Sandra C.C. Nunes

Erythritol: Crystal growth from the melt

The structural changes occurring on erythritol as it is cooled from the melt to low temperature, and then heated up to the melting point have been investigated by differential scanning calorimetry (DSC), polarized light thermal microscopy (PLTM), X-ray powder diffraction (PXRD) and Fourier transform infrared spectroscopy (FTIR). By DSC, it was possible to set up the conditions to obtain an amorphous solid, a crystalline solid, or a mixture of both materials in different proportions. Two crystalline forms have been identified: a stable and a metastable one with melting points of 117 and 104 degrees C, respectively. The fusion curve decomposition of the stable form revealed the existence of three conformational structures. The main paths of the crystallization from the melt were followed by PLTM. The texture and colour changes allowed the characterization of the different phases and transitions in which they are involved on cooling as well as on heating processes. The type of crystallization front and its velocity were also followed by microscopic observation. These observations, together with the data provided by PXRD, allowed elucidating the transition of the metastable form into the stable one. The structural changes occurring upon the cooling and subsequent heating processes, namely those arising from intermolecular hydrogen bonds, were also accompanied by infrared spectroscopy. Particular attention was given to the spectral changes occurring in the OH stretching region.

Thermal Behaviour of Human Stratum Corneum

Purpose: To use high-speed differential scanning calorimetry (DSC) in the identification of transitions in human stratum corneum (SC). Several scanning rates (100°C/min to 400°C/min) are used. Results: Eight transitions from 0 to 120°C are detected in a significant number of samples. Most of these transitions have already been identified in previous studies, but have been labeled considering essentially that only four are present. Results also indicate some degree of reversibility for transitions occurring at temperatures above 90°C. Dehydrated SC samples displayed slightly more defined transition peaks and a less frequent presence of the transitions below 50°C. In turn, the delipidised SC matrix showed two major endothermic signals, centered around 55 and 100°C, in conjunction with other much less marked features. Conclusions: The interpretation of DSC traces in terms of four main transition temperatures must be complemented having in mind the occurrence of other transitions, some of them at physiological temperatures. This work further suggests that transitions at temperatures above 90°C may to a large degree be associated to lipids, while transition at approx. 55°C is probably related to lipids covalently linked to proteins, as previously suggested.

A study of the structure of the pindolol based on infrared spectroscopy and natural bond orbital theory.

Beta-adrenoceptor-blocking agents (beta-blockers) are on the list of the top selling drugs. Pindolol is a representative of this type of compound, either from the structural point of view, or as reference for comparison of the pharmacokinetic properties of the beta-blockers. A study of the pindolol structure based on infrared spectroscopy and natural bond orbital (NBO) theory is the main aim of the present research. FTIR spectra of the solid pindolol were recorded from 4000 to 400cm(-1), at temperatures between 25 and -170 degrees C. For spectral interpretation, the theoretical vibrational spectra of the conformer present in the solid was obtained at the B3LYP/6-31G* level of theory. NBO analysis of the reference conformer, before and after optimization, was carried out at the same level of theory referred above. Characteristic absorption vibrational bands of the spectra of solid pindolol and of the isolated conformer were identified. Intra- and intermolecular interactions in pindolol were confirmed by the frequency shift of the vibrational modes and by the NBO theory. A detailed molecular picture of pindolol and of its intermolecular interactions was obtained from spectroscopy and NBO theory. The combination of both methods gives a deeper insight into the structure.

