The leishmanicidal flavonols quercetin and quercitrin target Leishmania (Leishmania) amazonensis arginase

Polyamine biosynthesis enzymes are promising drug targets for the treatment of leishmaniasis, Chagas' disease and African sleeping sickness. Arginase, which is a metallohydrolase, is the first enzyme involved in polyamine biosynthesis and converts arginine into ornithine and urea. Ornithine is used in the polyamine pathway that is essential for cell proliferation and ROS detoxification by trypanothione. The flavonols quercetin and quercitrin have been described as antitrypanosomal and antileishmanial compounds, and their ability to inhibit arginase was tested in this work. We characterized the inhibition of recombinant arginase from Leishmania (Leishmania) amazonensis by quercetin, quercitrin and isoquercitrin. The IC50 values for quercetin, quercitrin and isoquercitrin were estimated to be 3.8, 10 and 4.3 mu M, respectively. Quercetin is a mixed inhibitor, whereas quercitrin and isoquercitrin are uncompetitive inhibitors of L. (L.) amazonensis arginase. Quercetin interacts with the substrate L-arginine and the cofactor Mn2+ at pH 9.6, whereas quercitrin and isoquercitrin do not interact with the enzyme's cofactor or substrate. Docking analysis of these flavonols suggests that the cathecol group of the three compounds interact with Asp129, which is involved in metal bridge formation for the cofactors Mn-A(2+) and Mn-B(2+) in the active site of arginase. These results help to elucidate the mechanism of action of leishmanicidal flavonols and offer new perspectives for drug design against Leishmania infection based on interactions between arginase and flavones. (C) 2012 Elsevier Inc. All rights reserved.

Application of magnetically induced hyperthermia in the model protozoan Crithidia fasciculata as a potential therapy against parasitic infections

Background: Magnetic hyperthermia is currently a clinical therapy approved in the European Union for treatment of tumor cells, and uses magnetic nanoparticles (MNPs) under time-varying magnetic fields (TVMFs). The same basic principle seems promising against trypanosomatids causing Chagas disease and sleeping sickness, given that the therapeutic drugs available have severe side effects and that there are drug-resistant strains. However, no applications of this strategy against protozoan-induced diseases have been reported so far. In the present study, Crithidia fasciculata, a widely used model for therapeutic strategies against pathogenic trypanosomatids, was targeted with Fe3O4 MNPs in order to provoke cell death remotely using TVMFs. Methods: Iron oxide MNPs with average diameters of approximately 30 nm were synthesized by precipitation of FeSO4 in basic medium. The MNPs were added to C. fasciculata choanomastigotes in the exponential phase and incubated overnight, removing excess MNPs using a DEAE-cellulose resin column. The amount of MNPs uploaded per cell was determined by magnetic measurement. The cells bearing MNPs were submitted to TVMFs using a homemade AC field applicator (f = 249 kHz, H = 13 kA/m), and the temperature variation during the experiments was measured. Scanning electron microscopy was used to assess morphological changes after the TVMF experiments. Cell viability was analyzed using an MTT colorimetric assay and flow cytometry. Results: MNPs were incorporated into the cells, with no noticeable cytotoxicity. When a TVMF was applied to cells bearing MNPs, massive cell death was induced via a nonapoptotic mechanism. No effects were observed by applying TVMF to control cells not loaded with MNPs. No macroscopic rise in temperature was observed in the extracellular medium during the experiments. Conclusion: As a proof of principle, these data indicate that intracellular hyperthermia is a suitable technology to induce death of protozoan parasites bearing MNPs. These findings expand the possibilities for new therapeutic strategies combating parasitic infection.

Surgical management of dentigerous cyst and keratocystic odontogenic tumor in children: a conservative approach and 7-year follow-up

Dentigerous cyst (DC) is one of the most common odontogenic cysts of the jaws and rarely recurs. On the other hand, keratocystic odontogenic tumor (KCOT), formerly known as odontogenic keratocyst (OKC), is considered a benign unicystic or multicystic intraosseous neoplasm and one of the most aggressive odontogenic lesions presenting relatively high recurrence rate and a tendency to invade adjacent tissue. Two cases of these odontogenic lesions occurring in children are presented. They were very similar in clinical and radiographic characteristics, and both were treated by marsupialization. The treatment was chosen in order to preserve the associated permanent teeth with complementary orthodontic treatment to direct eruption of the associated permanent teeth. At 7-years of follow-up, none of the cases showed recurrence.

