Roberto B. Faria

Interactions between Irrigants Commonly Used in Endodontic Practice: A Chemical Analysis

INTRODUCTION The aim of this work was to characterize the by-products formed in the associations between the most commonly used irrigants in endodontic practice through electrospray ionization quadrupole time-of-flight mass spectrometry analyses. METHODS Sodium hypochlorite (NaOCl) (0.16%, 1%, 2.5%, and 5.25%) was associated with 2% chlorhexidine (CHX) solution and gel, 17% EDTA, 10% citric acid, 37% phosphoric acid, saline solution, ethanol, and distilled water. CHX solution and gel were also associated with all above mentioned irrigants. The solutions were mixed in a 1:1 ratio, and electrospray ionization quadrupole time-of-flight mass spectrometry was used to characterize the precipitates when formed. RESULTS CHX produced an orange-brown precipitate when associated with NaOCl from 1%-5.25% and an orange-white precipitate when associated with 0.16% NaOCl. When associated with EDTA, CHX produced a white milky precipitate, and when associated with saline solution and ethanol, a salt precipitation was produced. No precipitation was observed when CHX was associated with citric acid, phosphoric acid, or distilled water. In the NaOCl associations, precipitation occurred only when CHX was present. CONCLUSION The orange-brown precipitate observed in the association between CHX and NaOCl occurs because of the presence of NaOCl, an oxidizing agent causing chlorination of the guanidino nitrogens of the CHX. The precipitates formed in the reaction of CHX with EDTA, saline solution, and ethanol were associated with acid-base reactions, salting-out process, and lower solubility, respectively. NaOCl associated with EDTA, citric acid, and phosphoric acid leads mainly to chlorine gas formation. Intermediate flushes with distilled water seem to be appropriate to prevent or at least reduce formation of by-products.

Revisiting the Kinetics and Mechanism of Bromate-Bromide Reaction

The bromate-bromide reaction was investigated in an acidity range not studied yet. The reaction was followed at the Br2/Br3- isosbestic point (l = 446 nm). It was observed a first-order behavior for bromate and bromide ions and a second-order behavior for H+ ion that results in the rate law n = k[BrO3-][Br- ][H+]2. This rate law suggests a mechanism involving two successive protonation of bromate followed by the interaction of the intermediate species H2BrO3+ with bromide. These results disagree with the obtained by other authors who observed a second-order behavior for the bromide and first-order for H+, and have proposed intermediate species like H2Br2O3 and HBr2O3-. The second-order for [H+] observed in the range 0.005 £ [H+] £ 2.77 mol L-1 sets down that the pKa of bromic acid, HBrO3, must be lower than -0.5 (T = 25 °C), different from all other values for this pKa proposed in the literature.

A new oxo-vanadium complex employing an imidazole-rich tripodal ligand: a bioinspired bromide and hydrocarbon oxidation catalyst.

A vanadyl complex with the ligand (bis(1-methylimidazol-2-yl)methyl)(2-(pyridyl-2-yl)ethyl)amine was synthesized and fully characterized by X-ray crystallography, elemental analyses, cyclic voltammetry and infrared, electronic and electron paramagnetic resonance spectroscopies. This compound was designed under the so called hybrid concept. It shows to be able to promiscuously use hydrogen peroxide to oxidize bromide and to catalyze the oxidation of benzene and cyclohexane with very good selectivities.

Azido- and chlorido-cobalt complex as carrier-prototypes for antitumoral prodrugs.

Cobalt(III) complexes are well-suited systems for cytotoxic drug release under hypoxic conditions. Here, we investigate the effect of cytotoxic azide release by cobalt-containing carrier-prototypes for antitumoral prodrugs. In addition, we study the species formed after reduction of Co(3+) → Co(2+) in the proposed models for these prodrugs. Three new complexes, [Co(III)(L)(N3)2]BF4(1), [{Co(II)(L)(N3)}2](ClO4)2(2), and [Co(II)(L)Cl]PF6(3), L=[(bis(1-methylimidazol-2-yl)methyl)(2-(pyridyl-2-yl)ethyl)amine], were synthesized and studied by several spectroscopic, spectrometric, electrochemical, and crystallographic methods. Reactivity and spectroscopic data reveal that complex 1 is able to release N3(-) either after reduction with ascorbic acid, or by ambient light irradiation, in aqueous phosphate buffer (pH6.2, 7.0 and 7.4) and acetonitrile solutions. The antitumoral activities of compounds 1-3 were tested in normoxia on MCF-7 (human breast adenocarcinoma), PC-3 (human prostate) and A-549 (human lung adenocarcinoma epithelial) cell lines, after 24h of exposure. Either complexes or NaN3 presented IC50 values higher than 200 μM, showing lower cytotoxicity than the clinical standard antitumoral complex cisplatin, under the same conditions. Complexes 1-3 were also evaluated in hypoxia on A-549 and results indicate high IC50 data (>200 μM) after 24h of exposure. However, an increase of cancer cell susceptibility to 1 and 2 was observed at 300 μM. Regarding complex 3, no cytotoxic activity was observed in the same conditions. The data presented here indicate that the tridentate ligand L is able to stabilize both oxidation states of cobalt (+3 and +2). In addition, the cobalt(III) complex generates the low cytotoxic cobalt(II) species after reduction, which supports their use as as carrier prototypes for antitumoral prodrugs.

