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  • Springer  (36)
  • Wiley  (5)
  • American Association for the Advancement of Science  (4)
  • 1995-1999  (27)
  • 1985-1989  (10)
  • 1965-1969  (8)
  • 11
    Electronic Resource
    Electronic Resource
    Theoretical Studies of the Electrostatic Effect on Carbocations Adsorbed over Zeolitic Sites (1997)
    Springer
    Molecular engineering 7 (1997), S. 429-438 
    Details
    ISSN: 1572-8951
    Keywords: Carbocation ; zeolite ; acidity ; electrostatic field
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Theoretical studies, at MNDO level, of carbocations adsorbed over a zeolite active site were carried out, taking into account the effect of electrostatic fields. Clusters 1 and 2 were placed between two electrostatic charges of opposite sign. The distance from each charge to the center of the oxygen atom bearing the proton or alkyl group was varied in order to find the minimum in energy. The ionicity, expressed by the decrease in bond order, slightly increased with the inclusion of the charges. A more pronounced effect was observed for a positive charge of +2. The energy of the system decreased with the introduction of electrostatic charges. This fact may be partly explained by the electrostatic interaction between the opposed charges and also to an observed increase in the Si–-O–-Al bond angle, decreasing the steric repulsions between the substituents and the acid site. The bond orders remained high, indicating a predominant covalent character for the adsorption of the carbocations. The calculations are in agreement with experimental data of exchange of polyvalent cations, which pointed to an increase in the acid strength of the zeolite, due to decrease in the electron density in the aluminum atom.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1008224428489
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  • 12
    Electronic Resource
    Electronic Resource
    Mechanism of superacid catalyzed alkane activation: theoretical ab initio studies of pentacoordinated carbonium ion rearrangement (1998)
    Springer
    Topics in catalysis 6 (1998), S. 163-168 
    Details
    ISSN: 1572-9028
    Keywords: carbonium ion ; superacid ; rearrangement ; alkane activation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract A theoretical ab initio study of the interconversion of pentacoordinated carbonium ions was carried out. For the isobutonium cations it was found that the respective C-carbonium ions were lower in energy than the H-isobutonium ions. Nevertheless, the interconversion of the 1-H-isobutonium cation in the C-isobutonium ion is a barrierless process. This suggests that product arisen from C–C protonation in liquid superacid and zeolite catalyzed alkane activation may be formed by protonation in the outer and more accessible primary C–H bonds of isobutane, rather than by direct protonation of the inner and more steric demanding C–C bonds.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1019166423058
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  • 13
    Electronic Resource
    Electronic Resource
    Compared chemical properties of dermonecrotic and lethal toxins from spiders of the genusLoxosceles (Araneae) (1996)
    Springer
    The protein journal 15 (1996), S. 337-343 
    Details
    ISSN: 1573-4943
    Keywords: Loxosceles ; spider ; dermonecrotic toxin ; lethal toxin ; amino acid sequence
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Loxosceles spider venom usually causes a typical dermonecrotic lesion in bitten patients, but it may also cause systemic effects that may be lethal. Gel filtration on Sephadex G-100 ofLoxosceles gaucho, L. laeta, orL. intermedia spider venoms resulted in three fractions (A, containing higher molecular mass components, B containing intermediate molecular mass components, and C with lower molecular mass components). The dermonecrotic and lethal activities were detected exclusively in fraction A of all three species. Analysis by SDS-PAGE showed that the major protein contained in fraction A has molecular weight approximately 35 kDa inL. gaucho andL. intermedia, but 32 kDa inL. laeta venom. These toxins were isolated from venoms ofL. gaucho, L. laeta, andL. intermedia by SDS-PAGE followed by blotting to PVDF membrane and sequencing. A database search showed a high level of identity between each toxin and a fragment of theL. reclusa (North American spider) toxin. A multiple sequence alignment of theLoxosceles toxins showed many common identical residues in their N-terminal sequences. Identities ranged from 50.0% (L. gaucho andL. reclusa) to 61.1% (L. intermedia andL. reclusa). The purified toxins were also submitted to capillary electrophoresis peptide mapping afterin situ partial hydrolysis of the blotted samples. The results obtained suggest thatL. intermedia protein is more similar toL. laeta toxin thanL. gaucho toxin and revealed a smaller homology betweenL. intermedia andL gaucho. Altogether these findings suggest that the toxins responsible for most important activities of venoms ofLoxosceles species have a molecular mass of 32–35 kDa and are probably homologous proteins.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01886859