Surgical treatment of globe subluxation in the active phase of the myogenic type of Graves orbitopathy: case reports

The purpose of the present article is to present and discuss two cases of globe subluxation in the active phase of myogenic Graves' orbitopathy and to evaluate the prevalence of this phenomenon. Two patients with the myogenic variant of Graves' orbitopathy that had being treated with oral and intravenous steroid pulses developed globe subluxation. Both had to have urgent eyelid and orbital decompression. After these observations, we reviewed the medical records of a sample of 284 patients (482 orbits) who had had orbital decompression at our Institution from 1992 to 2010, with a search for cases presenting severe proptosis or globe subluxation in the active phase of myogenic Graves' orbitopathy. No patient had to have decompression for globe subluxation in the active phase of Graves' orbitopathy. The prevalence of this event as an indication for surgery in the myogenic variant of Graves' orbitopathy was therefore 0.7% (2/284) or even less. The combination of lowering the upper eyelid and orbital decompression had a dramatic therapeutic effect on these patients despite the presence of intense inflammatory signs in the orbits. In conclusion, patients affected with the myogenic variant of Graves' orbitopathy may develop globe subluxation. Urgent surgical treatments should not be postponed despite the presence of active inflammation.

Antidepressants prevent hierarchy destabilization induced by lipopolysaccharide administration in mice: a neurobiological approach to depression

In spite of the high prevalence and negative impact of depression, little is known about its pathophysiology. Basic research on depression needs new animal models in order to increase knowledge of the disease and search for new therapies. The work presented here aims to provide a neurobiologically validated model for investigating the relationships among sickness behavior, antidepressants treatment, and social dominance behavior. For this purpose, dominant individuals from dyads of male Swiss mice were treated with the bacterial endotoxin lipopolysaccharide (LPS) to induce social hierarchy destabilization. Two groups were treated with the antidepressants imipramine and fluoxetine prior to LPS administration. In these groups, antidepressant treatment prevented the occurrence of social destabilization. These results indicate that this model could be useful in providing new insights into the understanding of the brain systems involved in depression.

Crystal structure of dihydroorotate dehydrogenase from Leishmania major

Dihydroorotate dehydrogenase (DHODH) is the fourth enzyme in the de novo pyrimidine biosynthetic pathway and has been exploited as the target for therapy against proliferative and parasitic diseases. In this study, we report the crystal structures of DHODH from Leishmania major, the species of Leishmania associated with zoonotic cutaneous leishmaniasis, in its apo form and in complex with orotate and fumarate molecules. Both orotate and fumarate were found to bind to the same active site and exploit similar interactions, consistent with a ping-pong mechanism described for class 1A DHODHs. Analysis of LmDHODH structures reveals that rearrangements in the conformation of the catalytic loop have direct influence on the dimeric interface. This is the first structural evidence of a relationship between the dimeric form and the catalytic mechanism. According to our analysis, the high sequence and structural similarity observed among trypanosomatid DHODH suggest that a single strategy of structure-based inhibitor design can be used to validate DHODH as a druggable target against multiple neglected tropical diseases such as Leishmaniasis, Sleeping sickness and Chagas' diseases. (C) 2012 Elsevier Masson SAS. All rights reserved.

