Transcrição cooperativa de genes ribossomais em Escherichia coli usando um modelo estocástico e dependente de sequência

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Fungos endofíticos: prospecção de atividade biocatalítica e aplicação biotecnológica

Pós-graduação em Biotecnologia - IQ

New insights on reaction pathway selectivity promoted by crown ether phase-transfer catalysis: Model ab initio calculations of nucleophilic fluorination

Crown ethers have the ability of solubilizing inorganic salts in apolar solvents and to promote chemical reactions by phase-transfer catalysis. However, details on how crown ethers catalyze ionic S(N)2 reactions and control selectivity are not well understood. In this work, we have used high level theoretical calculations to shed light on the details of phase-transfer catalysis mechanism of KF reaction with alkyl halides promoted by 18-crown-6. A complete analysis of the of the model reaction between KF(18-crown-6) and ethyl bromide reveals that the calculations can accurately predict the product ratio and the overall kinetics. Our results point out the importance of the K* ion and of the crown ether ring in determining product selectivity. While the K* ion favors the S(N)2 over the E2 anti pathway, the crown ether ring favors the S(N)2 over E2 syn route. The combination effects lead to a predicted 94% for the S(N)2 pathway in excellent agreement with the experimental value of 92%. A detailed analysis of the overall mechanism of the reaction under phase-transfer conditions also reveals that the KBr product generated in the nucleophilic fluorination acts as an inhibitor of the 18-crown-6 catalyst and it is responsible for the observed slow reaction rate. (C) 2012 Elsevier B.V. All rights reserved.

Period adding cascades: Experiment and modeling in air bubbling

Period adding cascades have been observed experimentally/numerically in the dynamics of neurons and pancreatic cells, lasers, electric circuits, chemical reactions, oceanic internal waves, and also in air bubbling. We show that the period adding cascades appearing in bubbling from a nozzle submerged in a viscous liquid can be reproduced by a simple model, based on some hydrodynamical principles, dealing with the time evolution of two variables, bubble position and pressure of the air chamber, through a system of differential equations with a rule of detachment based on force balance. The model further reduces to an iterating one-dimensional map giving the pressures at the detachments, where time between bubbles come out as an observable of the dynamics. The model has not only good agreement with experimental data, but is also able to predict the influence of the main parameters involved, like the length of the hose connecting the air supplier with the needle, the needle radius and the needle length. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3695345]

Enzyme Microheterogeneous Hydration and Stabilization in Supercritical Carbon Dioxide

Supercritical carbon dioxide is a promising green-chemistry solvent for many enzyme-catalyzed chemical reactions, yet the striking stability of some enzymes in such unconventional environments is not well understood. Here, we investigate the stabilization of the Candida antarctica Lipase B (CALB) in supercritical carbon dioxide-water biphasic systems using molecular dynamics simulations. The preservation of the enzyme structure and optimal activity depend on the presence of small amounts of water in the supercritical dispersing medium. When the protein is at least partially hydrated, water molecules bind to specific sites on the enzyme surface and prevent carbon dioxide from penetrating its catalytic core. Strikingly, water and supercritical carbon dioxide cover the protein surface quite heterogeneously. In the first solvation layer, the hydrophilic residues at the surface of the protein are able to pin down patches of water, whereas carbon dioxide solvates preferentially hydrophobic surface residues. In the outer solvation shells, water molecules tend to cluster predominantly on top of the larger water patches of the first solvation layer instead of spreading evenly around the remainder of the protein surface. For CALB, this exposes the substrate-binding region of the enzyme to carbon dioxide, possibly facilitating diffusion of nonpolar substrates into the catalytic funnel. Therefore, by means of microheterogeneous solvation, enhanced accessibility of hydrophobic substrates to the active site can be achieved, while preserving the functional structure of the enzyme. Our results provide a molecular picture on the nature of the stability of proteins in nonaqueous media.

