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- Ligand size polydispersity effect on SSCP signal interpretationPublication . Rocha, Luciana S.; Botero, Wander G.; Alves, Nuno G.; Moreira, José; Costa, Ana M. Rosa da; Pinheiro, J. P.The present study aims to establish unambiguously the conditions required for the validity of the average diffusion (D) approximation in fully labile systems with significant ligand size polydispersity. The average diffusion coefficient is a key parameter in mass transfer that affects signal interpretation in dynamic electroanalytical techniques. To achieve this goal, the binding of Cd(II) and Pb(II) to binary and ternary mixtures containing chemically homogenous (PSS)n-COOH polymers (ligand excess conditions were required) of different sizes (4, 10 and 30 KDa) was evaluated. It was experimentally evidenced that the average diffusion coefficient (D), can indeed be computed as the weighted average of several metal-polymer complexes of diverse sizes.
- Importance of phenols structure on their activity as antinitrosating agents: a kinetic studyPublication . Moreira, José; Pessêgo, Márcia; Costa, Ana M. Rosa daObjective : Nitrosative deamination of DNA bases induced by reaction with reactive nitrogen species (RNS) has been pointed out as a probable cause of mutagenesis. (Poly)phenols, present in many food items from the Mediterranean diet, are believed to possess antinitrosating properties due to their RNS scavenging ability, which seems to be related to their structure. It has been suggested that phenolic compounds will react with the above-mentioned species more rapidly than most amino compounds, thus preventing direct nitrosation of the DNA bases and their transnitrosation from endogenous N-nitroso compounds, or most likely from the transient N-nitrosocompounds formed in vivo. Materials and Methods : In order to prove that assumption, a kinetic study of the nitroso group transfer from a N-methyl-N-nitrosobenzenesulfonamide (N-methyl-N-nitroso-4-methylbenzenesulfonamide, MeNMBS) to the DNA bases bearing an amine group and to a series of phenols was carried out. In the transnitrosation of phenols, the formation of nitrosophenol was monitored by Ultraviolet (UV) / Visible spectroscopy, and in the reactions of the DNA bases, the consumption of MeNMBS was followed by high performance liquid chromatography (HPLC). Results : The results obtained point to the transnitrosation of DNA bases being negligible, as well as that of phenols bearing electron-withdrawing groups. Phenols with methoxy substituents in positions 2, 4, and / or 6, although they seemed to react, did not afford the expected product. Phenols with electron-releasing substituents, unless these blocked the oxygen atom, reacted with our model compound at an appreciable rate. O-nitrosation of the phenolate ion followed by rearrangement of the C-nitrosophenol seemed to be involved. Conclusion : This study provided evidence that the above compounds might actually act as antinitrosating agents in vivo.
- Mechanism for basic hydrolysis of N-nitrosoguanidines in aqueous solutionPublication . Moreira, José; Leis, J. R.; Norberto, F.; García-Río, Luís; Araujo, E.; Ribeiro, LauraA kinetic study was carried out on the hydrolysis of two N-nitrosoguanidines, 1-nitroso-1-methyl- 3-tolylsulfonylguanidine (TSGNO) and 1-nitroso-1-methyl-3-benzoylguanidine (BCGNO). We observed an absence of buffer catalysis using H2PO4 -/HPO4 2-, H3BO3/H2BO3 -, and HCO3 -/CO3 2- regulators and a complex dependency of the rate constant on the pH. We discovered the existence of three simultaneous reaction paths: spontaneous decomposition of the neutral form of the N-nitrosoguanidine, decomposition of the monoanion, and decomposition through the form of the dianion. The analysis of the kinetic data has allowed us to obtain the acidity constant for the formation of the monoanion of the N-nitrosoguanidine, with values of pKa I ) 11.5. The reaction rate for the process through the monoanion, k2, decreases as the acidity increases. The application of the principle of nonperfect synchronization shows that the basicity and reactivity do not correlate when there exists a possibility of stabilization of the negative charge by resonance. This behavior is consistent with the mechanism E1cB whereby the stabler the negative charge, the slower the elimination reaction. When dealing with the case of the elimination through the neutral form we observe that the reaction rate increases together with the capacity of stabilization of the positive charge on the nitrogen atom adjacent to the imino group. For the reaction through the dianion we used a maximum value of k3 ) 1010 s-1 to estimate the value of pKa II for the formation of the dianion of the N-nitrosoguanidine, obtaining values of pKa II < 24.
