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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
11

Infrared-Visible Sum Frequency Genergation Studies of Water at the Polymer/Sapphire Interface

Zhou, Jing 20 September 2013 (has links)
No description available.
12

Grammar teaching - to be or not to be, that is the question : En kvalitativ studie om grammatikundervisningens plats i språkundervisning i grundskolans tidigare år

Noresund, Lilia January 2010 (has links)
This research traces the trajectory of those methods of teaching grammar that five different teachers in Swedish from Stockholm municipality use in their practice. Their both constructive and negative attitudes towards grammar were in fokus when looking after the connection between their own experience of learning/teaching native language grammar and the methods that they are actually using in their own classes. I was trying to gain knowledge and achieve the purpose of this research by means of a qualitativ method. My empirical data consists of five interviews with the pedagogues who are teaching Swedish. I was also reviewing the scientific research related to the topic. This study has scratched the surface of the systemic funktional grammar (SFG) pedagogy. SFG is a model of grammar developed by the British linguist Michael Halliday. The meta-language informed by SFG seems to be the keyword of the model because of its implicit meaning. Only one of five teachers has been incorporating SFG into the teaching of history. When working with the third grade pupils the teacher has been using the modeling and deconstruktion of the kids own texts. the other four teachers have never heard about SFG before. The methods they are using are not updated that much, it is also easy to notice that they stich to the teaching of grammar as a separate part of language.
13

Réseaux ordonnés de nanoparticules étudiés par la combinaison de microscopie STM et spectroscopie SFG / Oordered arrays of nanoparticles studied by the combination of STM microscopy and SFG spectroscopy

Zakaria, Abdoul mouize 30 May 2018 (has links)
Ce manuscrit décrit l’élaboration d’un réseau dense et ordonné de nanoparticules épitaxiées sur couche mince d’oxyde (Al₂O₃) préparée sur un cristal de Ni₃Al (111). Des nanoparticules de Pd, et des nanoparticules à cœur de palladium et coquille d’or, ont été réalisées avec une distribution de taille étroite, grâce au fait que les centres de nucléation sont régulièrement espacés. La couche mince d’oxyde, puis le réseau de nanoparticules, sont étudiés par microscopie STM en fonction des paramètres d’oxydation du substrat et de dépôt de Pd, en particulier les températures d’oxydation et recuit, et le flux d’atomes de palladium. Au-delà de 1 ML de Pd et 1 ML d’or, le réseau devient progressivement désordonné.En plus de la microscopie STM, la faible distribution de taille permet d’étudier les nanoparticules par des mesures spectroscopiques d’ensemble : la spectroscopie vibrationnelle par génération de fréquence somme (SFG) est utilisée pour caractériser la distribution des sites d’adsorption en fonction de la taille et de la nature chimique des nanoparticules ; la spectroscopie de réflectance différentielle (DRS) est utilisée pour caractériser les propriétés plasmoniques. La SFG permet de discriminer les sites de facettes et d’arêtes ; ceci permet de montrer que les nanoparticules croissent rapidement en hauteur. Sur les particules cœur-coquille, la SFG permet d’observer la ségrégation d’atomes de Pd à travers la coquille d’or, les sites de Pd et d’or se distinguant par la fréquence de CO. CO s’adsorbe sélectivement sur les atomes de Pd à pression modérée (inférieure à 10-3 mbar), puis également sur les atomes d’or aux pressions élevées (1 mbar). De plus, à haute pression CO induit la ségrégation, réversible thermiquement, d’un grand nombre d’atomes de Pd.La DRS permet d’observer l’apparition d’une résonance plasmon pour une coquille plus large que 3 ML d’or. La résonance est d’autant plus intense que le cœur de Pd est petit, et la coquille épaisse. Mais ne devient vraiment intense qu’au voisinage de la coalescence des NPs.Ces expériences ouvrent la voie à des expériences plus fines, notamment pour sonder plus précisément les propriétés en fonction de la taille, ou l’adsorption de CO en présence d’oxygène. Elles ouvrent aussi la voie à l’étude ultérieure de l’adsorption de molécules organiques sur le réseau de nanoparticules. L’adsorption de molécules en pont sera recherchée. Le transfert d’électron dans le réseau hybride ainsi constitué sera étudié, des expériences pompe-sonde où des électrons seront excités dans les nanoparticules. La SFG permettra de sonder l’état transitoire des molécules sous l’effet du transfert de charge. / This manuscript describes the development of a dense and ordered network of epitaxially grown nanoparticles on an oxide thin-film (Al₂O₃) prepared on a Ni₃Al (111) crystal. Pd nanoparticles, and nanoparticles with palladium core and gold shell, were made with a narrow size distribution, thanks to the fact that the nucleation centers are regularly spaced. The thin oxide layer and then the nanoparticle network are studied by STM microscopy according to the substrate oxidation and Pd deposition parameters, in particular the oxidation and annealing temperatures, and the flow of palladium atoms. Beyond 1 ML of Pd and 1 ML of gold, the network becomes progressively disordered.In addition to STM microscopy, the small size distribution makes it possible to study the nanoparticles by means of overall spectroscopic measurements: the sum frequency generation (SFG) vibrational spectroscopy is used to characterize the distribution of the adsorption sites according to the size and chemical nature of the nanoparticles; differential reflectance spectroscopy (DRS) is used to characterize plasmonic properties. SFG discriminates between facet and edge sites, allowing to show that the nanoparticles grow rapidly in height. On core-shell particles, SFG allows to observe the segregation of Pd atoms across the gold shell, with Pd and gold sites distinguished by CO frequency. CO is selectively adsorbed on Pd atoms at moderate pressure (less than 10-3 mbar), then also on Au atoms at high pressures (1 mbar). Moreover, at high pressure CO induces the thermally reversible segregation of a large number of Pd atoms.The DRS makes it possible to observe the appearance of a plasmon resonance for a shell wider than 3 ML of gold. The resonance is more intense as the core of Pd is small, and the shell thick. But only becomes really intense in the vicinity of the coalescence of NPs.These experiments pave the way for finer experiments, in particular to probe more precisely the properties as a function of size, or the adsorption of CO in the presence of oxygen. They also open the way for the subsequent study of the adsorption of organic molecules on the nanoparticle network. The adsorption of molecules in bridge between nanoparticles will be sought. The electron transfer in the hybrid network thus formed will be studied using pump-probe experiments where electrons will be excited in the nanoparticles. SFG will probe the transient state of molecules under the effect of charge transfer.
14