Compact and autonomous multiwavelength microanalyzer for in-line and in situ colorimetric determinations

Nowadays, the attainment of microsystems that integrate most of the stages involved in an analytical process has raised an enormous interest in several research fields. This approach provides experimental set-ups of increased robustness and reliability, which simplify their application to in-line and continuous biomedical and environmental monitoring. In this work, a novel, compact and autonomous microanalyzer aimed at multiwavelength colorimetric determinations is presented. It integrates the microfluidics (a three-dimensional mixer and a 25 mm length "Z-shape" optical flow-cell), a highly versatile multiwavelength optical detection system and the associated electronics for signal processing and drive, all in the same device. The flexibility provided by its design allows the microanalyzer to be operated either in single fixed mode to provide a dedicated photometer or in multiple wavelength mode to obtain discrete pseudospectra. To increase its reliability, automate its operation and allow it to work under unattended conditions, a multicommutation sub-system was developed and integrated with the experimental set-up. The device was initially evaluated in the absence of chemical reactions using four acidochromic dyes and later applied to determine some key environmental parameters such as phenol index, chromium(VI) and nitrite ions. Results were comparable with those obtained with commercial instrumentation and allowed to demonstrate the versatility of the proposed microanalyzer as an autonomous and portable device able to be applied to other analytical methodologies based on colorimetric determinations.

Self-similarities of periodic structures for a discrete model of a two-gene system

We report self-similar properties of periodic structures remarkably organized in the two-parameter space for a two-gene system, described by two-dimensional symmetric map. The map consists of difference equations derived from the chemical reactions for gene expression and regulation. We characterize the system by using Lyapunov exponents and isoperiodic diagrams identifying periodic windows, denominated Arnold tongues and shrimp-shaped structures. Period-adding sequences are observed for both periodic windows. We also identify Fibonacci-type series and Golden ratio for Arnold tongues, and period multiple-of-three windows for shrimps. (C) 2012 Elsevier B.V. All rights reserved.

OSCILLATORY DYNAMICS IN SYSTEMS CONTAINING BROMATE AND 1,4-CYCLOHEXANEDIONE IN ACIDIC MEDIA. I. THE EFFECT OF TEMPERATURE.

OSCILLATORY DYNAMICS IN SYSTEMS CONTAINING BROMATE AND 1,4-CYCLOHEXANEDIONE IN ACIDIC MEDIA. I. THE EFFECT OF TEMPERATURE. We present in this work the influence of temperature on the dynamics of homogeneous chemical systems containing bromate and 1,4-cyclohexanedione (1,4-CHD) in acidic media. In particular, the following systems were studied: bromate/1,4-CHD/acid, bromate/1,4-CHD/ferroin/acid and bromate/1,4-CHD/trisbipyridine ruthenium/acid. Investigations were carried out by means of an electrochemical probe, at five temperatures between 5 and 45 degrees C. Activation energies (E-a) were estimated in different ways for the pre-oscillatory and oscillatory regimes. In any case, the E-a was found to depend on the catalyst, composition and initial concentrations. In addition, it was observed that ferroin and trisbipyridine ruthenium act as catalysts only during the transition between the induction period and oscillatory regime.

Selectivity and Mechanisms Driven by Reaction Dynamics: The Case of the Gas-Phase OH-+CH3ONO2 Reaction

Well-established statistical approaches such as transition-state theory based on high-level calculated potential energy profiles are unable to account for the selectivity observed in the gas-phase OH- + CH3ONO2 reaction. This reaction can undergo bimolecular nucleophilic displacement at either the carbon center (S(N)2@C) or the nitrogen center (S(N)2@N) as well as a proton abstraction followed by dissociation (E(CO)2) pathway. Direct dynamics simulations yield an S(N)2:E(CO)2 product ratio in close agreement with experiment and show that the lack of reactivity at the nitrogen atom is due to the highly negative electrostatic potential generated by the oxygen atoms in the ONO2 group that scatters the incoming OH-. In addition to these dynamical effects, the nonstatistical behavior of these reactions is attributed to the absence of equilibrated reactant complexes and to the large number of recrossings, which might be present in several ion-molecule gas-phase reactions.