Redundant nerve roots of the cauda equina: review of the literature

In imaging diagnosis, redundant nerve roots of the cauda equina are characterized by the presence of elongated, enlarged and tortuous nerve roots in close relationship with a high-grade lumbar spinal canal stenosis. This is not an independent entity, but it is believed to be a consequence of the chronic compression at the level of the lumbar canal stenosis and thus may be part of the natural history of lumbar spinal stenosis. The present paper is aimed at reviewing the histopathological, electrophysiological and imaging findings, particularly at magnetic resonance imaging, as well as the clinical meaning of this entity. As the current assessment of canal stenosis and root compression is preferably performed by means of magnetic resonance imaging, this is the imaging method by which the condition is identified. The recognition of redundant nerve roots at magnetic resonance imaging is important, particularly to avoid misdiagnosing other conditions such as intradural arteriovenous malformations. The literature approaching the clinical relevance of the presence of redundant nerve roots is controversial. There are articles suggesting that the pathological changes of the nerve roots are irreversible at the moment of diagnosis and therefore neurological symptoms are less likely to improve with surgical decompression, but such concept is not a consensus.

Fatores prognósticos associados ao tratamento cirúrgico da mielorradiculopatia espondilótica cervical

OBJETIVO: Identificar os fatores clínicos dos indivíduos, fatores sociais, ambientais e dos exames de imagem que se correlacionam ao resultado final de melhora neurológica em pacientes submetidos ao tratamento cirúrgico da mielopatia espondilótica cervical. MÉTODOS: A avaliação clínica foi quantificada pela escala deficitária da JOA. Analisamos 200 casos de mielorradiculopatia cervical, operados no HC-FMUSP, no período de janeiro de 1993 a janeiro de 2007. A média de segmento foi de 06 anos e 08 meses. A análise radiológica foi baseada nos critérios de instabilidade de White e scala de Kellgren. RESULTADOS: Em 80% houve melhora, 14% estabilização e em 6% piora do quadro neurológico. A piora neurológica não foi associada com nenhum fator clínico, ambiental ou de imagem. A melhora neurológica foi diretamente proporcional a menor idade na cirurgia, ausência de co-morbidade, sinal de Hoffman, atrofia muscular, hipersinal medular na RNM, menor período de evolução pré-operatório, melhor status neurológico pré-operatório e inversamente proporcional ao diâmetro AP do canal medular e multiplicidade de compressões. Identificou-se associação com o tabagismo. Mais de 70 anos, evolução superior a 24 meses, atrofia muscular, pontuação JOA igual ou inferior a sete pontos e diâmetro AP do canal inferior ou igual a seis mm não foram associado à melhora.

Incapacidade para o trabalho: análise dos benefícios auxílio-doença concedidos no estado de Rondônia

Trata-se de um estudo epidemiológico descritivo com objetivo de estimar a prevalência e a duração dos benefícios auxílio-doença, concedidos pelo Instituto Nacional de Seguro Social, bem como dos fatores associados à incapacidade para o trabalho entre os trabalhadores empregados em Rondônia em 2008. As prevalências foram apresentadas por 10.000 trabalhadores e estratificadas por sexo, idade, diagnóstico, atividade econômica, espécie e duração dos benefícios. Os homens apresentaram maiores prevalência e duração dos benefícios. Os benefícios previdenciários mostraramse 2,5 mais prevalentes do que os acidentários e ambos aumentaram com a idade. Os acidentários foram mais prevalentes na faixa etária de 50-59 anos. Silvicultura, construção civil e locação de mão de obra apresentaram as maiores prevalências. Os principais grupos diagnósticos foram lesões; doenças do sistema osteomuscular e doenças do aparelho digestivo. A duração dos benefícios auxílio-doença foi maior entre homens, com > 40 anos e na espécie previdenciária. A atividade econômica é um importante fator de risco para a incapacidade para o trabalho, e precisa ser mais bem explorada como tal visando melhorar a prevenção de doenças e diminuir o impacto na saúde dos trabalhadores decorrentes das condições ambientais e dos processos de trabalho.

