The link between partial melting, granitization and granulite development in central Ribeira Fold Belt, SE Brazil: New evidence from elemental and Sr-Nd isotopic geochemistry

Elemental and Sr-Nd isotopic data on metatexites, diatexites, orthogneisses and charnockites from the central Ribeira Fold Belt indicate that they are LILE-enriched weakly peraluminous granodiorites. Harker and Th-Hf-La correlation trends suggest that these rocks represent a co-genetic sequence, whereas variations on CaO, MnO, Y and HREE for charnockites can be explained by garnet consumption during granulitic metamorphism. Similar REE patterns and isotopic results of epsilon(565)(Nd) = -5.4 to -7.3 and (87)Sr/(86)Sr(565) = 0.706-0.711 for metatexites, diatexites, orthogneisses and charnockites, as well as similar T(DM) ages between 2.0 and 1.5 Ga are consistent with evolution from a relatively homogeneous and enriched common crustal (metasedimentary) protolith. Results suggest a genetic link between metatexites, diatexites, orthogneisses and charnockites and a two-step process for charnockite development: (a) generation of the hydrated igneous protoliths by anatexis of metasedimentary rocks; (b) continuous high-grade metamorphism that transformed the ""S-type granitoids"" (leucosomes and diatexites) into orthogneisses and, as metamorphism and dehydration progressed, into charnockites. (C) 2011 Elsevier Ltd. All rights reserved.

Thermochronology of central Ribeira Fold Belt, SE Brazil: Petrological and geochronological evidence for long-term high temperature maintenance during Western Gondwana amalgamation

The studied sector of the central Ribeira Fold Belt (SE Brazil) comprises metatexites, diatexites, charnockites and blastomylonites. This study integrates petrological and thermochronological data in order to constrain the thermotectonic and geodynamic evolution of this Neoproterozoic-Ordovician mobile belt during Western Gondwana amalgamation. New data indicate that after an earlier collision stage at similar to 610 Ma (zircon, U-Pb age), peak metamorphism and lower crust partial melting, coeval with the main regional high grade D(1) thrust deformation, occurred at 572-562 Ma (zircon, U-Pb ages). The overall average cooling rate was low (<5 degrees C/Ma) from 750 to 250 degrees C (at similar to 455 Ma; biotite-WR Rb-Sr age), but disparate cooling paths indicate differential uplift between distinct lithotypes: (a) metatexites and blastomylonites show a overall stable 3-5 degrees C/Ma cooling rate; (b) charnockites and associated rocks remained at T>650 degrees C during sub-horizontal D(2) shearing until similar to 510-470 Ma (garnet-WR Sm-Nd ages) (1-2 degrees C/Ma), being then rapidly exhumed/cooled (8-30 degrees C/Ma) during post-orogenic D(3) deformation with late granite emplacement at similar to 490 Ma (zircon, U-Pb age). Cooling rates based on garnet-biotite Fe-Mg diffusion are broadly consistent with the geochronological cooling rates: (a) metatexites were cooled faster at high temperatures (6 degrees C/Ma) and slowly at low temperatures (0.1 degrees C/Ma), decreasing cooling rates with time; (b) charnockites show low cooling rates (2 degrees C/Ma) near metamorphic peak conditions and high cooling rates (120 degrees C/Ma) at lower temperatures, increasing cooling rates during retrogression. The charnockite thermal evolution and the extensive production of granitoid melts in the area imply that high geothermal gradients were sustained fora long period of time (50-90 Ma). This thermal anomaly most likely reflects upwelling of asthenospheric mantle and magma underplating coupled with long-term generation of high HPE (heat producing elements) granitoids. These factors must have sustained elevated crustal geotherms for similar to 100 Ma, promoting widespread charnockite generation at middle to lower crustal levels. (C) 2010 Elsevier B.V. All rights reserved.

The Guarguaraz Complex and the Neoproterozoic-Cambrian evolution of southwestern Gondwana: Geochemical signatures and geochronological constraints