Durability of crystalline monumental stones in terms of their petrophysical characteristics

The durability of stone building materials is an issue of utmost importance in the field of monument conservation. In order to be able to preserve our built cultural heritage, the thorough knowledge of its constituent materials and the understanding of the processes that affect them are indispensable. The main objective of this research was to evaluate the durability of a special stone type, the crystalline stones, in correlation with their intrinsic characteristics, the petrophysical properties. The crystalline stones are differentiated from the cemented stones on the basis of textural features. Their most important specific property is the usually low, fissure-like porosity. Stone types of significant monumental importance, like the marble or granite belong to this group. The selected materials for this investigation, indeed, are a marble (Macael marble, Spain) and a granite (Silvestre Vilachán granite, Spain). In addition, an andesite (Szob andesite, Hungary) also of significant monumental importance was selected. This way a wide range of crystalline rocks is covered in terms of petrogenesis: stones of metamorphic, magmatic and volcanic origin, which can be of importance in terms of mineralogical, petrological or physical characteristics. After the detailed characterization of the petrophysical properties of the selected stones, their durability was assessed by means of artificial ageing. The applied ageing tests were: the salt crystallization, the frost resistance in pure water and in the presence of soluble salts, the salt mist and the action of SO2 in the presence of humidity. The research aimed at the understanding of the mechanisms of each weathering process and at finding the petrophysical properties most decisive in the degradation of these materials. Among the several weathering mechanisms, the most important ones were found to be the physical stress due to crystallization pressure of both salt and ice, the thermal fatigue due to cyclic temperature changes and the chemical reactions (mostly the acidic attack) between the mineral phases and the external fluids. The properties that fundamentally control the degradation processes, and thus the durability of stones were found to be: the mineralogical and chemical composition; the hydraulic properties especially the water uptake, the permeability and the drying; the void space structure, especially the void size and aperture size distribution and the connectivity of the porous space; and the thermal and mechanical properties. Because of the complexity of the processes and the high number of determining properties, no mechanisms or characteristics could be identified as typical for crystalline stones. The durability or alterability of each stone type must be assessed according to its properties and not according to the textural or petrophysical classification they belong to. Finally, a critical review of standardized methods is presented, based on which an attempt was made for recommendations of the most adequate methodology for the characterization and durability assessment of crystalline stones.

