No / A microemulsion electrokinetic chromatography (MEEKC) method has been developed and validated for the determination of 4-hydroxybenzoates and their impurities. These materials are commonly known as parabens and are widely used as preservatives in foods, cosmetics and pharmaceuticals. The method was shown to be selective and quantitative for the methyl, ethyl, propyl and butyl esters of 4-hydroxybenzoic acid. An internal standard, 4-hydroxyacetophenone, was employed to improve injection precision and detector linearity. In addition, 4-hydroxybenzoic acid, the major degradent, could also be monitored at the 0.1% (m/m) level. The method was successfully validated for assay and detection of the impurities in 4-hydroxybenzoic acid methyl ester and 4-hydroxybenzoic acid propyl ester samples and for the determination of 4-hydroxybenzoic acid methyl ester in a liquid pharmaceutical formulation. The determination of paraben content by MEEKC in a liquid sample was consistent with HPLC analysis. This work is the first reported validated MEEKC method and shows that the methodology can be successfully implemented into routine quality control testing.

Microalgae being photosynthetic and having quick growth cycles can prove to be excellent candidates as biofactories for the production of aromatic compounds like para-hydroxybenzoic acid (pHBA) that act as a monomer in liquid crystal polymers. We developed transgenic lines of the model alga Chlamydomonas reinhardtii by performing nuclear transformation using electroporation. The transgenic cell lines expressed the ubiC gene that utilized chorismate from the shikimate pathway as a substrate to produce pHBA. The maximum yield of pHBA measured in these lines was 80 mg/L. Accruing pHBA can be toxic to the cells and the mechanism by which C. reinhardtii could detoxify pHBA is not known. C. reinhardtii genome was thus scanned for sequences similar to UDP-glucosyltransferase (UGT) that can transfer the glucose moiety to pHBA, rendering it non-toxic to the cell lines. Our analysis suggested the absence of any potential UGTs that could glycosylate pHBA and detoxify it. We further performed feeding experiments to test the ability of wt-type C. reinhardtii cells to detoxify pHBA and understand its fate. C. reinhardtii cells were fed with varying concentrations of pHBA and harvested at different time intervals. The HPLC chromatograms indicated a majority of the pHBA was catabolized. Based on these results, literature was reviewed to find a suitable UGT candidate to enable the engineering of the glycosylation mechanism in the alga. A transgenic algal line with gene encoding UGT89B1 was created and fed with varying concentrations of pHBA. HPLC chromatograms from the extracts revealed the presence of phenolic glucoside. Following this, C. reinhardtii was co-transformed with ubiC and gene encoding UGT89B1; this led to the production of pHBA and further glycosylation to pHBA glucoside (phenolic glucoside). The maximum yield of pHBA yields in these cell lines was 180 mg/L. Growing C. reinhardtii lines producing pHBA on a large scale can lead to problems like contamination with bacteria and other algal species, a decline in pH, and a rise in temperature. To circumvent these problems, we explored the plausibility of using other algal strains. We analyzed the genome of Galdieria sulphuraria, a photosynthetic thermophile, that can use more than fifty different carbon sources for its growth and metabolism. We hypothesized several of these traits to have arisen by acquiring genes through horizontal gene transfer. We used a segmentation and clustering algorithm to identify regions of the genome that have atypical nucleotide composition. The atypical segments identified by the method were further analyzed using phylogenetic methods to further support claims of their alien origin. Our method identified 69 genes that were not previously reported as alien genes, some of which could be implicated in imparting resistance to environmental stress factors. Our experiments also revealed G. sulphuraria has a greater extent of alien genes compared to C. reinhardtii.