The Guarguaraz Complex, in western Argentina, comprises a metasedimentary assemblage, associated with mafic sills and ultramafic bodies intruded by basaltic dikes, which are interpreted as Ordovician dismembered ophiolites. Two kinds of dikes are recognized, a group associated with the metasediments and the other ophiolite-related. Both have N-MORB signatures, with epsilon(Nd) between +3.5 and +8.2, indicating a depleted source, and Grenville model ages between 0.99 and 1.62 Ga. A whole-rock Sm-Nd isochron yielded an age of 655 +/- 76 Ma for these mafic rocks, which is compatible with cianobacteria and acritarchae recognized in the clastic metasedimentary platform sequences, that indicate a Neoproterozoic (Vendian)-Cambrian age of deposition. The Guarguaraz metasedimentary-ophiolitic complex represents, therefore, a remnant of an oceanic basin developed to the west of the Grenville-aged Cuyania terrane during the Neoproterozoic. The southernmost extension of these metasedimentary sequences in Cordon del Portillo might represent part of this platform and not fragments of the Chilenia terrane. An extensional event related to the fragmentation of Rodinia is represented by the mafic and ultramafic rocks. The Devonian docking of Chilenia emplaced remnants of ocean floor and slices of the Cuyania terrane (Las Yaretas Gneisses) in tectonic contact with the Neoproterozoic metasediments, marking the Devonian western border of Gondwana. (C) 2009 Elsevier Ltd. All rights reserved.

Metamorphic P-T evolution of granulites in the central Ribeira Fold Belt, SE Brazil

Pseudosections, geothermobarometric estimates and careful petrographic observations of gneissic migmatites and granulites from Neoproterozoic central Ribeira Fold Belt (SE Brazil) were performed in order to quantify the metamorphic P-T conditions during prograde and retrograde evolution of the Brasiliano Orogeny. Results establish a prograde metamorphic trajectory from amphibolite facies conditions to metamorphic peak (T = 850 +/- 50 A degrees C; P = 8 +/- 1 kbar) that promoted widespread dehydrationmelting of 30 to 40% of the gneisses and high-grade granitization. After the metamorphic peak, migmatites evolved with cooling and decompression to T a parts per thousand 500 A degrees C and P a parts per thousand 5 kbar coupled with aH2O increase, replacing the high-grade paragenesis plagioclase-quartz-K-feldspar-garnet by quartz-biotite-sillimanite-(muscovite). Cordierite absence, microtextural observations and P-T results constrain the migmatite metamorphic evolution in the pseudosections as a clockwise P-T path with retrograde cooling and decompression. High-temperature conditions further dehydrated the lower crust with biotite and amphibole-dehydration melting and granulite formation coupled with 10% melt generation. Granulites can thus be envisaged as middle to lower crust dehydrated restites. Granulites were slowly (nearly isobarically) cooled, followed by late exhumation/retrograde rapid decompression and cooling, reflecting a two step P-T path. This retrograde evolution, coupled with water influx, chemically reequilibrated the rocks from granulite to amphibolite/greenschist facies, promoting the replacement of the plagioclase-quartz-garnet-hypersthene peak assemblage by quartz-biotite- K-feldspar symplectites.

P-T-Fluid evolution and graphite deposition during retrograde metamorphism in Ribeira Fold Belt, SE Brazil: Oxygen fugacity, fluid inclusions and C-O-H isotopic evidence

Combined fluid inclusion (FI) microthermometry, Raman spectroscopy, X-ray diffraction, C-O-H isotopes and oxygen fugacities of granulites from central Ribeira Fold Belt, SE Brazil, provided the following results: i) Magnetite-Hematite fO(2) estimates range from 10(-11.5) bar (QFM + 1) to 10(-18.3) bar (QFM - 1) for the temperature range of 896 degrees C-656 degrees C, implying fO(2) decrease from metamorphic peak temperatures to retrograde conditions; ii) 5 main types of fluid inclusions were observed: a) CO(2) and CO(2)-N(2) (0-11 mol%) high to medium density (1.01-0.59 g/cm(3)) FI; b) CO(2) and CO(2)-N(2) (0-36 mol%) low density (0.19-0.29 g/cm(3)) FI; c) CO(2) (94-95 mol%)-N(2) (3 mol%)-CH(4) (2-3 mol%)-H(2)O (water phi(v) (25 degrees C) = 0.1) FI; d) low-salinity H(2)O-CO(2) FI; and e) late low-salinity H(2)O FI; iii) Raman analyses evidence two graphite types in khondalites: an early highly ordered graphite (T similar to 450 degrees C) overgrown by a disordered kind (T similar to 330 degrees C); iv) delta(18)O quartz results of 10.3-10.7 parts per thousand, imply high-temperature CO(2) delta(18)O values of 14.4-14.8 parts per thousand, suggesting the involvement of a metamorphic fluid, whereas lower temperature biotite delta(18)O and delta D results of 7.5-8.5 parts per thousand and -54 to -67 parts per thousand respectively imply H(2)O delta(18)O values of 10-11 parts per thousand and delta D(H2O) of -23 to -36 parts per thousand suggesting delta(18)O depletion and increasing fluid/rock ratio from metamorphic peak to retrograde conditions. Isotopic results are compatible with low-temperature H(2)O influx and fO(2) decrease that promoted graphite deposition in retrograde granulites, simultaneous with low density CO(2), CO(2)-N(2) and CO(2)-N(2)-CH(4)-H(2)O fluid inclusions at T = 450-330 degrees C. Graphite delta(13)C results of -10.9 to -11.4 parts per thousand imply CO(2) delta(13)C values of -0.8 to -1.3 parts per thousand suggesting decarbonation of Cambrian marine carbonates with small admixture of lighter biogenic or mantle derived fluids. Based on these results, it is suggested that metamorphic fluids from the central segment of Ribeira Fold Belt evolved to CO(2)-N(2) fluids during granulitic metamorphism at high fO(2), followed by rapid pressure drop at T similar to 400-450 degrees C during late exhumation that caused fO(2) reduction induced by temperature decrease and water influx, turning carbonic fluids into CO(2)-H(2)O (depleting biotite delta(18)O and delta D values), and progressively into H(2)O. When fO(2) decreased substantially by mixture of carbonic and aqueous fluids, graphite deposited forming khondalites. (C) 2010 Elsevier Ltd. All rights reserved.