Hur sätter vi ord på vår artros på nätet? : En jämförande analys av processer, deltagare och omständigheter relaterat till besvär med artros / How do we describde our osteoarthritis online? : A comparative analysis about processes, participants and circumstances related to complaints of osteoarthritis.

Lundberg, Sara January 2022 (has links)
Background: Today osteoarthritis is considered a public disease, meanwhile good health literacy is directly connected to high quality of life. This study examines how this can be worded in posts online and in texts from two different health care institutions websites. Purpose: To differentiate between how complaints of osteoarthritis is worded by health care institutions, in this case 1177.se and Kry.se, versus posts online made by private persons. Method: SFG together with a transitivity analysis, where extra focus was on processes, participants, and circumstances. Results: The complaints are more commonly worded as materiel processer and with more explicit participants in the texts collected from the health care institutions and in the online posts there were more of both materiel and relational processes and the participants can sometimes be implicit.
15

Ab initio molecular dynamics study of ion and pH effects at silica/liquid water interfaces : structure, acid-base reactivity and vibrational spectroscopy / Etude par dynamique moléculaire ab initio des effets d'ions et de pH aux interfaces silice/eau liquide : structure, réactivité acido-basique et spectroscopie vibrationnelle

Pfeiffer-Laplaud, Morgane 16 September 2016 (has links)
L'interface (0001) alpha-quartz hydroxylé/eau liquide est modélisée par dynamique moléculaire dans le formalisme de la théorie de la fonctionnelle de la densité (DFT-MD). Poursuivant une étude déjà publiée sur la structure et la réactivité acido-basique de cette interface, nous élargissons l'analyse aux effets d'ions simples et de pH sur la structure et les propriétés de l'interface avec une attention particulière aux phénomènes d'adsorption ainsi qu'aux modifications de l'eau interfaciale et des sites de surface. Nous caractérisons en particulier les changements dans la réactivité de surface dus à la présence d'ions (cations alcalins et halogénures) par calcul direct de pKa et détaillons la structure de la double couche électrique dans le cas de paires d'ions.Nous cherchons de plus à calculer les spectres SFG (Sum Frequency Generation) vibrationnels à ces interfaces, ce qui serait une première pour une interface solide/liquide traitée au niveau DFT-MD. L'approche théorique de cette spectroscopie vibrationnelle non linéaire du second ordre permettrait de proposer une interprétation claire des bandes d'élongation O-H, alors que les études expérimentales continuent à diverger sur ce sujet. / We use Density-Functional-Theory-based molecular dynamics simulations to investigate the hydroxylated (0001) alpha-quartz/liquid water interface. As a follow up of an already published study on the structural and acid/base properties of this interface, we now focus on simple ion and pH effects on these properties and characterize adsorption behaviors and interfacial changes on both solid and liquid sides. In particular, we directly calculate surface pKa's in presence of ions (alkaline cations and/or halide anions) and provide microscopic details on the structure of the electric double layer when ion pairs are concerned.Besides, we try to apply DFT-MD simulations to the computation of vibrational Sum-Frequency Generation spectra at a solid/liquid interface. Indeed, calculations would be necessary to provide a clear interpretation of the vibrational bands in the OH stretching region since experimental band assignment is still a matter of debate.
16

Den tunna blå linjen : En kritisk diskursanalys av hur polis respektive medborgare konstrueras i polisens inlägg på Instagram

Åhlander, Isak January 2021 (has links)
Den svenska polisens användande av sociala medier är en viktig del i deras arbete med att främja rättvisa och trygghet. På sina 250 officiella konton på Facebook, Instagram och Twitter berättar de om sitt vardagliga arbete, med syfte att bidra till ökad trygghet bland allmänheten. Denna studie intresserar sig för dessa berättelser eftersom de kan ge en inblick i den bild polisen målar upp av sig själva och av allmänheten, vilket är intressant både på grund av deras roll som myndighet och mot bakgrund av den kritik som riktats mot sättet som de använder sociala medier på. Med kritisk diskursanalys som teoretisk och metodologisk ansats undersöker denna studie hur polis respektive medborgare konstrueras i polisens Instagram-inlägg och vilken relation detta skapar mellan dem. Analysmaterialet, bestående av 6 Instagram-inlägg från officiella poliskonton runt om i landet, analyseras med hjälp av SFG:s transitivitetssystem samt substantivistiska benämningar, vars resultat sedan diskuteras utifrån Faircloughs tredimensionella modell. Studiens resultat visar bland annat att inläggens form stämmer väl överens med polisens eget syfte med sitt användande av sociala medier. Resultaten visar också hur polisen både konstrueras som en självständig och aktivt brottsbekämpande myndighet och som aktiva brottsbekämpande individer medan medborgarna i stället konstrueras som gärningspersoner eller som en del av en allmänhet i behov av polisens skydd.
17

Nonlinear optical spectroscopic studies of dense gases, supercritical fluid solutions, and self-assembled monolayer interfaces