Atypical human infections by animal Trypanosomes

The two classical forms of human trypanosomoses are sleeping sickness due to Trypanosoma brucei gambiense or T. brucei rhodesiense, and Chagas disease due to T. cruzi. However, a number of atypical human infections caused by other T. species (or sub-species) have been reported, namely due to T. brucei brucei, T. vivax, T. congolense, T. evansi, T. lewisi, and T. lewisi-like. These cases are reviewed here. Some infections were transient in nature, while others required treatments that were successful in most cases, although two cases were fatal. A recent case of infection due to T. evansi was related to a lack of apolipoprotein L-I, but T. lewisi infections were not related to immunosuppression or specific human genetic profiles. Out of 19 patients, eight were confirmed between 1974 and 2010, thanks to improved molecular techniques. However, the number of cases of atypical human trypanosomoses might be underestimated. Thus, improvement, evaluation of new diagnostic tests, and field investigations are required for detection and confirmation of these atypical cases.

Structure-based drug design studies on a series of aldolase inhibitors

Human African trypanosomiasis, also known as sleeping sickness, is a major cause of death in Africa, and for which there are no safe and effective treatments available. The enzyme aldolase from Trypanosoma brucei is an attractive, validated target for drug development. A series of alkyl‑glycolamido and alkyl-monoglycolate derivatives was studied employing a combination of drug design approaches. Three-dimensional quantitative structure-activity relationships (3D QSAR) models were generated using the comparative molecular field analysis (CoMFA). Significant results were obtained for the best QSAR model (r2 = 0.95, non-cross-validated correlation coefficient, and q2 = 0.80, cross-validated correlation coefficient), indicating its predictive ability for untested compounds. The model was then used to predict values of the dependent variables (pKi) of an external test set,the predicted values were in good agreement with the experimental results. The integration of 3D QSAR, molecular docking and molecular dynamics simulations provided further insight into the structural basis for selective inhibition of the target enzyme.

Inhibitors of Trypanosoma brucei trypanothione reductase: comparative molecular field analysis modeling and structural basis for selective inhibition

Background: Sleeping sickness is a major cause of death in Africa. Since no secure treatment is available, the development of novel therapeutic agents is urgent. In this context, the enzyme trypanothione reductase (TR) is a prominent molecular target that has been investigated in drug design for sleeping sickness. Results: In this study, comparative molecular field analysis models were generated for a series of Trypanosoma brucei TR inhibitors. Statistically significant results were obtained and the models were applied to predict the activity of external test sets, with good correlation between predicted and experimental results. We have also investigated the structural requirements for the selective inhibition of the parasite's enzyme over the human glutathione reductase. Conclusion: The quantitative structure-activity relationship models provided valuable information regarding the essential molecular requirements for the inhibitory activity upon the target protein, providing important insights into the design of more potent and selective TR inhibitors.

The Emergence of the Post-Socialist Welfare State - The Case of the Baltic States : Estonia, Latvia and Lithuania