- Non-volatile memory device using a polymer modified nanocrystalPublication . Kiazadeh, Asal; Gomes, Henrique L.; Costa, Ana M. Rosa da; Moreira, José; De Leeuw, Dago M.; Meskers, S. C. J.Thin-film planar structures using AgCl nanocrystals embedded in a polymer blend; exhibit reliable and reproducible switching between different non-volatile conductance states. It is shown that resistive switching in these systems cannot be related with migration diffusion or aggregation of metals to form metallic filaments. This is supported by temperature-dependent measurement showing that the current in the high conductance state is thermal activated (0.6 eV).
- Synthesis and catalytic properties in olefin epoxidation of octahedral dichloridodioxidomolybdenum(VI) complexes bearing N,N-dialkylamide ligands: crystal structure of [Mo2O4(mu(2)-O)Cl-2(dmf)(4)]Publication . Gago, Sandra; Neves, Patrícia; Monteiro, Bernardo; Pessêgo, Márcia; Lopes, Andre D.; Valente, Anabela A.; Almeida Paz, Filipe A.; Pillinger, Martyn; Moreira, José; Silva, Carlos M.; Gonçalves, Isabel S.The catalytic performance of the complexes [MoO2Cl2(L)2][L = N,N-dimethylformamide (dmf), N,N-dimethylacetamide(dma), N,N-dimethylpropionamide (dmpa), N,N-diethylformamide(def) and N,N-diphenylformamide (dpf)] was examined in the epoxidation of cis-cyclooctene with tert-butyl hydroperoxide(tbhp) at 55 °C and in the absence of a cosolvent.The complexes showed high turnover frequencies in the range of 561–577 molmolMo–1h–1, giving the epoxide as the only product in 98% yield after 6 h. The reaction rates decreased significantly in consecutive runs carried out by recharging the reactors with olefin and oxidant. On the basis of the IR spectroscopic characterisation of the solids recovered at the end of the catalytic reactions, the decrease in activity is attributed to the formation of dioxido(μ-oxido)-molybdenum(VI) dimers. Accordingly, the treatment of [MoO2Cl2(dmf)2] with an excess amount of tbhp led to the isolation of [Mo2O4(μ2-O)Cl2(dmf)4], which was characterised by single-crystal X-ray diffraction and found to exhibit a catalytic performance very similar to that found in the second runs for the mononuclear complexes. The kinetics of the reaction of [MoO2Cl2(dmf)2] with tbhp was further examined by UV/Vis spectroscopy, allowing rate constants and activation parameters to be determined. For the dpf adduct, the effect of different solvents on cyclooctene epoxidation and the epoxidation of other olefins, namely, (R)-(+)-limonene, α-pinene and norbornene, were investigated
- Supramolecular catalysis by cucurbit[7]uril and cyclodextrins: similarity and differencesPublication . Basilio, Nuno; García-Río, Luís; Moreira, José; Pessêgo, MárciaTo understand the analogies and differences between the cucurbituril and cyclodextrin cavities different solvolytic reactions have been studied in the presence of cucurbit[7]uril, CB7, and β-CD or its methylated derivative, DM-β-CD. Solvolysis of 1-bromoadamantane has been used as a test to evaluate the ability of the cavities to solvate the Br- leaving group. Obtained results show that in both cases the polarity inside the cavity is similar to that of a 70% ethanol:water mixture. Solvolysis of substituted benzoyl chlorides shows a great difference between the CB7 and DM-β-CD cavity. Solvolysis of electron withdrawing substituted benzoyl chlorides (associative mechanism) is catalyzed by DM-β-CD and inhibited by CB7. However, solvolysis of electron donating substituted benzoyl chlorides(dissociative mechanism) is catalyzed by CB7 and inhibited by DM-β-CD. These experimental behaviors have been explained on the basis of different solvolytic mechanisms. Participation of the hydroxyl groups of the cyclodextrin as a nucleophile can explain the catalytic effect observed for solvolysis of benzoyl chlorides reacting by an associative mechanism. Solvolysis of benzoyl chlorides reacting by a dissociative mechanism is catalyzed by CB7 due to the ability of the CB7 cavity to stabilize the acylium ion developed in the transition state by electrostatic interactions.
