use high surface area electrodes to attain a high surface/volume ratio (nanoparticles, nanostructures, highly hydrated films) and be more sensible to any chemical phenomena occurring at the surface.
We show that FEXRAV gives important information by itself but can also serve as a preliminary screening of the potential window or, more generally, for choosing the best experimental conditions for a better targeted XAS analysis. In fact, this work includes a detailed XAS study aimed to clarify the mechanism of iridium oxide as catalyst for water oxidation: for the first time we directly observed the co-existence of more than one Ir oxidation state at E >1.23V (RHE), that is consistent with the role of Ir as center of a catalytic cycle. This represents a crucial point for a better understanding of water electrolysis and photoelectrochemical (PEC) water splitting (2).
We completed this study by time-resolved energy dispersive XAS for better understanding the time-dependence of the interfacial phenomena occurring during pseudocapacitance charge/discharge and during the water oxidation catalysis.
(1) Minguzzi, A.; Lugaresi, O.; Locatelli, C.; Rondinini S.; d'Acapito, F.; Achilli, E.; Ghigna, P. Anal. Chem. 2013, 85, 7009-7013.
(2) Minguzzi A., Lugaresi O., Achilli E., Locatelli C., Vertova A., Ghigna P., Rondinini S., Chem. Sci. 2014, 5, 3591-3597
Long range structure and local Cu environment in KMg1-xCuxF3 solid solution probed by means of XRPD and EPR
No full textKCuF3 is a Mott-Hubbard insulator with a pseudocubic perovskite structure. In its most common crystalline form, it belongs to the I4/mcm space group. The structural distortion is due to orbital ordering associated with cooperative Jahn-Teller effect [1]. CuF6 octahedra are elongated in the ab plane along a or b axis, in an antiferrodistorsive pattern. The distortion corresponds to an alternate occupation of Cu-3dy2-z2 and Cu-3dx2-z2 hole states on Cu(3d9) ion. The orbital configuration results in quasi one-dimensional magnetic properties. Nearest-neighbour superexchange (NN-SE) interactions are strong and antiferromagnetic (AF) along the c axis and, for T>TN=38 K, weak and ferromagnetic in the ab plane; the ratio between NN-SE along and perpendicular to c is |Jc|/Ja100 [2]. For T<38 K KCuF3 shows three-dimensional antiferromagnetic order.
Melting of orbital order in KCuF3 is expected to occur well above room temperature [1], where the cooperative structural distortion should disappear and the undistorted cubic perovskite structure (space group Pm3m) should be favoured. Preliminary XRPD measurements carried out at beamline ID31 at ESRF did not detect any phase transition up to 730 °C.
A system where melting of the orbital order could be more easily observed is the complete solid solution KMg1-xCuxF3 (0<x<1). At room temperature, the structure of this system is cubic (space group Pm3m) for low Cu concentration, while it is isomorphic with KCuF3 for high Cu concentration [3]. The cubic to tetragonal phase transition with increasing x is accompanied by the onset of a cooperative Jahn-Teller distortion [3].
In the present paper we present a combined XRPD and EPR study on the relation between long range structure, local Cu environment and magnetic properties of KMg1-xCuxF3 (0<x<1) solid solution as a function of x and T.
KMg1-xCuxF3 is cubic (space group Pm3m) for x0.63. A miscibility gap is present for 0.63<x<0.73.
As shown by EPR spectroscopy, Cu environment is isotropic for x0.2 in the cubic domain. Its symmetry becomes axial and then orthorhombic with increasing Cu concentration. Collective orbital ordering becomes more and more important increasing x in the tetragonal domain, causing a symmetrisation of the EPR lineshape
Low-alkali metal content in beta-vanadium mixed bronzes: the crystal structures of β-Kx(V,Mo)6O15 (x = 0.23 and 0.32) by single-crystal X-ray diffraction
No full textThe vanadium–molybdenum mixed oxide bronzes of composition K0.23(V5.35Mo0.65)O15 and K0.32(V5.48Mo0.52)O15 have a monoclinic
structure with s.g. C2/m, Z 1⁄4 2, and unit-cell dimensions a 1⁄4 15.436(2), b 1⁄4 3.6527(5), c 1⁄4 10.150(1)A ̊ , b 1⁄4 108.604(3)1 and
a 1⁄4 15.452(2), b 1⁄4 3.6502(5), c 1⁄4 10.142(1)A ̊ , b 1⁄4 109.168(3)1, respectively, as determined by single-crystal X-ray diffraction. These
compounds show the b-NaxV6O15 tunnel structure, which is isostructural with bannermanite, natural sodium–potassium vanadate.