This dissertation takes a step towards providing a better understanding of post-socialist welfare state development from a theoretical as well as an empirical perspective. The overall analytical goal of this thesis has been to critically assess the development of social policies in Estonia, Latvia and Lithuania using them as illustrative examples of post-socialist welfare state development in the light of the theories, approaches and typologies that have been developed to study affluent capitalist democracies. The four studies included in this dissertation aspire to a common aim in a number of specific ways. The first study tries to place the ideal-typical welfare state models of the Baltic States within the well-known welfare state typologies. At the same time, it provides a rich overview of the main social security institutions in the three countries by comparing them with each other and with the previous structures of the Soviet period. It examines the social insurance institutions of the Baltic States (old-age pensions, unemployment insurance, short-term benefits, sickness, maternity and parental insurance and family benefits) with respect to conditions of eligibility, replacement rates, financing and contributions. The findings of this study indicate that the Latvian social security system can generally be labelled as a mix of the basic security and corporatist models. The Estonian social security system can generally also be characterised as a mix of the basic security and corporatist models, even if there are some weak elements of the targeted model in it. It appears that the institutional changes developing in the social security system of Lithuania have led to a combination of the basic security and targeted models of the welfare state. Nevertheless, as the example of the three Baltic States shows, there is diversity in how these countries solve problems within the field of social policy. In studying the social security schemes in detail, some common features were found that could be attributed to all three countries. Therefore, the critical analysis of the main social security institutions of the Baltic States in this study gave strong supporting evidence in favour of identifying the post-socialist regime type that is already gaining acceptance within comparative welfare state research. Study Two compares the system of social maintenance and insurance in the Soviet Union, which was in force in the three Baltic countries before their independence, with the currently existing social security systems. The aim of the essay is to highlight the forces that have influenced the transformation of the social policy from its former highly universal, albeit authoritarian, form, to the less universal, social insurance-based systems of present-day Estonia, Latvia and Lithuania. This study demonstrates that the welfare–economy nexus is not the only important factor in the development of social programs. The results of this analysis revealed that people's attitudes towards distributive justice and the developmental level of civil society also play an important part in shaping social policies. The shift to individualism in people’s mentality and the decline of the labour movement, or, to be more precise, the decline in trade union membership and influence, does nothing to promote the development of social rights in the Baltic countries and hinders the expansion of social policies. The legacy of the past has been another important factor in shaping social programs. It can be concluded that social policy should be studied as if embedded not only in the welfare-economy nexus, but also in the societal, historical and cultural nexus of a given society. Study Three discusses the views of the state elites on family policy within a wider theoretical setting covering family policy and social policy in a broader sense and attempts to expand this analytical framework to include other post-socialist countries. The aim of this essay is to explore the various views of the state elites in the Baltics concerning family policy and, in particular, family benefits as one of the possible explanations for the observed policy differences. The qualitative analyses indicate that the Baltic States differ significantly with regard to the motives behind their family policies. Lithuanian decision-makers seek to reduce poverty among families with children and enhance the parents’ responsibility for bringing up their children. Latvian policy-makers act so as to increase the birth rate and create equal opportunities for children from all families. Estonian policy-makers seek to create equal opportunities for all children and the desire to enhance gender equality is more visible in the case of Estonia in comparison with the other two countries. It is strongly arguable that there is a link between the underlying motives and the kinds of family benefits in a given country. This study, thus, indicates how intimately the attitudes of the state bureaucrats, policy-makers, political elite and researchers shape social policy. It confirms that family policy is a product of the prevailing ideology within a country, while the potential influence of globalisation and Europeanisation is detectable too. The final essay takes into account the opinions of welfare users and examines the performances of the institutionalised family benefits by relying on the recipients’ opinions regarding these benefits. The opinions of the populations as a whole regarding government efforts to help families are compared with those of the welfare users. Various family benefits are evaluated according to the recipients' satisfaction with those benefits as well as the contemporaneous levels of subjective satisfaction with the welfare programs related to the absolute level of expenditure on each program. The findings of this paper indicate that, in Latvia, people experience a lower level of success regarding state-run family insurance institutions, as compared to those in Lithuania and Estonia. This is deemed to be because the cash benefits for families and children in Latvia are, on average, seen as marginally influencing the overall financial situation of the families concerned. In Lithuania and Estonia, the overwhelming majority think that the family benefit systems improve the financial situation of families. It appears that recipients evaluated universal family benefits as less positive than targeted benefits. Some universal benefits negatively influenced the level of general satisfaction with the family benefits system provided in the countries being researched. This study puts forward a discussion about whether universalism is always more legitimate than targeting. In transitional economies, in which resources are highly constrained, some forms of universal benefits could turn out to be very expensive in relative terms, without being seen as useful or legitimate forms of help to families. In sum, by closely examining the different aspects of social policy, this dissertation goes beyond the over-generalisation of Eastern European welfare state development and, instead, takes a more detailed look at what is really going on in these countries through the examples of Lithuania, Latvia and Estonia. In addition, another important contribution made by this study is that it revives ‘western’ theoretical knowledge through ‘eastern’ empirical evidence and provides the opportunity to expand the theoretical framework for post-socialist societies.

