O estudo de interfaces carregadas é de extrema importância para compreensão de fenômenos não só na área de polímeros e colóides, mas também em biologia molecular. Apesar das extensas pesquisas nessa área ainda há muito a ser estudado, visto que detalhes da interação íon-íon ainda não foram satisfatoriamente esclarecidos. Esse trabalho insere-se neste contexto abordando o efeito da inversão de carga em filmes de Langmuir de DMPA (ácido dimiristoilfosfatídico - um fosfolipídio aniônico) formados sobre uma solução aquosa de LaCl3. Já foram observados efeitos similares com DMPA espalhado em soluções de BaCl2 e CsI, entretanto, a inversão de carga para monocamadas de DMPA espalhadas em subfase contendo LaCl3 foi recentemente observada com uma concentração de cloreto de lantânio quatro ordens de grandezas menor que o previsto por teorias de correlação íoníon. Com o objetivo de confirmar a ocorrência do fenômeno de inversão de carga e obter informações estruturais adicionais, foram realizados experimentos de tensão superficial, potencial de superfície, de espectroscopia SFG (sum-frequency generation) e microscopia a ângulo de Brewster, além de cálculos teóricos da ionização da monocamada. Verificou-se que o acréscimo de íons trivalentes na subfase leva a formação de domínios nos filmes, despertando o interesse na obtenção de imagens SFG da superfície dos mesmos. Por isso, além do estudo realizado com os filmes de Langmuir de DMPA formados sobre subfases aquosas de LaCl3, também foi desenvolvido o projeto de um microscópio SFG. / The study of charged interfaces is important for understanding several phenomena in polymer and colloid science, as well as in molecular biology. Even though this topic has benn extensively investigated, there are many open issues, such as details of ion-ion interactions. In this context, this work investigates charge inversion in Langmuir films of DMPA (dimyristoilphosphatidic acid - an anionic phospholipid) spread on aqueous solutions of LaCl3. Similar effects have been previously observed for DMPA films on subphases containing BaCl2 and CsI, but charge inversion for DMPA monolayers spread on LaCl3 solutions has been recently reported for Lanthanum Chloride concentrations that are four orders of magnitude lower than predicted by ion-ion correlation theories. We have performed surface tension, surface potential, sum-frequency generation (SFG) spectroscopy and Brewster angle microscopy experiments, as well as calculations of monolayer ionization, with the goal of confirming charge inversion and providing additional structural information for this system. It was observed that the addition of trivalent ions in the subphase induced the formation of domains in the DMPA monolayers, sparking our interest in obtaining images by SFG microscopy of these films. Therefore, besides the study of DMPA Langmuir films on subphases containing LaCl3, we have also designed an SFG microscope.

O estudo de interfaces carregadas é de extrema importância para compreensão de fenômenos não só na área de polímeros e colóides, mas também em biologia molecular. Apesar das extensas pesquisas nessa área ainda há muito a ser estudado, visto que detalhes da interação íon-íon ainda não foram satisfatoriamente esclarecidos. Esse trabalho insere-se neste contexto abordando o efeito da inversão de carga em filmes de Langmuir de DMPA (ácido dimiristoilfosfatídico - um fosfolipídio aniônico) formados sobre uma solução aquosa de LaCl3. Já foram observados efeitos similares com DMPA espalhado em soluções de BaCl2 e CsI, entretanto, a inversão de carga para monocamadas de DMPA espalhadas em subfase contendo LaCl3 foi recentemente observada com uma concentração de cloreto de lantânio quatro ordens de grandezas menor que o previsto por teorias de correlação íoníon. Com o objetivo de confirmar a ocorrência do fenômeno de inversão de carga e obter informações estruturais adicionais, foram realizados experimentos de tensão superficial, potencial de superfície, de espectroscopia SFG (sum-frequency generation) e microscopia a ângulo de Brewster, além de cálculos teóricos da ionização da monocamada. Verificou-se que o acréscimo de íons trivalentes na subfase leva a formação de domínios nos filmes, despertando o interesse na obtenção de imagens SFG da superfície dos mesmos. Por isso, além do estudo realizado com os filmes de Langmuir de DMPA formados sobre subfases aquosas de LaCl3, também foi desenvolvido o projeto de um microscópio SFG. / The study of charged interfaces is important for understanding several phenomena in polymer and colloid science, as well as in molecular biology. Even though this topic has benn extensively investigated, there are many open issues, such as details of ion-ion interactions. In this context, this work investigates charge inversion in Langmuir films of DMPA (dimyristoilphosphatidic acid - an anionic phospholipid) spread on aqueous solutions of LaCl3. Similar effects have been previously observed for DMPA films on subphases containing BaCl2 and CsI, but charge inversion for DMPA monolayers spread on LaCl3 solutions has been recently reported for Lanthanum Chloride concentrations that are four orders of magnitude lower than predicted by ion-ion correlation theories. We have performed surface tension, surface potential, sum-frequency generation (SFG) spectroscopy and Brewster angle microscopy experiments, as well as calculations of monolayer ionization, with the goal of confirming charge inversion and providing additional structural information for this system. It was observed that the addition of trivalent ions in the subphase induced the formation of domains in the DMPA monolayers, sparking our interest in obtaining images by SFG microscopy of these films. Therefore, besides the study of DMPA Langmuir films on subphases containing LaCl3, we have also designed an SFG microscope.