The growing development of nanotechnology has resulted in an increased use of nanoparticles (NPs) in various applications ranging from medicine, military, to daily consumer products. There is a concern that NPs can be dispersed into the environment in various ways, for example to air and water during manufacture, use, incineration or recycling of products and thus pose a risk to health and the environment. Risk assessments of NPs are hence necessary. One property of NPs, which may make them very useful and at the same time potentially harmful, is their small size (in nanometer range) and hence high surface area per NP mass.This study forms part of the National Mistra Environmental Nanosafety Research Program. The program provides an interdisciplinary platform for researchers from e.g. nanoscience, medicine, chemistry, material science, life cycle analysis, and social science. Specific aspects of this program involve characterization of NPs in different environmental settings, toxicity studies of aquatic organisms, integrated risk assessment of NPs, and societal dimensions of nanosafety. The contribution of this thesis within the program includes studies of stability and mobility of metal NPs and their extent of transformation/dissolution upon environmental interaction. Environmental risk assessments of NPs require a detailed understanding of how they change in terms of physical and chemical properties (charge, size, and surface oxide composition), important aspects for their stability, mobility, and reactivity in the environment. Generated data is highly relevant for the other activities of the Mistra Environmental Nanosafety program, e.g. to gain an improved understanding and design of particle dispersions and ecotoxicity studies, as any environmental interaction will result in the transformation/dissolution of the NPs and change the surface chemistry (e.g. adsorption of natural organic matter, changes in surface oxide properties), aspects that largely influence their speciation and potential toxicity.Common sonication protocols exist to prepare particle dispersions for different in vitro studies. The influence of key parameters stipulated by these protocols on the particle size, transformation/dissolution, and extent of sedimentation was investigated for bare metal NPs. Improved knowledge on these aspects is crucial for design and interpretation of results of NP-related investigations. Reactive metal NPs such as Cu and Mn NPs started to dissolve and release metals already during the probe sonication step of the stock solution, and that the presence of bovine serum albumin (often added as a stabilizing agent) enhanced this process. Even though prolonged sonication time i.e. increased delivered acoustic energy, reduced the size of formed agglomerates, sedimentation was still significant. As a consequence, administered doses from pipetted stock solutions were significantly lower (30-70%) than the nominal doses. The main reason behind the significant extent of agglomeration, with concomitant sedimentation, is related to the strong van der Waals forces prevailing between metal NPs. It is hence essential to determine the administrated dose of metallic NPs in e. g. nanotoxicological testing.Interactions between metallic NPs and natural organic matter (NOM) were studied in terms of stability, mobility and metal dissolution in order to mimic a potential exposure scenario. NOM was represented by humic acid (HA), a main component of organic matter in the environment, and by dihydroxybenzoic acid (DHBA), a small degradation product of NOM. Sedimentation of the Cu, and the Al NPs were slower in the presence of NOM in freshwater compared with freshwater only, whereas the effect of NOM was small for the Mn NPs. Stabilization was related to surface adsorption of NOM, which increased the steric repulsion between the particles, and in the case of HA also increased the magnitude of the zeta potential (resulting in increased electrostatic repulsion). Slight initial increase in particle stability wasobserved in freshwater containing DHBA, but after 24 h, sedimentation of the NPs was comparable to the conditions in freshwater only. The presence of HA (at a concentration of 20 mg/L) was found to stabilize the NPs in freshwater for more than 24 h. However, both the lower and higher HA concentration (2 and 40 mg/L) resulted in agglomeration of the Cu and Al NPs already within a few hours. Mn NPs were more stable in terms of sedimentation in freshwater at all three humic acid concentrations. This concludes that the concentration and type of NOM largely influence the stability of the studied metal NPs in solution. In contrast, SiO2 NPs were not influenced by the presence of NOM in terms of stability, most probably predominantly related to smaller attractive van der Waals forces and larger electrostatic repulsion (due to higher surface charge) compared with the metal NPs.Metal release from the Cu and Al NPs was enhanced in the presence of NOM, whereas no significant influence was observed for the Mn NPs. All metal NPs were dissolved relatively fast; 10% or more of the particle mass was dissolved within 24 h. Speciation predictions revealed rapid complexation between released Cu and Al in solution and NOM, reducing the bioavailability, whereas less complexation was evident for released Mn (as ions). In all, rapid agglomeration and sedimentation imply that any risks associated with the environmental dispersion of these metal NPs will be limited to the vicinity of their source. Mn NPs, having lower sedimentation rates than the Cu and Al NPs, and lack of solution complexation of released ions will likely have a relatively higher probability to be mobile and transported to other aquatic settings.

CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / CNPq - Conselho Nacional de Desenvolvimento Científico e Tecnológico / FAPEMIG - Fundação de Amparo a Pesquisa do Estado de Minas Gerais / Neste trabalho foi desenvolvido um nanocompósito polimérico de óxido de grafeno reduzido e poli(ácido 3 hidroxibenzóico) para a modificação de eletrodos de grafite, visando o desenvolvimento de um genossensor para a detecção do DNA genômico de Alicyclobacillus acidoterrestris. Esta é uma bactéria associada à deterioração de sucos ácidos, como o suco de laranja, do qual o Brasil é o maior produtor mundial. Neste contexto, os biossensores aparecem como dispositivos de detecção rápidos e fáceis de manusear, com grande potencial para serem utilizados em toda a cadeia produtiva do suco. Para a construção do genossensor, óxido de grafeno foi produzido pelo método de Hummers modificado, gotejado sobre a superfície do eletrodo de grafite e reduzido eletroquimicamente. O ácido 3-hidroxibenzóico foi eletropolimerizado sobre esta superfície contendo o nanomaterial. Análises por espectroscopia no infravermelho e voltametria cíclica comprovaram a redução do óxido de grafeno. Além disso, as análises eletroquímicas evidenciaram que o nanocompósito produzido apresenta propriedades eletrônicas superiores às do filme polimérico. Sobre este nanocompósito foi imobilizado um oligonucleotídeo sonda ALIC1, específico para A. acidoterrestris, o qual foi utilizado para detecção de um oligonucleotídeo alvo complementar ALIC2 pela técnica de voltametria de pulso diferencial (VPD), tanto direta quanto indiretamente, esta última utilizando-se o intercalante da dupla fita de DNA Hoechst 33258. Um lisado celular obtido a partir de uma cultura de A. acidoterrestris também foi detectado de maneira indireta pela técnica de VPD, e uma curva de calibração foi construída. O genossensor proposto apresentou um limite de detecção de 174 ng mL-1 e limite de quantificação de 581 ng mL-1, sendo capaz de detectar o DNA genômico em uma amostra real de suco de laranja e de discernir entre amostras de A. acidoterrestris e Escherichia coli. Deste modo, este bioeletrodo apresenta-se como a primeira plataforma de detecção eletroquímica do DNA genômico de A. acidoterrestris na literatura científica. / In this work a polymeric nanocomposite of reduced graphene oxide and poly (3- hydroxybenzoic acid) was developed for the modification of graphite electrodes, aiming the development of a genossensor for the detection of the Alicyclobacillus acidoterrestris genomic DNA. This bacterium is associated with the spoilage of acidic juices, such as orange juice, of which Brazil is the largest producer in the world. In this context, biosensors appear as fast and easy to handle detection devices, with great potential for use throughout the juice production chain. For the construction of the genosensor, graphene oxide was produced by the modified Hummers method, dripped onto the surface of the graphite electrode and reduced electrochemically. The 3-hydroxybenzoic acid was electropolymerized on this surface containing the nanomaterial. Analyzes by infrared spectroscopy and cyclic voltammetry proved the reduction of graphene oxide. In addition, the electrochemical analysis showed that the nanocomposite produced has higher electronic properties than the polymeric film. On this nanocomposite, an oligonucleotide probe ALIC1, specific for A. acidoterrestris, was immobilized, and was used to detect a complementary target oligonucleotide ALIC2, both directly and indirectly, the latter using the Hoechst 33258 double strand DNA intercalator, by the differential pulse voltammetry (DPV) technique. A cell lysate obtained from an A. acidoterrestris culture was also indirectly detected by DPV, and a calibration curve was constructed. The proposed genosensor presented a limit of detection of 174 ng mL-1 and limit of quantification of 581 ng mL-1, being able to detect the genomic DNA in a real sample of orange juice and to distinguish between the samples of A acidoterrestris and Escherichia coli. Thus, this bioelectrode presents as the first platform of electrochemical detection of the genomic DNA of A. acidoterrestris in the scientific literature. / Dissertação (Mestrado)