Cluster-derived Gold/Iron catalysts for environmental applications

During the last years we assisted to an exponential growth of scientific discoveries for catalysis by gold and many applications have been found for Au-based catalysts. In the literature there are several studies concerning the use of gold-based catalysts for environmental applications and good results are reported for the catalytic combustion of different volatile organic compounds (VOCs). Recently it has also been established that gold-based catalysts are potentially capable of being effectively employed in fuel cells in order to remove CO traces by preferential CO oxidation in H2-rich streams. Bi-metallic catalysts have attracted increasing attention because of their markedly different properties from either of the costituent metals, and above all their enhanced catalytic activity, selectivity and stability. In the literature there are several studies demostrating the beneficial effect due to the addition of an iron component to gold supported catalysts in terms of enhanced activity, selectivity, resistence to deactivation and prolonged lifetime of the catalyst. In this work we tried to develop a methodology for the preparation of iron stabilized gold nanoparticles with controlled size and composition, particularly in terms of obtaining an intimate contact between different phases, since it is well known that the catalytic behaviour of multi-component supported catalysts is strongly influenced by the size of the metal particles and by their reciprocal interaction. Ligand stabilized metal clusters, with nanometric dimensions, are possible precursors for the preparation of catalytically active nanoparticles with controlled dimensions and compositions. Among these, metal carbonyl clusters are quite attractive, since they can be prepared with several different sizes and compositions and, moreover, they are decomposed under very mild conditions. A novel preparation method was developed during this thesis for the preparation of iron and gold/iron supported catalysts using bi-metallic carbonyl clusters as precursors of highly dispersed nanoparticles over TiO2 and CeO2, which are widely considered two of the most suitable supports for gold nanoparticles. Au/FeOx catalysts were prepared by employing the bi-metallic carbonyl cluster salts [NEt4]4[Au4Fe4(CO)16] (Fe/Au=1) and [NEt4][AuFe4(CO)16] (Fe/Au=4), and for comparison FeOx samples were prepared by employing the homometallic [NEt4][HFe3(CO)11] cluster. These clusters were prepared by Prof. Longoni research group (Department of Physical and Inorganic Chemistry- University of Bologna). Particular attention was dedicated to the optimization of a suitable thermal treatment in order to achieve, apart from a good Au and Fe metal dispersion, also the formation of appropriate species with good catalytic properties. A deep IR study was carried out in order to understand the physical interaction between clusters and different supports and detect the occurrence of chemical reactions between them at any stage of the preparation. The characterization by BET, XRD, TEM, H2-TPR, ICP-AES and XPS was performed in order to investigate the catalysts properties, whit particular attention to the interaction between Au and Fe and its influence on the catalytic activity. This novel preparation method resulted in small gold metallic nanoparticles surrounded by highly dispersed iron oxide species, essentially in an amorphous phase, on both TiO2 and CeO2. The results presented in this thesis confirmed that FeOx species can stabilize small Au particles, since keeping costant the gold content but introducing a higher iron amount a higher metal dispersion was achieved. Partial encapsulation of gold atoms by iron species was observed since the Au/Fe surface ratio was found much lower than bulk ratio and a strong interaction between gold and oxide species, both of iron oxide and supports, was achieved. The prepared catalysts were tested in the total oxidation of VOCs, using toluene and methanol as probe molecules for aromatics and alchols, respectively, and in the PROX reaction. Different performances were observed on titania and ceria catalysts, on both toluene and methanol combustion. Toluene combustion on titania catalyst was found to be enhanced increasing iron loading while a moderate effect on FeOx-Ti activity was achieved by Au addition. In this case toluene combustion was improved due to a higher oxygen mobility depending on enhanced oxygen activation by FeOx and Au/FeOx dispersed on titania. On the contrary ceria activity was strongly decreased in the presence of FeOx, while the introduction of gold was found to moderate the detrimental effect of iron species. In fact, excellent ceria performances are due to its ability to adsorb toluene and O2. Since toluene activation is the determining factor for its oxidation, the partial coverage of ceria sites, responsible of toluene adsorption, by FeOx species finely dispersed on the surface resulted in worse efficiency in toluene combustion. Better results were obtained for both ceria and titania catalysts on methanol total oxidation. In this case, the performances achieved on differently supported catalysts indicate that the oxygen mobility is the determining factor in this reaction. The introduction of gold on both TiO2 and CeO2 catalysts, lead to a higher oxygen mobility due to the weakening of both Fe-O and Ce-O bonds and consequently to enhanced methanol combustion. The catalytic activity was found to strongly depend on oxygen mobility and followed the same trend observed for catalysts reducibility. Regarding CO PROX reaction, it was observed that Au/FeOx titania catalysts are less active than ceria ones, due to the lower reducibility of titania compared to ceria. In fact the availability of lattice oxygen involved in PROX reaction is much higher in the latter catalysts. However, the CO PROX performances observed for ceria catalysts are not really high compared to data reported in literature, probably due to the very low Au/Fe surface ratio achieved with this preparation method. CO preferential oxidation was found to strongly depend on Au particle size but also on surface oxygen reducibility, depending on the different oxide species which can be formed using different thermal treatment conditions or varying the iron loading over the support.