Structural analysis of the Itapucumi Group in the Vallemi region, northern Paraguay: Evidence of a new Brasiliano/Pan-African mobile belt

The Neoproterozoic (Ediacaran) Itapucumi Group in northern Paraguay is composed of carbonate and siliciclastic rocks, including ooid grainstones, marls, shales and sandstones, containing Cloudina fossils in the eastern region. It is almost undeformed over the Rio Apa Cratonic Block but shows a strong deformational pattern at its western edge. A detailed structural analysis of the Itapucumi Group was conducted in the Vallemi Mine, along with a regional survey in other outcrops downstream in the Paraguay River and in the San Alfredo, Cerro Paiva and Sargent Jose E. Lopez regions. In the main Vallemi quarry, the structural style is characterized by an axial-plane slaty cleavage in open to isoclinal folds, sometimes overturned, associated with N-S trending thrust faults and shear zones of E-vergence and with a low-grade chlorite zone metamorphism. The structural data presented here are compatible with the hypothesis of a newly recognized mobile belt on the western side of the Rio Apa Cratonic Block, with opposite vergence to that of the Paraguay Mobile Belt in Brazil. Both belts are related to the Late Brasiliano/Pan-African tectonic cycle with a Lower Cambrian deformation and metamorphism age. The deformation could be due to the late collision of the Amazonian Craton with the remainder of Western Gondwana or to the western active plate boundary related to the Pampean Belt. The structural and lithologic differences between the western Itapucumi Group in the Vallemi and Paraguay River region and the eastern region, near San Alfredo and Cerro Paiva, suggest that this group could be divided into two lithostratigraphic units, but more stratigraphic and geochronological analyses are required to confirm this possibility. (C) 2010 Elsevier Ltd. All rights reserved.

Cys mutants in functional regions of Sticholysin I clarify the participation of these residues in pore formation

Experimental evidence shows that the mechanism of pore formation by actinoporins is a multistep process, involving binding of the water-soluble monomer to the membrane and subsequent oligomerization on the membrane surface, leading to the formation of a functional pore. However, as for other eukaryotic pore-forming toxins, the molecular details of the mechanism of membrane insertion and oligomerization are not clear. In order to obtain further insight with regard to the structure-function relationship in sticholysins, we designed and produced three cysteine mutants of recombinant sticholysin I (rStI) in relevant functional regions for membrane interaction: StI E2C and StI F15C (in the N-terminal region) and StI R52C (in the membrane binding site). The conformational characterization derived from fluorescence and CD spectroscopic studies of StI E2C, StI F15C and StI R52C suggests that replacement of these residues by Cys in rStI did not noticeably change the conformation of the protein. The substitution by Cys of Arg(52) in the phosphocholine-binding site, provoked noticeable changes in rStI permeabilizing activity; however, the substitutions in the N-terminal region (Glu(2), Phe(15)) did not modify the toxin`s permeabilizing ability. The presence of a dimerized population stabilized by a disulfide bond in the StI E2C mutant showed higher pore-forming activity than when the protein is in the monomeric state, suggesting that sticholysins pre-ensembled at the N-terminal region could facilitate pore formation. (C) 2011 Elsevier Ltd. All rights reserved.