Rotondaro, Matthew C. 04 November 2022 (has links)
Three types of nonlinear optical spectroscopies, ultrafast two-dimensional infrared (2DIR) spectroscopy, transient infrared (IR) absorption/pump-probe spectroscopy, and sum-frequency generation (SFG) vibrational spectroscopy, are used to investigate molecular structure and dynamics in two distinct classes of materials. First, 2DIR and pump-probe spectroscopies are used to study ultrafast rotational and vibrational energy relaxation in dense gaseous and supercritical fluid solutions, special solvation effects near the critical point, and the evolution of cooperative, liquid-phase dynamics as a function of density for two different solvent systems. 2DIR’s demonstrated capabilities offer a unique tool for identifying co-existing free rotor and liquid-like populations within the same fluid sample, evaluating the adequacy of isolated binary collision (IBC) relaxation descriptions in dense gas and near-liquid density fluids, and learning about how solute-solvent intermolecular properties separately influence rotational and vibrational relaxation in these dynamic and heterogeneous environments. Analysis of the density-dependent 2DIR and pump-probe spectra of a quasi-free rotor (asymmetric stretch rovibrational band of N2O) in SF6 and Xe provides timescales for rotational energy relaxation rates (1 – 3 collisions), but much slower vibrational energy relaxation rates. A critical slowing effect on the rate of rotational relaxation is identified, and liquid-like solvation is observed in dense gaseous solutions at state points lower than the critical density. Solvent-dependent differences in energy relaxation and IBC model breakdown, as well as applications of this 2DIR methodology to other high density and supercritical solution dynamics and descriptions are discussed. In a second nonlinear spectroscopy project, SFG is used to study the role of substrate type, gold or silver, and surface roughness on the parity odd-even effect in n-alkanethiolate (n = 10 – 16) self-assembled monolayers (SAMs), materials of potential importance to molecular scale electronics. SFG methyl vibrational transition intensities, frequencies, and linewidths display parity and metal dependence attributable to the orientational differences of the interfacial ethyl group, which inverts for SAMs on Ag substrates relative to SAMs on Au. Substrate roughness, an often-underreported experimental parameter, is shown here to affect the extent of odd-even methyl orientation anisotropy, and this SFG analysis establishes a new roughness limit for the appearance of odd-even effects on Ag substrates.
18

Kontrastive Studie der Übersetzung schwedischer Modalverben in Viveca Stens Roman Offermakaren ins Deutsche : Kann eine Klassifizierung nach der Systemisch-Funktionellen Grammatik ein Muster aufzeigen? / A Contrastive Study on the German Translation of Swedish Modal Verbs in Viveca Sten’s Novel Offermakaren : Could a categorization based on Systemic Functional Grammar indicate a pattern?

Böiers, Ulrika January 2024 (has links)
Inom den systemisk-funktionella grammatiken delas modalitet in i fyra olika modalitetstyper. De är SANNOLIKHET, VANLIGHET, FÖRPLIKTELSE och VILLIGHET. I denna kontrastiva studie undersöks hur de tre svenska modalverben böra, skola och kunna tagna ur en svensk nutidsroman kan klassificeras i dessa modalitetstyper. Därefter undersöks hur de svenska modalverben har översatts i den tyska utgåvan av romanen. Målet med denna undersökning är att ta reda på om det finns någon regelbundenhet i vilket tyskt modalverb som används i översättningen av ett visst modalverb inom en viss modalitetstyp. Resultatet av denna studien är att det inte går att påvisa ett helt entydigt mönster som kan kopplas till ett enskilt modalverb och en särskild modalitätstyp. Däremot ses tendenser som kan undersökas närmre med hjälp av ytterligare data.
19

Mecanismos de interação molecular de polieletrólitos antimicrobianos em membranas modelo por espectroscopia vibracional não linear / Mechanisms of molecular interaction between antimicrobial polyelectrolytes and membrane models by nonlinear vibrational spectroscopy