Reactions of Nitrosoalkenes with Dipyrromethanes and Pyrroles: Insight into the Mechanistic Pathway

The reactivity of nitrosoalkenes toward dipyrromethanes, pyrrole, and 2,5-dimethylpyrrole is described. 1-(p-Bromophenyl)nitrosoethylene shows a different chemical behavior with these heterocycles than the previously reported reactions of ethyl nitrosoacrylate, which proceeds via a Diels-Alder reaction. 1-(p-Bromophenyl)nitrosoethylene reacts with dipyrromethanes and pyrrole to afford two isomeric oximes via conjugate addition followed by rearomatization of the pyrrole unit. On the other hand, this nitrosoalkene reacts with 2,5-dimethylpyrrole through an initial conjugate addition followed by intramolecular O- and N-nucleophilic addition with the formation of the corresponding bicyclic oxazine and five-membered cyclic nitrone, respectively. Quantum chemical calculations, at the DFT level of theory, indicate that the barriers associated with the Diels-Alder reactions of ethyl nitrosoacrylate are over 30 kJ/mol lower than those that would be required for the cycloadditions of 1-(p-bromophenyl)nitrosoethylene. Thus, calculations predict that the Diels-Alder reaction is privileged in the case of ethyl nitrosoacrylate and point to a different reaction pathway for 1-(p-bromophenyl)nitrosoethylene, corroborating the experimental findings.

Interpreting the Rich Behavior of Ternary DNA-PEI-Fe(III) Complexes

This work aims to shed light on the mechanism of interaction between components of ternary DNA-PEI-Fe(III) complexes, using experimental and theoretical approaches. In the experimental part, the chelation between PEI-Fe(III) was inspected by potentiometry and electrical conductance measurements and the respective importance for the condensation of DNA analyzed. To this end, three different mixing protocols for the components were imposed using different PEIs, branched (bPEI1.2 and bPEI10) and linear (lPEI2.5 and lPEI25). A delay in DNA condensation was observed when PEI and Fe(III) were premixed and then added to DNA. The set of observations was complemented by determination of the amount of Fe(III) included in the polyplexes, which was found to be dependent on the order of mixture and on the type of PEI used, decreasing with intrinsic PEI condensation efficiency. Overall, a coherent picture in which Fe(III) compensates PEI, probably modulating the respective charge, emerges. Some points arisen from the experimental part were rationalized using Monte Carlo simulations. Different architectured polycation (PC) chains were modeled and an interaction between PC and multivalent ions, mimicking the chelation of Fe(III) by the PEI, was imposed. It was found that chelation enhances polyanion (PA) compaction, irrespective of the PC architecture and charge density. The amount of multivalent ions in each polyplex compensates the negative charge unbalanced by the PC. The charge density and the ability of chelation of each PC dictate the disposition of each condensing agent along the PA backbone, and their coexistence strengthens PA compaction. The deep understanding of these ternary mixtures is a step forward in the optimization of such systems for application in gene delivery.

Conformational preferences of α,α-trehalose in gas phase and aqueous solution

This work presents an investigation on the conformational preferences of alpha,alpha-trehalose in gas phase and aqueous solution. Eighty-one systematically selected structures were studied at the B3LYP/6-311++G(d,p)//B3LYP/6-31G(d) level, giving rise to 40 unique conformers. The 19 lower energy structures and some selected other were further re-optimized at the B3LYP/6-311++G(d,p) level. The main factors accounting for the conformers stability were pointed out and discussed. NBO and QTAIM analyses were performed in some selected conformers in order to address the anomeric and exo-anomeric effects as well as intramolecular hydrogen bonding. The effect of solvent water on the relative stability of the conformers was accounted for by applying the conductor-like polarizable continuum model, CPCM.

A new perspective on correlated polyelectrolyte adsorption: Positioning, conformation, and patterns

This work focuses on multiple chain deposition, using a coarse-grained model. The phenomenon is assessed from a novel perspective which emphasizes the conformation and relative arrangement of the deposited chains. Variations in chain number and length are considered, and the surface charge in the different systems ranges from partially neutralized to reversed by backbone deposition. New tools are proposed for the analysis of these systems, in which focus is given to configuration-wise approaches that allow the interpretation of correlated multi-chain behavior. It is seen that adsorption occurs, with a minimal effect upon the bulk conformation, even when overcharging occurs. Also, chain ends create a lower electrostatic potential, which makes them both the least adsorbed region of the backbone, and the prevalent site of closer proximity with other chains. Additionally, adsorption into the most favorable region of the surface overrides, to a large degree, interchain repulsion.