Absolute configuration reassignment of two chromanes from Peperomia obtusifolia (Piperaceae) using VCD and DFT calculations

Previous analysis of the ECD spectra of two prenylated benzopyrans isolated from Peperomia obtusifolia, by means of the helicity rule for the chromane chromophore, resulted in the incorrect assignment of their absolute configuration, (5) instead of (R) for a deduced P-helicity of the chromane ring for the (+)-enantiomers. This was discovered by the application of DFT calculations and VCD spectroscopy. Experimental and calculated (B3LYP/6-31G(d)) VCD and IR spectra were compared, and a definitive absolute configuration of (+)-1 and (+)-2 is reassigned directly in solution as (R). The assumption of equatorial positioning of bulky groups, shown here to be invalid for the title molecules, is the underlying cause of the previous incorrect assignment of absolute configuration. Moreover, TDDFT (B3LYP/6-311++G(2d,2p)//B3LYP/6-31G(d)) calculations of ECD spectra have shown that both P- and M-helicity of the heterocyclic ring, for a given absolute configuration, lead to the same sign for the (1)L(b) ECD band, thus bringing into question the validity of the empirical ECD helicity rule for chromane molecules. (C) 2010 Elsevier Ltd. All rights reserved.

Solvatochromic Shift of the pi -> pi* Transition in All-Trans, Cis-13, Cis-11, Cis-9, and Cis-7 Retinal Isomers Induced by Water and Methanol

The solvatochromic shift of the lowest singlet it pi -> pi* electronic transition in the all-trans, cis-13, cis-11, cis-9, and cis-7 retinal isomers were computed under the influence of water, methanol, and benzene solvents. Excitation energies were calculated in gas phase and in solution. The calculations in solution were performed considering the sequential Monte Carlo (MC) /Quantum Mechanical approach. The MC simulations were performed considering the full retinal isomer molecules and 900 water molecules, 900 methanol, or 400 benzene ones. The OPLS/AA parametrization was chosen for retinal, methanol, and benzene molecules and the SPC model was used for water one. From the MC calculations 100 independent configurations were selected, with 100 solvent molecules in thermodynamical equilibrium at T = 298.15 K. Average point-charges were obtained from those independent configurations for water, methanol, and benzene solvent. TDDFT and CASSCF//CASPT2 methodologies were used to compute the vertical excitation energy of the retinal isomers in different environment. (C) 2010 Wiley Periodicals, Inc. Int J Quantum Chem 110: 2076-2087, 2010

Geranylation of benzoic acid derivatives by enzymatic extracts from Piper crassinervium (Piperaceae)

The ability to carry out geranylations on aromatic substrates using enzymatic extracts from the leaves of Piper crassinervium (Piperaceae) was evaluated. A literature analysis pointed out its importance as a source of prenylated bioactive molecules. The screening performed on aromatic acceptors (benzoic acids, phenols and phenylpropanoids) including geranyl diphosphate as prenyl donor, showed the biotransformation of the 3,4-dihydroxybenzoic acid by the crude extract, and the p-hydroxybenzoic acid by both the microsomal fraction and the crude extract, after treating leaves with glucose. The analysis of the products allowed the identification of C- and O-geranylated derivatives, and the protease (subtilisin and pepsin) inhibition performed on the O-geranylated compounds showed weak inhibition. Electrophoretic profiles indicated the presence of bands/spots among 56-58 kDa and pI 6-7, which are compatible with prenyltransferases. These findings show that P. crassinervium could be considered as a source of extracts with geranyltransferase activity to perform biotransformations on aromatic substrates. (C) 2010 Elsevier Ltd. All rights reserved.

Designing Novel Antitrypanosomal Agents from a Mixed Graph-Theoretical Substructural Approach

Chagas disease is nowadays the most serious parasitic health problem. This disease is caused by Trypanosoma cruzi. The great number of deaths and the insufficient effectiveness of drugs against this parasite have alarmed the scientific community worldwide. In an attempt to overcome this problem, a model for the design and prediction of new antitrypanosomal agents was obtained. This used a mixed approach, containing simple descriptors based on fragments and topological substructural molecular design descriptors. A data set was made up of 188 compounds, 99 of them characterized an antitrypanosomal activity and 88 compounds that belong to other pharmaceutical categories. The model showed sensitivity, specificity and accuracy values above 85%. Quantitative fragmental contributions were also calculated. Then, and to confirm the quality of the model, 15 structures of molecules tested as antitrypanosomal compounds (that we did not include in this study) were predicted, taking into account the information on the abovementioned calculated fragmental contributions. The model showed an accuracy of 100% which means that the ""in silico"" methodology developed by our team is promising for the rational design of new antitrypanosomal drugs. (C) 2009 Wiley Periodicals, Inc. J Comput Chem 31: 882-894. 2010