Rimoli, Caio Vaz 04 August 2015 (has links)
Pesquisa em novas moléculas e estratégias antimicrobianas é crucial devido ao aumento de resistência a antibióticos pelos microrganismos. Polímeros antimicrobianos tem várias vantagens quando comparados a outros biocidas pequenos: maiores tempo de vida, potência, especificidade e baixa toxicidade residual. Logo, outras aplicações tecnológicas como recobrimentos, embalagens ou produtos têxteis antimicrobianos poderem ser exploradas. Em particular, derivados hidrossolúveis de quitosana, como os oligômeros de quitosana (OQ), são biopolímeros catiônicos extraídos de fontes renováveis que são candidatos promissores a serem agentes antimicrobianos de amplo espectro (fungos, bactérias gram-positivas e bactérias gram-negativas). Diferentemente da quitosana, que é sobretudo bioativa em pHs ácidos, OQ permanece catiônico – e portanto ativo – em pH fisiológico. Não obstante, o mecanismo exato pelo qual o polímero age nas membranas celulares permanece desconhecido em nível molecular. Este trabalho visa investigar o mecanismo de interação entre os OQ e modelos de membrana biomiméticos (Filmes de Langmuir). Para comparação, outro polieletrólito catiônico sintético com propriedades antibacterianas, o PAH – poli(hidrocloreto de alilamina) – foi investigado. Nós realizamos a Espectroscopia por Geração de Soma de Frequência (SFG) em Filmes de Langmuir de fosfolipídeos em água pura e em subfases contendo antimicrobianos. A Espectroscopia SFG nos permite obter o espectro vibracional de moléculas interfaciais (filme lipídico e moléculas que estão interagindo com ele: água e antimicrobianos) sem nenhuma contribuição de moléculas do interior do volume e é muito sensível às conformações lipídicas da membrana. Um fosfolipídeo zwitteriônico (DPPC) foi usado para modelar membranas tipo-humana, enquanto outro carregado negativamente (DPPG) modelava a tipo-bacteriana. Isotermas em subfases contendo antimicrobianos mostraram que ambos PAH e OQ causam uma pequena expansão das monocamadas de DPPC. Entretanto, para as monocamadas de DPPG ambos os polieletrólitos geraram uma expansão significativa. Entre eles, os OQ causaram um efeito mais drástico. Espectros SFG dos estiramentos CH mostraram que a conformação lipídica permaneceu bem empacotada em todos os casos (ligeiramente menos ordenada com PAH) apesar das expansões da membrana. Isto indica que os OQ foram inseridos formaram ilhas de OQ dentro do filme lipídico. Mudanças na forma de linha dos estiramentos da água interfacial indicaram que a adsorção de PAH em ambos os filmes foram capazes de compensar as cargas negativas, gerando uma inversão de cargas na superfície. Os espectros SFG dos grupos fosfato também indicaram que, em água pura, as cabeças polares de DPPC estão com uma orientação mais ordenada do que no caso do DPPG. Contudo, quando interagindo com os polieletrólitos catiônicos, as cabeças dos DPPGs se ordenam, ficando preferencialmente perpendicular à interface. Experimentos com antimicrobianos injetados na subfase enquanto os filmes de Langmuir já estavam condensados indicaram que os OQ foram capazes de penetrar na monocamada, embora causando uma expansão no filme menor. Esta comparação evidencia que a escolha da metodologia experimental afeta o resultado, mas ambas podem ser complementares, visto que podem representar diferentes fases do ciclo celular das biomembranas. A visão detalhada provida aqui para as interações moleculares desses polieletrólitos com filmes lipídicos podem os elucidar mecanismos de atividade biocida deles e auxiliar no