Molecular interaction governing solubility and release profiles in supramolecular systems containing fenbufen, pluronics and cyclodextrins

In this work, the combination of two solubilizing agents, methyl-beta-cyclodextrin and Pluronic F68, is proposed and analyzed for a sparingly water soluble drug, fenbufen. Despite the large solubility enhancement achieved, ca. 70-fold, the synergistic effect promoted by the combination of these agents, is only effective over a narrow concentration range, being replaced by a competition between the drug and hydrophobic blocks of the copolymer when the concentration of the latter increases. Moreover, the detailed analysis of the release profiles, resorting to a model dependent approach, shows that the combination of these agents is a strong modulator of the overall profile and also of the total amount of drug that is released. Molecular dynamics simulation indicates a significant change in the cyclodextrin behavior, when alone in solution, situation in which the collapse of the conical structure is relatively frequent, or in the presence of each of the other components, and also shows that copolymer extension is decreased upon introduction of the drug, while there is an increase in extension when the cyclodextrin is added. Inclusion complexes are detected for both drug and copolymer. These play a definite role in solubilization by cyclodextrins, but are also responsible for the competitive behavior observed when polymeric micelles are present. Hydrophobic block copolymers compete with the drug for the cyclodextrin cavity through the formation of polypseudorotaxanes, which in turn modulates drug release. From the fundamental point of view, this work presents an in depth analysis of the molecular behavior in these systems, focusing on the cyclodextrin, inclusion complexes, polymeric micelles and supramolecular systems.

Expanding Transdermal Delivery with Lipid Nanoparticles: A New Drug-in-NLC-in-Adhesive Design

A monolithic drug-in-NLC-in-adhesive transdermal patch, with a novel design, was developed for codelivery of olanzapine (OL) and simvastatin (SV). Nanostructured lipid carriers (NLC) and enhancers were used as passive strategies, while the pretreatment of the skin with Dermaroller was tested as an active approach. The formulation was optimized for composition in a quality by design basis, in terms of enhancer and adhesive, with focus on permeation behavior, adhesion properties, and cytotoxicity. Propylene glycol promoted the best permeation rate for both drugs, with enhancement ratios of 8.1 and 12.9 for OL and SV, respectively, relative to the corresponding Combo-NLC patch without enhancer. Molecular dynamics results provided a rationale for these observations. The adhesive type displayed an important role in skin permeation, reinforced by the presence of the enhancer. Finally, Dermaroller pretreatment did not promote a significant improvement in permeation, which highlights the role of the combination of NLC with chemical enhancer in the transdermal patch as the main driving force in the process. It is also observed that NLC are able to reduce cytotoxicity, especially that associated with SV. This work provides a promising in vitro-in silico basis for a future in vivo development.

Confined polyelectrolytes: The complexity of a simple system.

The interaction between polyelectrolytes and counterions in confined situations and the mutual relationship between chain conformation and ion condensation is an important issue in several areas. In the biological field, it assumes particular relevance in the understanding of the packaging of nucleic acids, which is crucial in the design of gene delivery systems. In this work, a simple coarse‐grained model is used to assess the cooperativity between conformational change and ion condensation in spherically confined backbones, with capsides permeable to the counterions. It is seen that the variation on the degree of condensation depends on counterion valence. For monovalent counterions, the degree of condensation passes through a minimum before increasing as the confining space diminishes. In contrast, for trivalent ions, the overall tendency is to decrease the degree of condensation as the confinement space also decreases. Most of the particles reside close to the spherical wall, even for systems in which the density is higher closer to the cavity center. This effect is more pronounced, when monovalent counterions are present. Additionally, there are clear variations in the charge along the concentric layers that cannot be totally ascribed to polyelectrolyte behavior, as shown by decoupling the chain into monomers. If both chain and counterions are confined, the formation of a counterion rich region immediately before the wall is observed. Spool and doughnut‐like structures are formed for stiff chains, within a nontrivial evolution with increasing confinement.