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  • 14
    Electronic Resource
    Electronic Resource
    Thermal diffusivity measurements in C2H2-plasma polymerized films using photoacoustic techniques (1996)
    Springer
    Journal of materials science 15 (1996), S. 590-593 
    Details
    ISSN: 1573-4811
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00579260
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  • 15
    Electronic Resource
    Electronic Resource
    Genetic diversity and relationships of cacao (Theobroma cacao L.) in southern Mexico (1998)
    Springer
    Theoretical and applied genetics 96 (1998), S. 621-627 
    Details
    ISSN: 1432-2242
    Keywords: Key words Cacao ; Theobroma cacao ; Genetic diversity ; Crop evolution ; RAPD
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract  Neotropical tree crops are affected by a combination of biological and human factors that complicate the study of genetic diversity and crop evolution. Genetic diversity and relationships among southern Mexican populations and horticultural collections of Theobroma cacao (chocolate, cocoa, cacao) are examined in light of the agricultural practices of the Maya. Collections of cacao were obtained from the extremes of its geographic range including archeological sites in southern Mexico where cacao was first domesticated. Genetic diversity was assayed by 57 informative random amplified polymorphic DNA (RAPD) marker loci. A unique sample of the total diversity found in this study exists in the southern Mexican populations. These populations are significantly different from all other cacao with regards to their profile of RAPD bands, including the ‘criollo’ variety, their morphological and geographical group. A population of cacao found in a sinkhole (cenote) in northern Yucatan with genetic affinities to populations in Chiapas suggests the Maya maintained plants far away from their native habitat. This finding concurs with known agroforestry practices of the Maya. Modern efforts to increase germplasm of tropical tree crops such as cacao should carefully examine archeological sites where genetic diversity, either deliberately or by chance, was collected and maintained by ancient cultures.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s001220050780
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  • 16
    Electronic Resource
    Electronic Resource
    Disruption of the nuclear gene encoding the 20.8-kDa subunit of NADH:ubiquinone reductase ofNeurospora mitochondria (1996)
    Springer
    Molecular genetics and genomics 252 (1996), S. 177-183 
    Details
    ISSN: 1617-4623
    Keywords: Neurospora crassa ; Mitochondria ; Complex I ; Assembly ; Gene disruption
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract The nuclear gene coding for the 20.8-kDa subunit of the membrane arm of respiratory chain NADH:ubiquinone reductase (Complex I) fromNeurospora crassa, nuo-20.8, was localized on linkage group I of the fungal genome. A genomic DNA fragment containing this gene was cloned and a duplication was created in a strain ofN. crassa by transformation. To generate RIP (repeat-induced point) mutations in the duplicated sequence, the transformant was crossed with another strain carrying an auxotrophic marker on chromosome I. To increase the chance of finding an isolate with a non-functionalnuo-20.8 gene, random progeny from the cross were selected against this auxotrophy since RIP of the target gene will only occur in the nucleus carrying the duplication. Among these, we isolated and characterised a mutant strain that lacks the 20.8 kDa mitochondrial protein, indicating that this cysteine-rich polypeptide is not essential. Nevertheless, the absence of the 20.8-kDa subunit prevents the full assembly of complex I. It appears that the peripheral arm and two intermediates of the membrane arm of the enzyme are still formed in the mutant mitochondria. The NADH:ubiquinone reductase activity of sonicated mitochondria from the mutant is rotenone insensitive. Electron microscopy of mutant mitochondria does not reveal any alteration in the structure or numbers of the organelles.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02173218
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  • 17
    Electronic Resource
    Electronic Resource
    Disruption of the nuclear gene encoding the 20.8-kDa subunit of NADH:ubiquinone reductase of Neurospora mitochondria (1996)
    Springer
    Molecular genetics and genomics 252 (1996), S. 177-183 
    Details
    ISSN: 1617-4623
    Keywords: Key wordsNeurospora crassa ; Mitochondria ; Complex I ; Assembly ; Gene disruption