planejamento racional de novos polímeros antimicrobianos. / Research on new antimicrobial molecules and strategies is crucial due to the increasing microorganism resistance to antibiotics. Antimicrobial Polymers have many advantages when compared to other small biocides: increased lifetimes, potency, specificity and lower residual toxicity. Therefore, they have great potential for technological applications, such as antimicrobial coatings, packages, or textile products. In particular, water-soluble derivatives of chitosan, such as chitosan oligomers (CO), are cationic biopolymers obtained from renewable sources that are promising candidates to a wide-spectrum antimicrobial agent (fungi, gram positive and gram negative bacteria). Unlike chitosan, which is mainly bioactive at acidic pH, CO remain cationic - and therefore active - at physiological pH. Nevertheless, the exact mechanism by which this polymer acts on the cell membranes remains unknown at the molecular level. This work aims at investigating the molecular interaction between CO and a biomimetic cell membrane model (Langmuir Film). For comparison, another synthetic cationic polylelectrolyte with antibacterial properties, PAH – poly(alylamine hydrochloride), has been investigated. We have carried out Sum-Frequency Generation (SFG) Spectroscopy on Langmuir Films of phospholipids on pure water and on antimicrobial containing subphases. SFG Spectroscopy allows obtaining the vibrational spectrum of interfacial molecules (lipid Langmuir Film and molecules interacting with it – water and antimicrobials), without any contribution from the bulk molecules, and is quite sensitive to the conformation of membrane lipids. A zwitterionic phospholipid (DPPC) was used to model human-like membranes, while a negatively charged phospholipid (DPPG) modeled bacterial-like membranes. Surface pressure-area isotherms on antimicrobial-containing subphases showed that both PAH and CO led to a small expansion of DPPC monolayers. However, for DPPG monolayers both polyelectrolytes led to significant expansion, with CO causing a more dramatic effect. SFG spectra in the CH stretch range showed that the lipid chain conformation remained always well ordered in all cases (slightly less ordered upon interacting with PAH), despite membrane expansion. This indicates that CO were inserted in the monolayer, forming islands of CO within the lipid film. Changes in the SFG spectral lineshape of OH stretches for the interfacial water molecules indicated that PAH adsorption on both DPPC and DPPG films was able to overcompensate the lipid negative charge and led to an overall surface charge reversal. The SFG spectra of the phosphate groups also indicated that in pure water the DPPC headgroups had a more ordered orientation than in the case of DPPG. Nevertheless, upon interaction with the cationic polyelectrolytes, the DPPG headgroups also become ordered, with a preferential orientation towards the subphase. Experiments with the antimicrobials injected in the subphase under a condensed Langmuir film indicated that CO were also capable of monolayer penetration, albeit causing a reduced film expansion. This comparison indicates that the choice of experimental methodology affects the outcome, but both may be complementary, as they may represent different phases of a biomembrane lifecycle. The detailed view provided here for the molecular interaction of these polyelectrolytes with lipid films may shed light on the mechanism of their biocidal activity and aid on a rational design of new antimicrobial polymers.
20