Modeling the gas and aqueous phase chemistry of the marine boundary layer

Ein eindimensionales numerisches Modell der maritimenGrenzschicht (MBL) wurde erweitert, um chemische Reaktionenin der Gasphase, von Aerosolpartikeln und Wolkentropfen zu beschreiben. Ein Schwerpunkt war dabei die Betrachtung derReaktionszyklen von Halogenen. Soweit Ergebnisse vonMesskampagnen zur Verfuegung standen, wurden diese zurValidierung des Modells benutzt. Die Ergebnisse von frueheren Boxmodellstudien konntenbestaetigt werden. Diese zeigten die saeurekatalysierteAktivierung von Brom aus Seesalzaerosolen, die Bedeutung vonHalogenradikalen fuer die Zerstoerung von O3, diepotentielle Rolle von BrO bei der Oxidation von DMS und dievon HOBr und HOCl in der Oxidation von S(IV). Es wurde gezeigt, dass die Beruecksichtigung derVertikalprofile von meteorologischen und chemischen Groessenvon grosser Bedeutung ist. Dies spiegelt sich darin wider,dass Maxima des Saeuregehaltes von Seesalzaerosolen und vonreaktiven Halogenen am Oberrand der MBL gefunden wurden.Darueber hinaus wurde die Bedeutung von Sulfataerosolen beidem aktiven Recyceln von weniger aktiven zu photolysierbarenBromspezies gezeigt. Wolken haben grosse Auswirkungen auf die Evolution und denTagesgang der Halogene. Dies ist nicht auf Wolkenschichtenbeschraenkt. Der Tagesgang der meisten Halogene ist aufgrundeiner erhoehten Aufnahme der chemischen Substanzen in die Fluessigphase veraendert. Diese Ergebnisse betonen dieWichtigkeit der genauen Dokumentation der meteorologischenBedingungen bei Messkampagnen (besonders Wolkenbedeckungsgrad und Fluessigwassergehalt), um dieErgebnisse richtig interpretieren und mit Modellresultatenvergleichen zu koennen. Dieses eindimensionale Modell wurde zusammen mit einemBoxmodell der MBL verwendet, um die Auswirkungen vonSchiffemissionen auf die MBL abzuschaetzen, wobei dieVerduennung der Abgasfahne parameterisiert wurde. DieAuswirkungen der Emissionen sind am staerksten, wenn sie insauberen Gebieten stattfinden, die Hoehe der MBL gering istund das Einmischen von Hintergrundluft schwach ist.Chemische Reaktionen auf Hintergrundaerosolen spielen nureine geringe Rolle. In Ozeangebieten mit schwachemSchiffsverkehr sind die Auswirkungen auf die Chemie der MBL beschraenkt. In staerker befahrenen Gebieten ueberlappensich die Abgasfahnen mehrerer Schiffe und sorgen fuerdeutliche Auswirkungen. Diese Abschaetzung wurde mitSimulationen verglichen, bei denen die Emissionen alskontinuierliche Quellen behandelt wurden, wie das inglobalen Chemiemodellen der Fall ist. Wenn die Entwicklungder Abgasfahne beruecksichtigt wird, sind die Auswirkungendeutlich geringer da die Lebenszeit der Abgase in der erstenPhase nach Emission deutlich reduziert ist.