Metamorfismo ercinico di bassa-pressione: evoluzione tettonico-metamorfica del complesso di mandatoriccio (massiccio della Sila - Calabria)

Low-pressure/high-temperature (LP/HT) metamorphic belts are characterised by rocks that experienced abnormal heat flow in shallow crustal levels (T > 600 °C; P < 4 kbar) resulting in anomalous geothermal gradients (60-150 °C/km). The abnormal amount of heat has been related to crustal underplating of mantle-derived basic magmas or to thermal perturbation linked to intrusion of large volumes of granitoids in the intermediate crust. In particular, in this latter context, magmatic or aqueous fluids are able to transport relevant amounts of heat by advection, thus favouring regional LP/HT metamorphism. However, the thermal perturbation consequent to heat released by cooling magmas is responsible also for contact metamorphic effects. A first problem is that time and space relationships between regional LP/HT metamorphism and contact metamorphism are usually unclear. A second problem is related to the high temperature conditions reached at different crustal levels. These, in some cases, can completely erase the previous metamorphic history. Notwithstanding this problem is very marked in lower crustal levels, petrologic and geochronologic studies usually concentrate in these attractive portions of the crust. However, only in the intermediate/upper-crustal levels of a LP/HT metamorphic belt the tectono-metamorphic events preceding the temperature peak, usually not preserved in the lower crustal portions, can be readily unravelled. The Hercynian Orogen of Western Europe is a well-documented example of a continental collision zone with widespread LP/HT metamorphism, intense crustal anatexis and granite magmatism. Owing to the exposure of a nearly continuous cross-section of the Hercynian continental crust, the Sila massif (northern Calabria) represents a favourable area to understand large-scale relationships between granitoids and LP/HT metamorphic rocks, and to discriminate regional LP/HT metamorphic events from contact metamorphic effects. Granulite-facies rocks of the lower crust and greenschist- to amphibolite-facies rocks of the intermediate-upper crust are separated by granitoids emplaced into the intermediate level during the late stages of the Hercynian orogeny. Up to now, advanced petrologic studies have been focused mostly in understanding P-T evolution of deeper crustal levels and magmatic bodies, whereas the metamorphic history of the shallower crustal levels is poorly constrained. The Hercynian upper crust exposed in Sila has been subdivided in two different metamorphic complexes by previous authors: the low- to very low-grade Bocchigliero complex and the greenschist- to amphibolite-facies Mandatoriccio complex. The latter contains favourable mineral assemblages in order to unravel the tectono-metamorphic evolution of the Hercynian upper crust. The Mandatoriccio complex consists mainly of metapelites, meta-arenites, acid metavolcanites and metabasites with rare intercalations of marbles and orthogneisses. Siliciclastic metasediments show a static porphyroblastic growth mainly of biotite, garnet, andalusite, staurolite and muscovite, whereas cordierite and fibrolite are less common. U-Pb ages and internal features of zircons suggest that the protoliths of the Mandatoriccio complex formed in a sedimentary basin filled by Cambrian to Silurian magmatic products as well as by siliciclastic sediments derived from older igneous and metamorphic rocks. In some localities, metamorphic rocks are injected by numerous aplite/pegmatite veins. Small granite bodies are also present and are always associated to spotted schists with large porphyroblasts. They occur along a NW-SE trending transcurrent cataclastic fault zone, which represents the tectonic contact between the Bocchigliero and the Mandatoriccio complexes. This cataclastic fault zone shows evidence of activity at least from middle-Miocene to Recent, indicating that brittle deformation post-dated the Hercynian orogeny. P-T pseudosections show that micaschists and paragneisses of the Mandatoriccio complex followed a clockwise P-T path characterised by four main prograde phases: thickening, peak-pressure condition, decompression and peak-temperature condition. During the thickening phase, garnet blastesis started up with spessartine-rich syntectonic core developed within micaschists and paragneisses. Coevally (340 ± 9.6 Ma), mafic sills and dykes injected the upper crustal volcaniclastic sedimentary sequence of the Mandatoriccio complex. After reaching the peak-pressure condition (≈4 kbar), the upper crust experienced a period of deformation quiescence marked by the static overgrowths of S2 by Almandine-rich-garnet rims and by porphyroblasts of biotite and staurolite. Probably, this metamorphic phase is related to isotherms relaxation after the thickening episode recorder by the Rb/Sr isotopic system (326 ± 6 Ma isochron age). The post-collisional period was mainly characterised by decompression with increasing temperature. This stage is documented by the andalusite+biotite coronas overgrown on staurolite porphyroblasts and represents a critical point of the metamorphic history, since metamorphic rocks begin to record a significant thermal perturbation. Peak-temperature conditions (≈620 °C) were reached at the end of this stage. They are well constrained by some reaction textures and mineral assemblages observed almost exclusively within paragneisses. The later appearance of fibrolitic sillimanite documents a small excursion of the P-T path across the And-Sil boundary due to the heating. Stephanian U-Pb ages of monazite crystals from the paragneiss, can be related to this heating phase. Similar monazite U-Pb ages from the micaschist combined with the lack of fibrolitic sillimanite suggest that, during the same thermal perturbation, micaschists recorded temperatures slightly lower than those reached by paragneisses. The metamorphic history ended with the crystallisation of cordierite mainly at the expense of andalusite. Consequently, the Ms+Bt+St+And+Sill+Crd mineral assemblage observed in the paragneisses is the result of a polyphasic evolution and is characterised by the metastable persistence of the staurolite in the stability fields of the cordierite. Geologic, geochronologic and petrographic data suggest that the thermal peak recorded by the intermediate/upper crust could be strictly connected with the emplacement of large amounts of granitoid magmas in the middle crust. Probably, the lithospheric extension in the relatively heated crust favoured ascent and emplacement of granitoids and further exhumation of metamorphic rocks. After a comparison among the tectono-metamorphic evolutions of the different Hercynian crustal levels exposed in Sila, it is concluded that the intermediate/upper crustal level offers the possibility to reconstruct a more detailed tectono-metamorphic history. The P-T paths proposed for the lower crustal levels probably underestimate the amount of the decompression. Apart from these considerations, the comparative analysis indicates that P-T paths at various crustal levels in the Sila cross section are well compatible with a unique geologic scenario, characterized by post-collisional extensional tectonics and magmas ascent.

