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  • Springer  (28)
  • American Institute of Physics  (4)
  • Nature Publishing Group  (1)
  • 1995-1999  (27)
  • 1965-1969  (6)
  • 11
    Electronic Resource
    Electronic Resource
    Springer
    Molecular engineering 7 (1997), S. 429-438 
    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.
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  • 12
    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.
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  • 13
    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.
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  • 14
    Electronic Resource
    Electronic Resource
    Springer
    Journal of materials science 15 (1996), S. 590-593 
    ISSN: 1573-4811
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
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  • 15
    Electronic Resource
    Electronic Resource
    Springer
    Theoretical and applied genetics 96 (1998), S. 621-627 
    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.
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  • 16
    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.
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  • 17
    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.
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  • 18
    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.
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  • 19
    Electronic Resource
    Electronic Resource
    Springer
    Journal of low temperature physics 117 (1999), S. 1477-1481 
    ISSN: 1573-7357
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We have found a sharp transition at T c2 = 350 mK in the vortex creep rate of a single crystal of (U 1−x Th x )Be 13 with T c = 523 mK (x = 0.0275). For T ≪ T c2 , no creep of vortices is observed in a time scale of 105 s, while for T c2 〈 T 〈 T c , vortices creep at very high rates (30% of decay from a metastable configuration in the first 105 s at T = 400 mK). The sharp transition occurs at the same temperature at which the second jump in the specific heat appears in these samples. Similar low levels of creep rates have been reported by us in the low–T superconducting phase of UPt 3.1
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  • 20
    ISSN: 1617-4623
    Keywords: Key words Mitochondria ; Complex I ; Gene disruption ; Mutants ; Neurospora crassa
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract We have isolated and characterised the nuclear gene that codes for the 30.4-kDa subunit of the peripheral arm of complex I from Neurospora crassa. The single-copy gene was localised on chromosome VI of the fungal genome by restriction fragment length polymorphism mapping. An extra copy of the gene was introduced into a strain of N. crassa by transformation. This strain was crossed with another strain in order to inactivate, by repeat-induced point mutations, both copies of the duplication carried by the parental transformant. Ascospore progeny from the cross were analysed and a mutant strain lacking the 30.4-kDa protein, nuo30.4, was isolated and further characterised. The mutant appears to assemble the membrane arm of complex I, while formation of the peripheral arm is prevented. Nevertheless, the mutant grows reasonably well – indicating that this well conserved protein is not essential for vegetative growth – and is able to mate with other strains both as male or female. Strains with multiple mutations are readily obtained from heterozygous crosses between different complex I mutants of N. crassa. On the other hand, homozygous crosses between several mutants, including nuo30.4, fail to produce ascospores. These results suggest that complex I plays an essential role during the sexual phase of the life cycle of the fungus.
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