Tropospheric photochemistry of ozone, its precursors and the hydroxyl radical

Zusammenfassung Ein 3-dimensionales globales Modell der unterenAtmosphäre wurde für die Untersuchung derOzonchemie, sowie der Chemie des Hydroxylradikals (OH) undwichtiger Vorläufersubstanzen, wie reaktiverStickstoffverbindungen und Kohlenwasserstoffe, verwendet.Hierfür wurde die Behandlung vonNicht-Methan-Kohlenwasserstoffen (NMKW) hinzugefügt,was auch die Entwicklung einer vereinfachten Beschreibungihrer Chemie, sowie die Erfassung von Depositionsprozessenund Emissionen erforderte. Zur Lösung der steifengewöhnlichen Differentialgleichungen der Chemie wurdeeine schnelles Rosenbrock-Verfahren eingesetzt, das soimplementiert wurde, dass die Modell-Chemie fürzukünftige Studien leicht abgeändert werden kann. Zur Evaluierung des Modells wurde ein umfangreicherVergleich der Modellergebnisse mit Bodenmessungen, sowieFlugzeug-, Sonden- und Satelliten-Daten durchgeführt.Das Modell kann viele Aspekte der Beobachtungen derverschieden Substanzen realistisch wiedergeben. Es wurdenjedoch auch einige Diskrepanzen festgestellt, die Hinweiseauf fehlerhafte Emissionsfelder oder auf die Modell-Dynamikund auch auf fehlende Modell-Chemie liefern. Zur weiteren Untersuchung des Einflusses verschiedenerStoffgruppen wurden drei Läufe mit unterschiedlichkomplexer Chemie analysiert. Durch das Berücksichtigender NMKW wird die Verteilung mehrerer wichtiger Substanzensignifikant beeinflusst, darunter z.B. ein Anstieg desglobalen Ozons. Es wurde gezeigt, dass die biogene SubstanzIsopren etwa die Hälfte des Gesamteffekts der NMKWausmachte (mehr in den Tropen, weniger anderswo). In einer Sensitivitätsstudie wurden die Unsicherheitenbei der Modellierung von Isopren weitergehend untersucht.Dabei konnte gezeigt werden, dass die Unsicherheit beiphysikalischen Aspekten (Deposition und heterogene Prozesse)ebenso groß sein kann, wie die aus dem chemischenGasphasen-Mechanismus stammende, welche zu globalbedeutsamen Abweichungen führte. Lokal können sichnoch größere Abweichungen ergeben. Zusammenfassend kann gesagt werden, dass die numerischenStudien dieser Arbeit neue Einblicke in wichtige Aspekte derPhotochemieder Troposphäre ergaben und in Vorschläge fürweiter Studien mündeten, die die wichtigsten gefundenenUnsicherheiten weiter verringern könnten.

Untersuchungen zu ionenchemischen Reaktionen und Mobilitätsmessungen an schweren Elementen in einer Puffergaszelle