<p>Lipids are important cellular biomolecules that perform essential functional and biological roles. For instance, lipids in the cell are the compartmentalizer for the cytoplasm and an energy storage unit. The knowledge surrounding lipids is abundant, yet there is still so much to uncover. There are many categories of lipids and within each category the structural composition is extremely diverse. In turn, the dramatic structural complexity of lipids demands analytical methods capable of providing in-depth structural characterization of individual molecular structures. However, lipid structural elucidation has remained challenging, namely due to the presence of isomeric and isobaric species with a complex mixture. In particular, isomeric/isobaric lipid structures arise from variations in class, headgroup, fatty acyl chain, <i>sn</i>-position, and/or carbon-carbon double bond (C=C) position(s). Recently, recent research suggests C=C composition impacts lipid physical properties, metabolic fate, and intermolecular interactions. Thus, analytical strategies capable of localizing sites of unsaturation are of interest in the lipidomics community.</p> <p>Mass spectrometry (MS) is a leading tool for lipid analysis. Electrospray ionization (ESI), a soft ionization method, is the most commonly used method for lipid ionization as a means of taking the ions from liquid-phase to gas-phase without extensive decomposition of the species. Utilizing ESI-MS, lipids can be identified at a sum compositional level via accurate mass measurements. . With tandem mass spectrometers, lipid ions can be further probed, utilizing tandem-MS (MS/MS) to generate structurally informative product ion spectra that facilitate the assignment of lipid molecular structure. More so, gas-phase ion/ion reactions represent a unique MS-based technique that has improved the analysis of lipids structures. Gas-phase ion/ion reactions allow for lipid species to be charge inverted from one polarity to the opposite polarity. This reaction enables lipids to be ionized in a polarity that is optimal for class identification and further investigated in the opposite polarity where more structural information is obtained. All the information provided is captured without the requirement of multiple solution conditions which is necessary when analyzing in both polarities. In the case of charge inverted lipids from positive ion mode to negative ion mode, fatty acyl composition can be obtained; however, C=C information is lacking.</p> <p>MS can also be paired with other analytically technologies to assist with lipid analysis. One of those technologies is liquid chromatography (LC), which allows for the separation of lipids based on different characteristic depending on the column type being used. Reverse-phase LC (RPLC) allows for the separation of lipid molecular species based on structural composition. RPLC-MS/MS benefits from the ability to separate lipids and determine their fatty acyl chain composition but it is difficult to specify C=C location with the use of a synthetic standard that is identical to each molecular species being analyzed.</p> <p>Commonality between the gas-phase ion/ion reactions for charge inversion of lipids and RPLC-MS/MS approaches is the inability to provide C=C coverage. In-solution and unique ion activation techniques have been developed for seeking such information. The Paternò–Büchi reaction is a UV-initiated [2 + 2]-cycloaddition of an excited carbonyl containing compound onto an olefin group. This reaction can be initiated onto the alkene group within an unsaturated lipid aliphatic chain to form an oxetane ring modification. There are two product ions that can be formed upon each unsaturation site due to a lack of regioselectivity the reagent can attach at either side of the C=C. The modified lipids can be taken into gas-phase and collisionally activated via low-energy collision induced dissociation, generating product ions indictive of C=C position(s). The work herein shows the incorporation of the PB reaction into the gas-phases ion/ion reaction and RPLC-MS/MS apparatuses for C=C localization. The methods have been applied to the lipid extracts of bovine liver and human plasma for confident molecule species determination.</p>

Experiments show that MgSO4 salt has a non-monotonic effect as a function of MgSO4 concentration on the ejection of DNA from bacteriophage lambda. There is a concentration, N0, at which the minimum amount of DNA is ejected. At lower or higher concentrations, more DNA is ejected. We propose that this non-monotonic behavior is due to the overcharging of DNA at high concentration of Mg⁺² counterions. As the Mg⁺² concentration increases from zero, the net charge of ejected DNA changes its sign from negative to positive. N0 corresponds to the concentration at which DNA is neutral. Our theory fits experimental data well. The DNA-DNA electrostatic attraction is found to be -0.004 kBT/nucleotide. Simulations of DNA-DNA interaction of a hexagonal DNA bundle support our theory. They also show the non-monotonic DNA-DNA interaction and reentrant behavior of DNA condensation by divalent counterions. Three problems in understanding the capsid assembly for a retrovirus are studied: First, the way in which the viral membrane affects the structure of in vivo assembled HIV-1 capsid is studied. We show that conical and cylindrical capsids have similar energy at high surface tension of the viral membrane, which leads to the various shapes of HIV-1 capsids. Secondly, the problem of RNA genome packaging inside spherical viruses is studied using RNA condensation theory. For weak adsorption strength of capsid protein, most RNA genomes are located at the center of the capsid. For strong adsorption strength, RNA genomes peak near the capsid surface and the amount of RNA packaged is proportional to the capsid area instead its volume. Theory fits experimental data reasonably well. Thirdly, the condensation of RNA molecules by nucleocapsid (NC) protein is studied. The interaction between RNA molecules and NC proteins is important for the reverse transcription of viral RNA which relates to the viral infectivity. For strong adsorption strength of the NC protein, there is a screening effect by RNA molecules around a single NC protein.