36,009 research outputs found
The Tobermorite-Like Layer in Non-Tobermorite Minerals
No full textThe tobermorite-like layer (TLL) is a characteristic feature in all the structures of the natural and synthetic compounds of the tobermorite group /1/. In those structures eptahedra of calcium cations, characterized by ‘a pyramidal part on one side and a dome part on the other side joining the equatorial oxygen atoms’, form columns through edge sharing; the columns are connected each other, once again through edge sharing, and adjacent columns present the pyramidal apical ligands on opposite sides of the resulting infinite layers. These layers, decorated with wollastonite chains on both sides, build up the ‘complex layers’ which are the basic structural module in all the phases of the tobermorite group (Fig. 1). TLL is a recurrent feature in several natural phases belonging to distinct mineral groups. In the structures of the compounds of the rinkite group the TLL is decorated on both sides by disilicate groups and is accompanied by an infinite ‘octahedral’ layer in building up the structural arrangement. Dovyrenite /2/ and roumaite /3/ are closely related to the minerals of the rinkite group and differ only in the way of decoration of the TLL by the disilicate groups. Fukalite /4/ too presents the TLL, decorated on both sides by four-repeat silicate chains and carbonate groups; tilleyite-type polyhedral layers are also present as distinct modules in building up the structural arrangement. The ubiquitous occurrence of TLL is related to its chemical and structural flexibility: the chemical and geometrical variations of the TLL in the different structures are described and discussed. Key-words: layered silicates, crystal structures, mineralogical crystallography. References /1/ Merlino S., Bonaccorsi E., Armbruster T. (1999): American Mineralogist, 84, 1613–1621. /2/ Kadiyski M., Armbruster T., Galuskin E.V., Pertsev N.N., Zadov A.E., Galuskina I.O., Wrzalik R., Dzierżanowski P., Kislov E.V. (2008): American Mineralogist, 93, 456-462. /3/ Biagioni C., Bonaccorsi E., Merlino S., Parodi G.C., Perchiazzi N., Chevrier V. (2008): Plinius, 34, 211. /4/ Merlino S., Bonaccorsi E., Grabezhev A.I., Zadov A.E., Pertsev N.N., Chukanov N.V. (2009): American Mineralogist, 94, 323–333
MOSANDRITE: STRUCTURAL AND CRYSTAL-CHEMICAL RELATIONSHIPS WITH RINKITE
No full textemical (EPM data), TG–DSC studies and structural investigations indicate that mosandrite presents a particular chemical
composition (low Ca, Na and F contents, high amount of H2O), unit-cell parameters (a 7.398, b 5.595, c 18.662 Å, b 101.37°,
V 757.29 Å3; space-group symmetry P21/c; Z = 2), and a rinkite-type structure characterized by a low occupancy of the M(2) and M(3) sites
Essential features of the polytypic charoite-96 structure compared to charoite-90
No full textCharoite, ideally (K,Sr,Ba,Mn) 15-16(Ca,Na) 32[(Si 70(O,OH) 180)](OH,F) 4•nH 2O, is a rock-forming mineral from the Murun massif in Yakutia, Sakha Republic, Siberia, Russia, where it occurs in a unique alkaline intrusion. Charoite occurs as four different polytypes, which are commonly intergrown in nanocrystalline fibres. We report the structure of charoite-96 (a = 32.11(6), b = 19.77(4), c = 7.23(1) Å, β = 95.85(9)°, V = 4565(24) Å 3, space group P2 1/m), which was solved ab initio by direct methods on the basis of 2676 unique electron diffraction reflections collected by automated diffraction tomography and refined to R 1/wR 2 = 0.34/0.37. The structure of charoite-96 is related to that of the charoite-90, which was also solved recently. Both structures are composed of three different types of dreier silicate chains running along [001] and separated by ribbons of edge-sharing Ca-and Na-centred octahedra. In the structure of charoite-96, adjacent blocks formed by three different silicate chains and stacked along the x axis, are shifted by a translation of 1⁄2 c. The shifts involve a hybrid dreier quadruple chain, [Si 17O 43] 18- and a double dreier chain, [Si 6O 17] 10-. In charoite-90 adjacent blocks are stacked without shifts. © 2011 Mineralogical Society
Heterogeneous nucleation helps the search for initial crystallization conditions of γ-glutamyl transpeptidase from Bacillus licheniformis
No full textHere, the crystallization and preliminary X-ray diffraction studies of Bacillus licheniformis γ-glutamyl transpeptidase (BlGT) are reported. The serendipitous finding of heterogeneous nucleants in the initial experiments provided the first crystallization conditions for the protein. Crystals were grown by hanging-drop vapour diffusion using a precipitant solution consisting of 20%(w/v) PEG 3350, 0.2 M magnesium chloride hexahydrate, 0.1 M Tris-HCl pH 8.2. The protein crystallized in the orthorhombic space group P2(1)2(1)2(1), with one heterodimer per asymmetric unit and unit-cell parameters a = 60.90, b = 61.97, c = 148.24 Å. The BlGT crystals diffracted to 2.95 Å resolution
“Salmo salar ribonuclease: structural and functional evidences of an auto inhibition mechanism”
No full textThe pancreatic-type ribonucleases (ptRNases) constitute one of the most investigated family of homologous proteins [1]. PtRNases have been found in various organs and tissues of mammals, amphibians, reptiles, birds [2] and, more recently, fish. In particular, seven new RNases were identified in two teleosts: five from Zebrafish (ZF RNases) and two from Salmo Salar (Ss-RNase-1 and Ss-RNase-2) [3-5]. We have focused our attention on the latter enzyme, which is the most active fish RNase, although its activity is much lower than that of the other members of the ptRNase family.