Design of novel antituberculosis compounds using graph-theoretical and substructural approaches

The increasing resistance of Mycobacterium tuberculosis to the existing drugs has alarmed the worldwide scientific community. In an attempt to overcome this problem, two models for the design and prediction of new antituberculosis agents were obtained. The first used a mixed approach, containing descriptors based on fragments and the topological substructural molecular design approach (TOPS-MODE) descriptors. The other model used a combination of two-dimensional (2D) and three-dimensional (3D) descriptors. A data set of 167 compounds with great structural variability, 72 of them antituberculosis agents and 95 compounds belonging to other pharmaceutical categories, was analyzed. The first model showed sensitivity, specificity, and accuracy values above 80% and the second one showed values higher than 75% for these statistical indices. Subsequently, 12 structures of imidazoles not included in this study were designed, taking into account the two models. In both cases accuracy was 100%, showing that the methodology in silico developed by us is promising for the rational design of antituberculosis drugs.

Resolution and Absolute Configuration Assignment of a Natural Racemic Chromane from Peperomia obtusifolia (Piperaceae)

The resolution of the natural racemic chromane 3,4-dihydro-5-hydroxy-2,7-dimethyl-8-(3 ``-methyl-2 ``-butenyl)-2-(4`-methyl-1`,3`-pentadienyl)-2H-1-benzopyran-6-carboxylic acid (1) isolated from the leaves of Peperomia obtusifolia has been accomplished using stereoselective HPLC. The absolute coil figuration of the resolved enantiomers was determined by the analysis of optical rotations and CD spectra. The finding of a racemic mixture instead of an enantiomerically pure metabolite raises questions about the final steps in the biosynthesis of this class of natural products, suggesting that the intramolecular chromane ring formation step may not be enzymatically controlled at all in P. obtusifolia. Chirality 21:799-801, 2009. (C) 2008 Wiley-Liss, Inc.

In vitro Trypanocidal Activity of Phenolic Derivatives from Peperomia obtusifolia

The trypanocidal activity of crude extracts and fractions from the leaves and stems of Peperomia obtusifolia (Piperaceae) was evaluated in vitro against the epimastigote forms of Trypanosoma cruzi. Bioactivity-guided fractionation of the most active extracts afforded seven known compounds, including three chromanes, two furofuran lignans and two flavone C-diglycosides. The most active compounds were the chromanes peperobtusin A and 3,4-dihydro-5-hydroxy-2,7-dimethyl-8-(2 ``-methyl-2 ``-butenyl)-2-(4`-methyl-1`,3`-pentadienyl)-2H-1-benzopyran-6-carboxylic acid, with IC(50) values of 3.1 mu M (almost three times more active than the positive control benznidazole, IC(50) 10.4 mu M) and 27.0 mu M, respectively. Cytotoxicity assays using peritoneal murine macrophages indicated that the chromanes were not toxic at the level of the IC(50) for trypanocidal activity. This is the first report on the trypanocidal activity besides unspecific cytotoxicity of chromanes from Peperomia species. Additionally it represents the first time isolation of 3,4-dihydro5-hydroxy-2,7-dimethyl-8-(2 ``-methyl-2 ``-butenyl)-2-(4`-methyl-1`,3`-pentadienyl)-2H-1-benzopyran-6-carboxylic acid from P. obtusifolia.

In vitro activity of compounds isolated from Piper crassinervium against Trypanosoma cruzi

This study describes the antichagasic potential of five compounds isolated from leaves of Piper crassinervium (Piperaceae). Two prenylated benzoic acid derivatives, one prenylated hydroquinone and two flavanones, were evaluated. The in vitro trypanocidal activity was determined against epimastigote forms of Trypanosoma cruzi (Y strain), the etiologic agent of Chagas disease. The most active compound was the prenylated hydroquinone [1,4-dihydroxy-2-(3(0),7(0)-dimethyl-1(0)-oxo-2(0)-E,6(0)-octadienyl)benzene] with an IC(50) value of 6.10 g mL(-1), which was in the same order of activity if compared with the positive control benznidazole (IC(50) = 1.60 mu g mL(-1)). This is the first report of trypanocidal activity for prenylated hydroquinone and benzoic acid derivatives.