Fundação de Amparo a Pesquisa do Estado de Minas Gerais / The biossensores constitute analytical important devices for the clinical diagnosis of many diseases, enabling real-time analysis, with high specificity and sensitivity. Accordingly, the aim of this study was to develop a platform functionalized with the poly (3- hydroxybenzoic acid), to detection of nitrogenous bases, oligonucleotides and antibodies against the surface protein (Am1) of Anaplasma marginale.These biological recognition elements were immobilized on the functionalized platform and the biosensor was characterized by electrochemical and spectroscopic techniques. The detections of the antigen-antibody interaction were conducted using the signal obtained from the oxidation of protein interleaver, 4- aminoantipyrine (4 -AAP), using voltammetry technique of differential pulse.Theoretical chemistry studies were employed to analyze the Am1 interactions with the sensor platform and the web server model RaptorX to infer three-dimensional structure of the membrane proteins of A. marginale.The molecular modeling was conducted from methods semi- empirical, DFT and molecular mechanics.The transitions redoxes of polymer were classified as almost reversible, with band gap of 1.2 V, characteristic of semiconductor materials.The detections of poly CT and antibodies of A. marginale, using the graphite electrode functionalized with the probes, poly GA and Am1 respectivamente, were 66% higher than the graphite electrode not functionalized.The protein of superfice Am1 presented a structure stable in beta-sheet, while Am2 presented an unstable structure in α -helix.In electrochemical impedance spectroscopy was possible qualitatively differentiate the functionalized platform with: Am1, IgG+, IgG- and interfering. The time of storage of the immunosensor was satisfactory, with reduced peak current by 35%, after 90 days. These results demonstrate the excelent applicability of functionalized platform for the development of immunosensor for diagnosis of bovine anaplasmosis, leaving only the economic feasibility studies for commercial deployment of the said immunosensor. / Os biossensores constituem-se como dispositivos analíticos importantes para o diagnóstico clínico de inúmeras doenças, por permitirem uma análise em tempo real, além de suas altas especificidades e sensibilidades. Nesse sentido, o objetivo desse estudo foi desenvolver uma plataforma funcionalizada com o poli (ácido 3-hidroxibenzóico), para a detecção de bases nitrogenadas, oligonucleotídeos e anticorpos contra a proteína de superfície (Am1) da Anaplasma marginale. Esses elementos de reconhecimento biológico foram imobilizados na plataforma funcionalizada e o biossensor foi caracterizado por técnicas eletroquímicas e espectroscópicas. As detecções da interação antígeno-anticorpo foram realizadas através do sinal obtido da oxidação do intercalador de proteína, 4-aminoantipirina (4-AAP), utilizando a técnica de voltametria de pulso diferencial. Estudos de química teórica foram empregados para análise das interações da Am1 com a plataforma sensora e o modelo web server RaptorX para inferência da estrutura tridimensional das proteínas de membrana da A. marginale. A modelagem molecular foi realizada a partir de métodos semi-empíricos, DFT e mecânica molecular. As transições redoxes do polímero foram classificadas como quase reversíveis, com band gap de 1,2 V característico de materiais semicondutores. As detecções de poli CT e anticorpos da A. marginale, empregando o eletrodo de grafite funcionalizado com as sondas, poli GA e Am1 respectivamente, foram 66 % superior a do eletrodo de grafite não funcionalizado. A proteína de superfície Am1 apresentou uma estrutura estável em folha-beta e a Am2 uma estrutura instável em α-hélice. Na espectroscopia de impedância eletroquímica foi possível diferenciar qualitativamente a plataforma funcionalizada com: Am1, IgG+, IgG- e interferente. O tempo de estocagem do imunossensor mostrou-se satisfatório, com redução do pico de corrente em 35 %, após 90 dias. Esses resultados demonstraram a excelente aplicabilidade da plataforma funcionalizada para o desenvolvimento de imunossensor para diagnóstico da anaplasmose bovina, restando apenas os estudos de viabilidade econômica para implantação comercial do referido imunossensor. / Doutor em Química