Binuclear CuII complexes as catalysts for hydrocarbon and catechol oxidation reactions with hydrogen peroxide and molecular oxygen

The tridentate ligands HL1, [(2-hydroxybenzyl)(2-(imidazol-2-yl)ethyl)]amine, and HL2, [(2-hydroxybenzyl)(2-(pyridil-2-yl)ethyl]amine, were used to synthesize binuclear CuII complexes, [Cu2(L1)2]Cl2•2H2O, complex 1, and [Cu2(L2)2](ClO4)2•1.5H2O, complex 2, in order to obtain catalysts for oxidative processes. Both complexes were characterized by elemental analysis, IR, UV-Vis and EPR spectroscopies. In addition, they were studied by cyclic voltammetry and potentiometric titration in order to investigate their behavior in solution. The crystal structure of complex 1 revealed a binuclear cation where the metal centers are bridged by two phenoxo groups. This arrangement provides a Cu...Cu distance of 3.043(10) A, which is similar to the observed for catechol oxidase (2.90 A). The catalytic reactivities of both complexes were investigated for hydrocarbon and catechol oxidations. Complexes 1 and 2 led to low overall hydrocarbon oxidation conversion values of 6.34 % and 7.15 %, respectively. However, for complex 1, only cyclohexanol (Cy-OH) and cyclohexanone (Cy=O) were isolated as reaction products, with selectivities of 68.1% for Cy-OH. This low overall conversion is tentatively attributed to steric hindrance effects produced by the non-coplanar aromatic rings of the ligand scaffolds, which suggest that the access of the hydrocarbon molecule to the binuclear active center is a determinant step in the reaction mechanism. Investigation of catecholase activities has shown high efficiencies, with complex 2 being more active than complex 1. It indicates that the pyridine-containing ligand is able to stabilize the intermediate CuICuI center which is proposed to be formed in this process. This is corroborated by the strong participation of pyridine in the LUMO (lowest unoccupied molecular orbital) of complex 2, which can help to accommodate the additional negative charge when the complex is reduced from CuIICuII to CuICuI.

Vibrational investigation of the stretching region of bromate ion in solution.

The splitting of v1 (794 cm(-1)) and v3 (805 cm(-1)) modes in the stretching region of the bromate (C3v) was observed for the first time in dimethyl sulfoxide (DMSO) using vibrational spectroscopy. Depolarization measurements allowed to assign the asymmetric and symmetric modes in solution. The band at 805 cm(-1) that has been attributed to the symmetric stretching mode (A1) corresponds in fact to the asymmetric stretching mode (E) and the band at 794 cm(-1) corresponds to the symmetric stretching mode (A1).

The linear μ-oxo-bis[pentachlororuthenate(IV)] anion. Molecular orbital calculations

Abstract Ab initio calculations were done on the dinuclear linear diamagnetic [Cl5RuORuCl5]4− anion. The optimized geometry agrees with the X-ray structure. The energy sequence obtained for the molecular orbitals explains the diamagnetic properties of this anion and allows the assignment of the electronic transitions. The lowest allowed energy for the electronic transition is assigned as π*RuORu←πRu,Cl. The calculated bond order between the ruthenium atoms and the oxygen in the RuORu μ-oxo bridge is close to 2. This supports the idea that this fragment is linear. The results show that the LUMO-eu has antibonding character between the ruthenium atoms and the oxygen. This helps to explain why compounds in which the ruthenium atoms have oxidation states lower than IV are not linear. As the LUMO is populated the double bonds to the oxygen atom tend to a single bond character and the linear arrangement is no longer required. In addition, on decreasing the symmetry, the degeneracy of the eu orbital is removed and this stabilizes any additional electrons.

Oscillatory bromate-oxalic acid-Ce-acetone-sulfuric acid reaction, in CSTR

Sao relatadas, pela primeira vez, a observacao de oscilacoes periodicas, em reator agitado de fluxo continuo (CSTR), para o sistema bromato-acido oxalico-Ce(IV)-acetona-acido sulfurico. Identificou-se que a reacao entre Ce(IV) e a acetona, ate entao nao descrita na literatura e que ocorre antes dos reagentes serem adicionados ao reator, e um fator decisivo para o aparecimento de oscilacoes regulares.

The ozone-iodine-chlorate clock reaction.

This work presents a new clock reaction based on ozone, iodine, and chlorate that differs from the known chlorate-iodine clock reaction because it does not require UV light. The induction period for this new clock reaction depends inversely on the initial concentrations of ozone, chlorate, and perchloric acid but is independent of the initial iodine concentration. The proposed mechanism considers the reaction of ozone and iodide to form HOI, which is a key species for producing non-linear autocatalytic behavior. The novelty of this system lies in the presence of ozone, whose participation has never been observed in complex systems such as clock or oscillating reactions. Thus, the autocatalysis demonstrated in this new clock reaction should open the possibility for a new family of oscillating reactions.

Vibrational study of the contact ion pair formation in the NaClO3–DMF system

Abstract Infrared and Raman studies of sodium chlorate in N,N-dimethylformamide (DMF) at different concentrations allowed to observe changes that are interpreted in terms of the contact ion pair formation. As the DMF is an ionising solvent it was possible to observe the equilibrium between associated and non-associated chlorate. In addition, depolarisation measurements were used to distinguish between the asymmetric and symmetric band representations in almost all regions of this anion. The vibrational assignment is entirely satisfactory indicating that in the Na+ClO3− contact ion pair the local symmetry around ClO3− changes from C3v to CS. In addition, our results do not show any evidence for the formation of solvent separated ion pairs and aggregates in this system.