- Electrostatic repulsion between cucurbit[7]urils can be overcome in [3]pseudorotaxane without adding saltsPublication . Pessêgo, Márcia; Moreira, José; Costa, Ana M. Rosa da; Corrochano, P.; Poblete, F. J.; García-Río, LuísThe host−guest chemistry betweencucurbit[7]uril (CB7) and a series of bolaform (Bn) surfactants with different chain lengths, n = 12−22, was the target of our study. [3]Pseudorotaxanes are formed when the alkyl chain of the bolaform has more than 14 carbon atoms. In these cases, two CB7 molecules can be accommodated between the two head groups of the bolaform without addition of electrolytes to the medium. In the case of a bolaform with 12 carbon atoms, the electrostatic repulsion between the carbonyl groups of the CB7 molecules avoids the threading of a second CB7 molecule yielding a mixed structure formed by a [2]pseudorotaxane and an external host−guest complex. The assembly behavior was investigated using NMR spectroscopy, isothermal titration calorimetry (ITC), and kinetic measurements.
- Cucurbit[7]uril: surfactant host-guest complexes in equilibrium with micellar aggregatesPublication . Pessêgo, Márcia; Basilio, Nuno; Moreira, José; García-Río, LuísIn order to compare the formation of host–guest complexes between b-cyclodextrin (b-CD) or cucurbit[7]uril (CB7) and cationic surfactants we studied the hydrolysis of 4-methoxybenzenesulfonyl chloride (MBSC). The selected surfactants allowed the length of the hydrocarbon chain to be varied between 6 and 18 carbon atoms. Contrary to the expected behaviour, the values of the binding constants between CB7 and surfactants are independent of the alkyl chain length of the surfactant. In the case of b-CD, however, a clear dependence of the binding constant on the hydrophobic character of the surfactant was observed. The values obtained with CB7 are significantly higher than those obtained with b-CD and these differences are explained to be a consequence of electrostatic interactions of the surfactants with the portals of CB7. It was found that a small percentage of uncomplexed CB7 was in equilibrium with the cationic micelles and this percentage increased on increasing the hydrophobic character of the surfactant.
- Intrinsic and extrinsic resistive switching in a planar diode based on silver oxide nanoparticlesPublication . Kiazadeh, Asal; Gomes, Henrique L.; Rosa da Costa, Ana; Moreira, José; De Leeuw, Dago M.; Meskers, S. C. J.Resistive switching is investigated in thin-film planar diodes using silver oxide nanoparticles capped in a polymer. The conduction channel is directly exposed to the ambient atmosphere. Two types of switching are observed. In air, the hysteresis loop in the current–voltage characteristics is S-shaped. The high conductance state is volatile and unreliable. The switching is mediated by moisture and electrochemistry. In vacuum, the hysteresis loops are symmetric, N-shaped and exhibit a negative differential resistance region. The conductance states are non-volatile with good data retention, programming cycling endurance and large current modulation ratio. The switching is attributed to electroforming of silver oxide clusters.
- Differences in cucurbit[7]uril: surfactant complexation promoted by the cationic head groupPublication . Pessêgo, Márcia; Silva, José P. da; Moreira, José; García-Río, LuísThe formation of host/guest complexes between cucurbit[7]uril (CB7) and tetradecyltrialkylammonium bromide surfactants with trimethyl (C14TMA+), triethyl (C14TEA+), and tripropyl (C14TPA+) head groups was studied. Complexation was studied by ESI-MS, isothermal titration calorimetry (ITC), and NMR spectroscopy. The structure of the complexes is strongly dependent on the surfactant head group. Both 1:1 and 2:1 complexes are formed with all cationic surfactants. The magnitude of the K1:1 binding constants decreases with the alkyl length of the head group, whereas that of the K2:1 binding constants is independent of the nature of the surfactant head group. Distinct structures were found for the 1:1 and 2:1 complexes. C14TMA+ and C14TPA+ surfactants form 1:1 inclusion complexes that comprise the head group close to the CB7 portal and 2:1 external complexes with both host molecules near the head group. The 1:1 complexes between C14TEA+ and CB7 showed that the head group was centered inside the CB7 cavity, whereas in the 2:1 complex the second molecule of CB7 was located at the end of the main alkyl chain.