The Ph.D. project compiled in this thesis has focused on the role of the solvent in solid-liquid phase equilibria and in nucleation kinetics. Six organic substances have been selected as model compounds, viz. ortho-, meta- and para-hydroxybenzoic acid, salicylamide, meta- and para-aminobenzoic acid. The different types of crystal phases of these compounds have been explored, and their respective solid-state properties have been determined experimentally. The solubility of these crystal phases has been determined in various solvents between 10 and 50 oC. The kinetics of nucleation has been investigated for salicylamide by measuring the metastable zone width, in five different solvents under different experimental conditions. A total of 15 different crystal phases were identified among the six model compounds. Only one crystal form was found for the ortho-substituted compounds, whereas the meta-isomeric compounds crystallized as two unsolvated polymorphs. The para-substituted isomers crystallized as two unsolvated polymorphs and as several solvates in different solvents. It was discovered that the molar solubility of the different crystal phases was linked to the temperature dependence of solubility. In general, a greater molar solubility corresponds to a smaller temperature dependence of solubility. The generality of this relation for organic compounds was investigated using a test set of 41 organic solutes comprising a total of 115 solubility curves. A semi-empirical solubility model was developed based on how thermodynamic properties relate to concentration and temperature. The model was fitted to the 115 solubility curves and used to predict the temperature dependence of solubility. The model allows for entire solubility curves to be constructed in new solvents based on the melting properties of the solute and the solubility in that solvent at a single temperature. Based on the test set comprising the 115 solubility curves it was also found that the melting temperature of the solute can readily be predicted from solubility data in organic solvents. The activity of the solid phase (or ideal solubility) of four of the investigated crystal phases was determined within a rigorous thermodynamic framework, by combining experimental data at the melting temperature and solubility in different solvents and temperatures. The results show that the assumptions normally used in the literature to determine the activity of the solid phase may give rise to errors up to a factor of 12. An extensive variation in the metastable zone width of salicylamide was obtained during repeated experiments performed under identical experimental conditions. Only small or negligible effects on the onset of nucleation were observed by changing the saturation temperature or increasing the solution volume. The onset of nucleation was instead considerably influenced by different cooling rates and different solvents. A correlation was found between the supersaturation ratio at the average onset of nucleation and the viscosity of the solvent divided by the solubility of the solute. The trends suggest that an increased molecular mobility and a higher concentration of the solute reduce the metastable zone width of salicylamide. / QC 20100831