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract The nuclear gene coding for the 20.8k-kDa subunit of the membrane arm of respiratory chain NADH:ubiquinone reductase (Complex I) from Neurospora crassa, nuo-20.8 was localized on linkage group I of the fungal genome. A genomic DNA fragment containing this gene was cloned and a duplication was created in a strain of N. crassa by transformation. To generate RIP (repeat-induced point) mutations in the duplicated sequence, the transformant was crossed with another strain carrying an auxotrophic marker on chromosome I. To increase the chance of finding an isolate with a non-functional nuo-20.8 gene, random progeny from the cross were selected against this auxotrophy since RIP of the target gene will only occur in the nucleus carrying the duplication. Among these, we isolated and characterised a mutant strain that lacks the 20.8 kDa mitochondrial protein, indicating that this cysteine-rich polypeptide is not essential. Nevertheless, the absence of the 20.8-kDa subunit prevents the full assembly of complex I. It appears that the peripheral arm and two intermediates of the membrane arm of the enzyme are still formed in the mutant mitochondria. The NADH:ubiquinone reductase activity of sonicated mitochondria from the mutant is rotenone insensitive. Electron microscopy of mutant mitochondria does not reveal any alteration in the structure or numbers of the organelles.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s004389670020
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  • 18
    Electronic Resource
    Electronic Resource
    Mixed elements in the optimal design of plates (1989)
    Springer
    Structural and multidisciplinary optimization 1 (1989), S. 127-136 
    Details
    ISSN: 1615-1488
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract The theory of design sensitivity analysis of structures, based on mixed finite element models, is developed for static, dynamic and stability constraints. The theory is applied to the optimal design of plates with minimum weight, subject to displacement, stress, natural frequencies and buckling stresses constraints. The finite element model is based on an eight node mixed isoparametric quadratic plate element, whose degrees of freedom are the transversal displacement and three moments per node. The corresponding nonlinear programming problem is solved using the commercially available ADS (Automated Design Synthesis) program. The sensitivities are calculated by analytical, semi-analytical and finite difference techniques. The advantages and disadvantages of mixed elements in design optimization of plates are discussed with reference to applications.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01637670
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  • 19
    Electronic Resource
    Electronic Resource
    Inbreeding as measured by dispensations and isonymy in Rio de Onor, Portugal (1986)
    Springer
    International journal of anthropology 1 (1986), S. 225-228 
    Details
    ISSN: 1824-3096
    Keywords: inbreeding ; isonymy ; Rio de Onor ; Portugal
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract This study examines the coefficient of inbreeding in the population of Rio de Onor (Portugal), for a period of 120 years. Two distinct methods were used: the dispensations for consaguineous marriages and isonymy. The two methods produced different results and these differences are explained, for the period after 1960, in terms of emigration.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02442036
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  • 20
    Electronic Resource
    Electronic Resource
    Agricultural development vs biodiversity conservation: the Mediterranean semiarid vegetation in El Ejido (Almería, southeastern Spain) (1996)
    Springer
    Biodiversity and conservation 5 (1996), S. 1597-1617 
    Details
    ISSN: 1572-9710
    Keywords: environmental impact ; Maytenus europaeus ; dunes vegetation ; birds diversity ; restoration of degraded areas
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: The development of a greenhouse agriculture in the traditionally impoverished region of Poniente de Almería, on the Mediterranean coast of Spain, has caused an enormous rise in living standards. However, the environmental impact of this badly-planned growth threatens the every dynamics of the exploitation system. A special examination must be made of the use of the three major resources responsible for the functioning of greenhouse production and its impact on the ecosystems and particularly on the vegetation. These resources are: clayey soils, sand from fossil dunes and ground water. While the use of the clayey soils and sand have negative effects on the conservation of ecologically valuable communities found nowhere else in Europe, ground water overexploitation has produced an increase in salinity in most of the aquifers. Of these, sand has been by far the best monitored resource and restoration programmes have been implemented in the extraction zones. This survey deals with the recent evolution of areas where the arto (Maytenus senegalensis subsp. europaeus) and the sabina (Juniperus phoenicea subsp. turbinata) have long been the dominant species, although the presence of the former is nowadays notoriously diminished. The study is based on aerial photographs taken in 1957, 1977 and 1985, together with our own field work. Curiously enough, all this man-made process of degradation has stimulated ornithological diversity. Finally, we propose here some measures which aim to preserve the most important enclaves of these Mediterranean shrub formations, specially those of the arto, since sabina-dominated communities already belong to existing conservation areas.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00052118
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