The crystallographic structure of Ss-RNase-2 (SS2) shows the typical V-shape structure of pancreatic-like ribonucleases, with three helices and six β-strands connected by loops and turns. The electron density in the active site region is discontinuous. In particular, the position of His 113, one of the residues of the catalytic triad that is usually very well defined, cannot be unambiguously identified. Furthermore, the segment Val117-Ile121, which is well-structured and anchored to the protein body, partially obstructs the active site. These structural features suggest that SS2 is in a sort of inhibited state. It is reasonable to hypothesize that, in consequence of an interaction with a specific ligand, the C-terminal segment moves to free the active site. To define the activation mechanism of SS2, we have designed and characterized two deletion mutants: SS2-des117-121, in which the chain segment that in the wild-type protein obstructs the active site has been removed, and SS2-des119-126, in which the elimination of the last eight residues of the chain might allow the sliding of the obstructing segment out of the active site. Interestingly, the crystallographic structure of SS2-des117-121 shows a well-defined active site, almost free of obstructions. Furthermore, the enzymatic activity assays show an improvement of the specific activity of SS2-des117-121 with respect to the wild-type protein. On the bases of these findings, we have proposed an intriguing auto-inhibition mechanism for SS2. The details will be discussed at the Meeting.
[1] L. Aravind, E.V. Koonin Methods Enzymol. 2001, 341, 3.
[2] S. Cho, J.J. Beintema, J. Zhang Genomics. 2005, 85, 208.
[3] E. Pizzo, P. Buonanno, A. Di Maro, S. Ponticelli, S. De Falco, N. Quarto, M.V. Cubellis, G. D'Alessio J Biol Chem. 2006, 281, 27454.
[4] Pizzo, E., Merlino, A., Turano, M., Russo Krauss, I., Coscia, F., Zanfardino, A., Varcamonti, M., Furia, A., Giancola, C., Mazzarella, L., Sica, F., D'Alessio, G. Bioch. J. 2010, 433(2), 345-355
[5] E. Pizzo, M. Varcamonti, A. Di Maro, A. Zanfardino, C. Giancola, G. D'Alessio FEBS J. 2008, 275, 1283
Effectiveness of surgical treatment of severe macrocheilia in a patient with orofacial granulomatosis
No full textOrofacial granulomatosis (OFG) is the term given to a group of diseases characterized by the presence of non-necrotizing granulomatous inflammation affecting the soft tissues of the orofacial region. Treatment of OFG is often challenging and unsatisfactory. We report on a 32-year-old man with a 2-year history of oedema and swelling of the upper lip without systemic symptoms. The history, clinical features and histopathological findings led to the diagnosis of cheilitis granulomatosa (CG), a disease included in the spectrum of OFG. The patient was treated with oral diaminodiphenyl sulfone (DDS) and clofazimine without success. Oral doxycycline led to a slight improvement of the disease. Because the volume of the upper lip was twice normal size, surgical reduction was performed, followed by administration of oral doxycycline for 3 months. This therapeutic approach led to complete remission, with no recurrence after 3 years
Chukanovite, Fe2(CO3)(OH)2, a new mineral from the weathered iron meteorite Dronino
No full textThe new mineral chukanovite, Fe2(CO3)(OH)2, occurs in cavities of weathered fragments of the Dronino ataxite iron
meteorite found near the Dronino village, Kasimov district, Ryazan’ Oblast, Russia. It is a product of terrestrial alteration of
meteorite iron. Associated minerals are goethite, akaganéite, hematite, hibbingite, reevesite, honessite, etc. Chukanovite forms acicular to fibrous individuals (up to 0.5 mm long and up to 2–3 μm thick) combined in spherulites up to 1 mm in diameter,
botryoidal spherulitic clusters and parallel- or radial-columnar aggregates which form crusts up to 1 mm thick. Unaltered
chukanovite is transparent, pale-green or colourless. The surface of aggregates is brownish-green. Streak is white. Lustre
is vitreous. Cleavage is perfect, probably on {0–21}, fracture is uneven. The mineral is brittle, the Mohs’ hardness is
3.5–4, the calculated density is 3.60 g/cm3. It is optically biaxial (–) with α 1.673(3), β 1.770(5), γ 1.780(5), 2Vmeas. 10(5)◦.
Average chemical composition (wt. %; electron probe, H2O by modified Penfield method, CO2 by selective sorption) is:
MgO 0.1, FeO 68.8, NiO 0.6, CO2 19.8, H2O 10.9, total 100.2. The empirical formula calculated on the basis of two metal
atoms is (Fe2+
1.97Ni0.02Mg0.01)Σ2.00(CO3)0.93(OH)2.14·0.18H2O, ideally Fe2(CO3)(OH)2. Chukanovite is monoclinic P21/a, with a =
12.396(1) Å, b = 9.407(1) Å, c = 3.2152(3) Å, β = 97.78◦. The strongest lines of the X-ray powder pattern [d(Å), I, (hkl)] are:
6.14, 40, (200); 5.15, 60, (231); 3.73, 80, (310); 2.645, 100, (230); 2.361, 40, (510); 2.171, 40, (520). The structure of chukanovite was refined on synchrotron data by the Rietveld method up to Rp = 3.43 %, wRp = 4.51 %, RBragg = 2.48 %. Chukanovite is closely related to the minerals of the malachite-rosasite group. It was named in honour of Nikita V. Chukanov (b. 1953), Russian physicist and mineralogist. The holotype specimen is deposited in the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow
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