Untersuchungen zu ionenchemischen Reaktionen und Mobilitätsmessungen an schweren Elementen in einer Puffergaszelle Die vorgelegte Arbeit beschreibt vorbereitende Untersuchungen zu ionenchemischen Reaktionen und Mobilitätsmessungen schwerer Elemente (Z>100) in einer mit Argon gefüllten Puffergaszelle. Dazu wurden am Element Erbium (Z=68), dem chemischen Homolog von Fermium (Z=100), zunächst in einem Fourier-Transformations-Massenspektrometer (FT/MS) Reaktionen mit Sauerstoff (O₂), Methan (CH₄) und Butylen (C₄H₈) untersucht und deren Reaktionskonstanten zu k(Er+O2)=(3,6±0,3)•10^-10cm³/s, k(Er+C4H8)=(1,3±0,1)•10^-10cm³/s gemessen. Für die Reaktion Er⁺+CH₄ wurde eine Obergrenze der Reaktionskonstante von k(Er+CH4)?,3•10^-15cm³/s bestimmt. Dieselben Reaktionen wurden anschließend in einer mit 60 mbar Argon gefüllten Puffergaszelle am Tandembeschleuniger des Max-Planck-Instituts für Kernphysik in Heidelberg studiert.Das in die Zelle eingeschossene Erbium wurde nach der Thermalisierung in einem zweistufigen Laserprozess resonant ionisiert. Diese Messungen führten zu gleichen Ergebnissen wie die FT/MS-Messungen (k(Er+O2)=3,3±0,4)•10^-10cm³/s, k(Er+CH4)?2•10^-17cm³/s). Die Reaktion von Erbium mit Butylen wurde ebenfalls beobachtet, eine Reaktionskonstante konnte jedoch nicht bestimmt werden. Die Reaktion von Erbium mit Sauerstoff wurde auch mit den direkt in die Puffergaszelle eingeschossenen Ionen ohne Laserionisation untersucht. Eine reproduzierbare Reaktionskonstante konnte nicht bestimmt werden, mögliche Ursachen werden diskutiert.Aus der Driftzeit der Ionen im Puffergas können Ionenmobilitäten bestimmt werden. Dies erlaubt Rückschlüsse auf die Ionenradien und damit auch auf Bindungslängen in Molekülen. Zwischen Plutonium und Americium wurde bei einer Driftzeit von (1,88±0,01) ms ein Driftzeitunterschied von (0,07±0,02) ms gemessen und daraus eine relative Verringerung des Ionenradius von Americium gegenüber dem von Plutonium um (3,1±1,3)% bestimmt. Relativistische Rechnungen sagen für den atomaren Radius von Americium gegenüber Plutonium eine Kontraktion in gleicher Größenordnung voraus; für Ionenradien existieren zur Zeit noch keine Rechnungen. Aus den gemessenen Driftzeiten des Plutoniums von (1,85±0,01) ms und Plutoniumoxids von (2,38±0,01) ms wurde eine Zunahme des Ionenradius des Plutoniumoxids gegenüber dem Plutonium um (28±2)% bestimmt.Außerdem wurden Reaktionen von Ruthenium (Z=44) und Osmium (Z=76), beides chemische Homologe von Hassium (Z=108), mit Sauerstoff in der FT/MS-Apparatur untersucht, mit dem Ziel widersprüchliche Messungen der Reaktionskonstanten aufzuklären.

Simulations of physics and chemistry of polar stratospheric clouds with a general circulation model

A polar stratospheric cloud submodel has been developed and incorporated in a general circulation model including atmospheric chemistry (ECHAM5/MESSy). The formation and sedimentation of polar stratospheric cloud (PSC) particles can thus be simulated as well as heterogeneous chemical reactions that take place on the PSC particles. For solid PSC particle sedimentation, the need for a tailor-made algorithm has been elucidated. A sedimentation scheme based on first order approximations of vertical mixing ratio profiles has been developed. It produces relatively little numerical diffusion and can deal well with divergent or convergent sedimentation velocity fields. For the determination of solid PSC particle sizes, an efficient algorithm has been adapted. It assumes a monodisperse radii distribution and thermodynamic equilibrium between the gas phase and the solid particle phase. This scheme, though relatively simple, is shown to produce particle number densities and radii within the observed range. The combined effects of the representations of sedimentation and solid PSC particles on vertical H2O and HNO3 redistribution are investigated in a series of tests. The formation of solid PSC particles, especially of those consisting of nitric acid trihydrate, has been discussed extensively in recent years. Three particle formation schemes in accordance with the most widely used approaches have been identified and implemented. For the evaluation of PSC occurrence a new data set with unprecedented spatial and temporal coverage was available. A quantitative method for the comparison of simulation results and observations is developed and applied. It reveals that the relative PSC sighting frequency can be reproduced well with the PSC submodel whereas the detailed modelling of PSC events is beyond the scope of coarse global scale models. In addition to the development and evaluation of new PSC submodel components, parts of existing simulation programs have been improved, e.g. a method for the assimilation of meteorological analysis data in the general circulation model, the liquid PSC particle composition scheme, and the calculation of heterogeneous reaction rate coefficients. The interplay of these model components is demonstrated in a simulation of stratospheric chemistry with the coupled general circulation model. Tests against recent satellite data show that the model successfully reproduces the Antarctic ozone hole.