Structure refinements from diffracted intensities collected in the 2–381y range converged to final R 1⁄4 5.58% and 7.48% for the two
crystals, respectively. The V atoms are distributed on three different crystallographic sites. Partial substitution of V with Mo occurs in
only one of these positions. Oxygen atoms involved in vanadyl groups point toward the tunnels. The K ions in the tunnels are
coordinated by seven oxygen atoms. The alkali metal content in these crystals is much lower than the solubility limit found for the
analogous Na containing compound
Study of photoelectrochemical behavior of copper oxides based materials using X-ray absorption spectroscopy
No full textThe use of sunlight to convert water into fuel is very attractive and ambitious since H2 is considered to be the energy carrier of the future thanks to its high mass energy density and its environmental friendliness [1,2]. Copper oxides-based photocathodes are attractive for their absorption in the visible range, low cost, high abundance and easy synthetic protocols as well as high photoactivity [3,4].
Two p-type semiconducting copper based materials has been prepared, characterized and tested as a photocathode for H2 production: CuO and Cu2O. The first one is prepared by thermal treatment of nanocrystalline CuI, which shows high efficiency in light conversion and interesting self-protection properties [5].
Cu2O instead was prepared by electrochemical deposition from a lactate-stabilized Cu++ bath [3]. Viceversa the main drawback of Cu(I) oxide is its lack of stability during photoelectrochemical conditions. For this material the influence of a metallic underlayer (Au, Cu) between the semiconductor itself and the FTO support was studied, together with the presence of a small load of Pt catalyst.
In-situ and in-operando techniques like X-ray absorption near edge structure (XANES), Extended X-Ray Absorption Fine Structure (EXAFS) and Fixed Energy X-ray Absorption Voltammetry (FEXRAV) [6] allow us to better understanding materials behavior. We observe changes in copper oxidation states upon light and/ or applied potential. Moreover, the role of methanol as hole-scavenger during photoelectrochemical experiment has been studied. FEXRAV measurements allow following the material degradation processes and defining the stability windows. With differential light and dark XANES spectra, we investigated the local changes in electronic structure upon spectroelectrochemical conditions. These results will allow us obtaining more stable system for photoelectrochemical hydrogen production.
References
[1] G. Centi, S. Perathoner, ChemSusChem. 3 (2010) 195–208.
[2] F. Malara, A. Minguzzi, M. Marelli, S. Morandi, R. Psaro, V. Dal Santo, A. Naldoni, ACS Catal. 5 (2015) 5292–5300.
[3] A. Paracchino, V. Laporte, K. Sivula, M. Grätzel, E. Thimsen, Nat. Mater. 10 (2011) 456–461.
[4] C. Li, T. Hisatomi, O. Watanabe, M. Nakabayashi, N. Shibata, K. Domen, J.-J. Delaunay, Energy Environ. Sci. 8 (2015) 1493–1500.
[5] T. Baran, S. Wojtyła, C. Lenardi, P. Ghigna, E. Achilli, S. Rondinini, A. Minguzzi, ACS Appl. Mater. Interfaces. (submitted).
[6] A. Minguzzi, O. Lugaresi, C. Locatelli, S. Rondinini, F. D’Acapito, E. Achilli. P. Ghigna. Anal. Chem. (2013), 85, 7009-7013
Electron Magnetic Resonance of f-elements-containing perovskite- and ferrite-like compounds
No full textHigh temperature structural behaviour of Li2VOSiO4
No full textStructural investigations at high temperature were carried out on compound Li2VOSiO4, which crystallises with a natisite-type structure. Unit-cell parameters were measured and diffracted intensities collected at regular intervals in the temperature range 25-500°C using single crystal X-ray diffraction techniques. Thermal expansion coefficients showed positive expansion of lattice constants, greater along the c direction, and of cell volume. Reversibility of thermal expansion in the investigated temperature range was checked by measuring unit-cell parameters after the crystal was cooled down from 500°C to room temperature. Structure refinements revealed that the [VOSiO4]n2- layers are almost rigid and that the most relevant modifications occur at the intercalated ion layer. © by Oldenbourg Wissenschaftsverlag
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