Ce travail, sur la dépollution de l’eau par OVHC sur charbon actif (CA), étudie l'interaction oxydation - adsorption : comment l'oxydation sur charbon de produits aromatiques modifie ses propriétés et en particulier sa capacité d'adsorption et d'oxydation (désactivation). Divers charbons commerciaux, mais aussi issus de boues activées, ont été caractérisés avant et après oxydation : propriétés texturales, composition élémentaire (CHNSO), fonctions de surface, pH au point de charge nulle et allure des courbes ATG. Seule leur teneur en cendres semble différer notablement. Les charbons actifs de boues ont une surface spécifique bien moindre (de 90 à 265 m².g-1) et un taux de cendres supérieur à 50% qui en font de moins bons adsorbants. Cependant la présence de métal, notamment le fer, a un effet positif sur l'oxydation catalytique. L'étude comparative en oxydation catalytique de 4 CA commerciaux en grains montre une désactivation intense (qui augmente avec la concentration de polluant) sur les 3-4 premières utilisations, puis une stabilisation. Le charbon résultant est nettement moins adsorbant. Les CA S23 et F22 ainsi stabilisés ont ensuite été utilisés pour déterminer la cinétique d'oxydation catalytique apparente et intrinsèque du phénol, grâce à un modèle incluant la diffusion. La comparaison de l'OVHC de 4 polluants aromatiques seuls et en divers mélanges synthétiques met en évidence des effets de mélange significatifs: les polluants les plus réfractaires en oxydation catalytique se dégradent beaucoup plus vite en mélange (ex 4NP) et inversement, les polluants les plus oxydés, lorsqu'ils sont seuls en solution, voient leur cinétique diminuer lorsqu'ils sont mélangés aux autres polluants (ex phénol, 4chlorophénol). Le mélange tend donc à uniformiser les comportements particuliers. Enfin, les essais de faisabilité du procédé de régénération oxydante AD-OX ont été effectués sur des effluents industriels. La régénération du charbon actif comme adsorbant est moins limitée qu'avec des mélanges de phénols substitués. / This work, on water treatment by catalytic Wet Air Oxidation (CWAO) using activated carbon (AC), aimed to study the interaction between oxidation-adsorption phenomena: how the oxidation of aromatic compounds on AC can modify its properties and specially its adsorption and oxidation characteristics (deactivation). Various commercials AC, and AC produced from activated sludge, have been characterized before and after oxidation by according methods such as textural characterization, elemental analysis (CHNSO), functional groups on carbon surfaces analysis, pH of the point of zero charge (pHpzc) analysis and thermogravimetric analysis (TGA); however, only their ash level was significantly different. In addition, the sludge based AC (SBAC) have a specific surface range of 90 to 265 m².g-1, which is lower than commercial AC, and with an ash level over 50%. Consequently they are less good adsorbents. However, the presence of metal, e.g. iron, has a positive affect on catalytic oxidation. The comparative study of catalytic oxidation with 4 commercial AC, show a severe deactivation (which increase with the pollutant concentration) on the first 3-4 utilizations before stabilization. The residual AC is clearly poor adsorbent. After stabilization, 2 commercials AC, only S23 and F22 CA, were studied on catalytic oxidation of phenol in term of apparent and intrinsic kinetics with including the diffusion effect in the model. The CWAO of 4 pure aromatic pollutants, and their various synthetic mixtures, showed mixing effects significantly: the most oxidation catalytic refractory pollutants were degraded rapidly when they mixed (e.g. 4-nitrophenol) but the kinetic was slow down for the pure pollutant whereas, the most oxidised pollutants, for pure solution, have an oxidation kinetic slower than when they mixed with others pollutants (e.g. Phenol, 4-chlorophenol). Therefore, the mixture tends to standardize from the individual behaviour. In conclude, the feasibility study of the oxidative regeneration process (AD-OX) was performed on industrial waste water. The regeneration AC as adsorbent is less limited than with mixtures of substituted phenols.