Comparison of model potentials for molecular dynamics simulation of crystalline silica

Simulationen von SiO2 mit dem von van Beest, Kramer und vanSanten (BKS) entwickelten Paarpotenzial erzeugen vielezufriedenstellende Ergebnisse, aber auch charakteristischeSchwachstellen. In dieser Arbeit wird das BKS-Potenzial mitzwei kürzlich vorgeschlagenen Potenzialen verglichen, dieeffektiv Mehrteilchen-Wechselwirkungen beinhalten. Der ersteAnsatz erlaubt dazu fluktuierende Ladungen, der zweiteinduzierbare Polarisierungen auf den Sauerstoffatomen. Die untersuchten Schwachstellen des BKS Potenzialsbeinhalten das Verhältnis der zwei Gitterkonstanten a und cim Quarzübergang, das von BKS falsch beschrieben wird.Cristobalit und Tridymit erscheinen instabil mit BKS.Weiterhin zeigt die BKS-Zustandsdichte charakteristischeAbweichungen von der wahren Zustandsdichte. DerÜbergangsdruck für den Stishovit I-II Übergang wird deutlichüberschätzt. Das Fluktuierende-Ladungs-Modell verbesserteinige der genannten Punkte, reproduziert aber viele andereEigenschaften schlechter als BKS. DasFluktierende-Dipol-Modell dagegen behebt alle genanntenArtefakte. Zusätzlich wird der druckinduzierte Phasenübergang imalpha-Quarz untersucht. Alle Potentiale finden die selbeStruktur für Quarz II. Bei anschliessender Dekompressionerzeugt BKS eine weitere Phase, während die beiden anderenPotentiale wieder zum alpha-Quarz zurückkehren. Weiterhinwerden zwei Methoden entwickelt, um die piezoelektrischenKonstanten bei konstantem Druck zu bestimmen. Die Ergebnissegeben Hinweise auf eine möglicherweisenicht-elektrostatische Natur der Polarisierungen imFluktuierende-Dipole-Modell. Mit dieser Interpretation scheint das Fluktuierende-DipolPotential alle verfügbaren experimentellen Daten am bestenvon allen drei untersuchten Ansätzen zu reproduzieren.