Mecanismos de interação molecular de polieletrólitos antimicrobianos em membranas modelo por espectroscopia vibracional não linear / Mechanisms of molecular interaction between antimicrobial polyelectrolytes and membrane models by nonlinear vibrational spectroscopy

Caio Vaz Rimoli 04 August 2015 (has links)
Pesquisa em novas moléculas e estratégias antimicrobianas é crucial devido ao aumento de resistência a antibióticos pelos microrganismos. Polímeros antimicrobianos tem várias vantagens quando comparados a outros biocidas pequenos: maiores tempo de vida, potência, especificidade e baixa toxicidade residual. Logo, outras aplicações tecnológicas como recobrimentos, embalagens ou produtos têxteis antimicrobianos poderem ser exploradas. Em particular, derivados hidrossolúveis de quitosana, como os oligômeros de quitosana (OQ), são biopolímeros catiônicos extraídos de fontes renováveis que são candidatos promissores a serem agentes antimicrobianos de amplo espectro (fungos, bactérias gram-positivas e bactérias gram-negativas). Diferentemente da quitosana, que é sobretudo bioativa em pHs ácidos, OQ permanece catiônico – e portanto ativo – em pH fisiológico. Não obstante, o mecanismo exato pelo qual o polímero age nas membranas celulares permanece desconhecido em nível molecular. Este trabalho visa investigar o mecanismo de interação entre os OQ e modelos de membrana biomiméticos (Filmes de Langmuir). Para comparação, outro polieletrólito catiônico sintético com propriedades antibacterianas, o PAH – poli(hidrocloreto de alilamina) – foi investigado. Nós realizamos a Espectroscopia por Geração de Soma de Frequência (SFG) em Filmes de Langmuir de fosfolipídeos em água pura e em subfases contendo antimicrobianos. A Espectroscopia SFG nos permite obter o espectro vibracional de moléculas interfaciais (filme lipídico e moléculas que estão interagindo com ele: água e antimicrobianos) sem nenhuma contribuição de moléculas do interior do volume e é muito sensível às conformações lipídicas da membrana. Um fosfolipídeo zwitteriônico (DPPC) foi usado para modelar membranas tipo-humana, enquanto outro carregado negativamente (DPPG) modelava a tipo-bacteriana. Isotermas em subfases contendo antimicrobianos mostraram que ambos PAH e OQ causam uma pequena expansão das monocamadas de DPPC. Entretanto, para as monocamadas de DPPG ambos os polieletrólitos geraram uma expansão significativa. Entre eles, os OQ causaram um efeito mais drástico. Espectros SFG dos estiramentos CH mostraram que a conformação lipídica permaneceu bem empacotada em todos os casos (ligeiramente menos ordenada com PAH) apesar das expansões da membrana. Isto indica que os OQ foram inseridos formaram ilhas de OQ dentro do filme lipídico. Mudanças na forma de linha dos estiramentos da água interfacial indicaram que a adsorção de PAH em ambos os filmes foram capazes de compensar as cargas negativas, gerando uma inversão de cargas na superfície. Os espectros SFG dos grupos fosfato também indicaram que, em água pura, as cabeças polares de DPPC estão com uma orientação mais ordenada do que no caso do DPPG. Contudo, quando interagindo com os polieletrólitos catiônicos, as cabeças dos DPPGs se ordenam, ficando preferencialmente perpendicular à interface. Experimentos com antimicrobianos injetados na subfase enquanto os filmes de Langmuir já estavam condensados indicaram que os OQ foram capazes de penetrar na monocamada, embora causando uma expansão no filme menor. Esta comparação evidencia que a escolha da metodologia experimental afeta o resultado, mas ambas podem ser complementares, visto que podem representar diferentes fases do ciclo celular das biomembranas. A visão detalhada provida aqui para as interações moleculares desses