This work concerns the interrelationship between thermodynamic, kinetic and structural aspects of crystal polymorphism. It is both experimental and theoretical, and limited with respect to compounds to substituted monocyclic aromatics. Two polymorphs of the compound m-aminobenzoic acid have been experimentally isolated and characterized by ATR-FTIR spectroscopy, X-ray powder diffraction and optical microscopy. In addition, two polymorphs of the compound m-hydroxybenzoic acid have been isolated and characterized by ATR-FTIR spectroscopy, high-temperature XRPD, confocal Raman, hot-stage and scanning electron microscopy. For all polymorphs, melting properties and specific heat capacity have been determined calorimetrically, and the solubility in several pure solvents measured at different temperatures with a gravimetric method. The solid-state activity (ideal solubility), and the free energy, enthalpy and entropy of fusion have been determined as functions of temperature for all solid phases through a thermodynamic analysis of multiple experimental data. It is shown that m-aminobenzoic acid is an enantiotropic system, with a stability transition point determined to be located at approximately 156°C, and that the difference in free energy at room temperature between the polymorphs is considerable. It is further shown that m-hydroxybenzoic acid is a monotropic system, with minor differences in free energy, enthalpy and entropy. 1393 primary nucleation experiments have been carried out for both compounds in different series of repeatability experiments, differing with respect to solvent, cooling rate, saturation temperature and solution preparation and pre-treatment. It is found that in the vast majority of experiments, either the stable or the metastable polymorph is obtained in the pure form, and only for a few evaluated experimental conditions does one polymorph crystallize in all experiments. The fact that the polymorphic outcome of a crystallization is the result of the interplay between relative thermodynamic stability and nucleation kinetics, and that it is vital to perform multiple experiments under identical conditions when studying nucleation of polymorphic compounds, is strongly emphasized by the results of this work. The main experimental variable which in this work has been found to affect which polymorph will preferentially crystallize is the solvent. For m-aminobenzoic acid, it is shown how a significantly metastable polymorph can be obtained by choosing a solvent in which nucleation of the stable form is sufficiently obstructed. For m-hydroxybenzoic acid, nucleation of the stable polymorph is promoted in solvents where the solubility is high. It is shown how this partly can be rationalized by analysing solubility data with respect to temperature dependence. By crystallizing solutions differing only with respect to pre-treatment and which polymorph was dissolved, it is found that the immediate thermal and structural history of a solution can have a significant effect on nucleation, affecting the predisposition for overall nucleation as well as which polymorph will preferentially crystallize. A set of polymorphic crystal structures has been compiled from the Cambridge Structural Database. It is found that statistically, about 50% crystallize in the crystallographic space group P21/c. Furthermore, it is found that crystal structures of polymorphs tend to differ significantly with respect to either hydrogen bond network or molecular conformation. Molecular mechanics based Monte Carlo simulated annealing has been used to sample different potential crystal structures corresponding to minima in potential energy with respect to structural degrees of freedom, restricted to one space group, for each of the polymorphic compounds. It is found that all simulations result in very large numbers of predicted structures. About 15% of the predicted structures have excess relative lattice energies of <=10% compared to the most stable predicted structure; a limit verified to reflect maximum lattice energy differences between experimentally observed polymorphs of similar compounds. The number of predicted structures is found to correlate to molecular weight and to the number of rotatable covalent bonds. A close study of two compounds has shown that predicted structures tend to belong to different groups defined by unique hydrogen bond networks, located in well-defined regions in energy/packing space according to the close-packing principle. It is hypothesized that kinetic effects in combination with this structural segregation might affect the number of potential structures that can be realized experimentally. The experimentally determined crystal structures of several compounds have been geometry-optimized (relaxed) to the nearest potential energy minimum using ten different combinations of common potential energy functions (force fields) and techniques for assigning nucleus-centred point charges used in the electrostatic description of the energy. Changes in structural coordinates upon relaxation have been quantified, crystal lattice energies calculated and compared with experimentally determined enthalpies of sublimation, and the energy difference before and after relaxation computed and analysed. It is found that certain combinations of force fields and charge assignment techniques work reasonably well for modelling crystal structures of small aromatics, provided that proper attention is paid to electrostatic description and to how the force field was parameterized. A comparison of energy differences for randomly packed as well as experimentally determined crystal structures before and after relaxation suggests that the potential energy function for the solid state of a small organic molecule is highly undulating with many deep, narrow and steep minima. / QC 20110527