polieletrólitos com filmes lipídicos podem os elucidar mecanismos de atividade biocida deles e auxiliar no planejamento racional de novos polímeros antimicrobianos. / Research on new antimicrobial molecules and strategies is crucial due to the increasing microorganism resistance to antibiotics. Antimicrobial Polymers have many advantages when compared to other small biocides: increased lifetimes, potency, specificity and lower residual toxicity. Therefore, they have great potential for technological applications, such as antimicrobial coatings, packages, or textile products. In particular, water-soluble derivatives of chitosan, such as chitosan oligomers (CO), are cationic biopolymers obtained from renewable sources that are promising candidates to a wide-spectrum antimicrobial agent (fungi, gram positive and gram negative bacteria). Unlike chitosan, which is mainly bioactive at acidic pH, CO remain cationic - and therefore active - at physiological pH. Nevertheless, the exact mechanism by which this polymer acts on the cell membranes remains unknown at the molecular level. This work aims at investigating the molecular interaction between CO and a biomimetic cell membrane model (Langmuir Film). For comparison, another synthetic cationic polylelectrolyte with antibacterial properties, PAH – poly(alylamine hydrochloride), has been investigated. We have carried out Sum-Frequency Generation (SFG) Spectroscopy on Langmuir Films of phospholipids on pure water and on antimicrobial containing subphases. SFG Spectroscopy allows obtaining the vibrational spectrum of interfacial molecules (lipid Langmuir Film and molecules interacting with it – water and antimicrobials), without any contribution from the bulk molecules, and is quite sensitive to the conformation of membrane lipids. A zwitterionic phospholipid (DPPC) was used to model human-like membranes, while a negatively charged phospholipid (DPPG) modeled bacterial-like membranes. Surface pressure-area isotherms on antimicrobial-containing subphases showed that both PAH and CO led to a small expansion of DPPC monolayers. However, for DPPG monolayers both polyelectrolytes led to significant expansion, with CO causing a more dramatic effect. SFG spectra in the CH stretch range showed that the lipid chain conformation remained always well ordered in all cases (slightly less ordered upon interacting with PAH), despite membrane expansion. This indicates that CO were inserted in the monolayer, forming islands of CO within the lipid film. Changes in the SFG spectral lineshape of OH stretches for the interfacial water molecules indicated that PAH adsorption on both DPPC and DPPG films was able to overcompensate the lipid negative charge and led to an overall surface charge reversal. The SFG spectra of the phosphate groups also indicated that in pure water the DPPC headgroups had a more ordered orientation than in the case of DPPG. Nevertheless, upon interaction with the cationic polyelectrolytes, the DPPG headgroups also become ordered, with a preferential orientation towards the subphase. Experiments with the antimicrobials injected in the subphase under a condensed Langmuir film indicated that CO were also capable of monolayer penetration, albeit causing a reduced film expansion. This comparison indicates that the choice of experimental methodology affects the outcome, but both may be complementary, as they may represent different phases of a biomembrane lifecycle. The detailed view provided here for the molecular interaction of these polyelectrolytes with lipid films may shed light on the mechanism of their biocidal activity and aid on a rational design of new antimicrobial polymers.

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