Marcos B. Gonçalves

Oxindole-Schiff base copper(II) complexes interactions with human serum albumin: Spectroscopic, oxidative damage, and computational studies

CD and EPR were used to characterize interactions of oxindole-Schiff base copper(II) complexes with human serum albumin (HSA). These imine ligands form very stable complexes with copper, and can efficiently compete for this metal ion towards the specific N-terminal binding site of the protein, consisting of the amino acid sequence Asp-Ala-His. Relative stability constants for the corresponding complexes were estimated from CD data, using the protein as competitive ligand, with values of log K(CuL) in the range 15.7-18.1, very close to that of [Cu(HSA)] itself, with log K(CuHSA) 16.2. Some of the complexes are also able to interfere in the alpha-helix structure of the protein, while others seem not to affect it. EPR spectra corroborate those results, indicating at least two different metal species in solution, depending on the imine ligand. Oxidative damage to the protein after incubation with these copper(II) complexes, particularly in the presence of hydrogen peroxide, was monitored by carbonyl groups formation, and was observed to be more severe when conformational features of the protein were modified. Complementary EPR spin-trapping data indicated significant formation of hydroxyl and carbon centered radicals, consistent with an oxidative mechanism. Theoretical calculations at density functional theory (DFT) level were employed to evaluate Cu(II)-L binding energies, L-->Cu(II) donation, and Cu(II)-->L back-donation, by considering the Schiff bases and the N-terminal site of HSA as ligands. These results complement previous studies on cytotoxicity, nuclease and pro-apoptotic properties of this kind of copper(II) complexes, providing additional information about their possibilities of transport and disposition in blood plasma.

Interactions of di-imine copper(II) complexes with albumin: competitive equilibria, promoted oxidative damage and DFT studies

Interactions of some diimine copper(II) complexes with bovine serum albumin (BSA) were investigated by spectroscopic techniques in order to compare the stability of the complexes and their capability of causing oxidative damage to the protein. The tri- and tetradentate imine ligands employed in this work contain pyridine, pyrazine or imidazole moieties, which are ubiquitous in biological systems. The relative thermodynamic stabilities of the copper(II) complexes were estimated by circular dichroism (CD) using BSA as the competitive ligand. The apparent stability constants determined for the complexes are very similar to one another and to that of the Cu(BSA) complex itself, for which log KCu(BSA) = 12.9 has already been described in the literature, indicating that the complexes are quite stable under physiological conditions. Two different copper binding sites were evidenced on BSA by spectroscopic measurements (CD, UV-Vis and EPR), depending on the ligand and on the [CuL]:[protein] stoichiometric ratio. Metal binding to any of the sites gives rise to significant protein oxidative damage, especially in the presence of hydrogen peroxide, indicating an oxidative process based on reactive oxygen species (ROS). A small amidated peptide, Asp-Thr-His-NH2, corresponding to the N-terminal region of BSA was synthesized, and its interaction with all the diimine-copper(II) complexes was also investigated in order to clarify the copper imine complex-albumin interactions. Electronic structure calculations at the density functional theory (DFT) level were made to compare the copper-ligand binding energies for each complex with that of the metal coordinated at the N-terminal site of the protein.

Spectroscopic characterization of schiff base-copper complexes immobilized in smectite clays

Herein, the immobilization of some Schiff base-copper(II) complexes in smectite clays is described as a strategy for the heterogenization of homogeneous catalysts. The obtained materials were characterized by spectroscopic techniques, mostly UV/Vis, EPR, XANES and luminescence spectroscopy. SWy-2 and synthetic Laponite clays were used for the immobilization of two different complexes that have previously shown catalytic activity in the dismutation of superoxide radicals, and disproportionation of hydrogen peroxide. The obtained results indicated the occurrence of an intriguing intramolecular redox process involving copper and the imine ligand at the surface of the clays. These studies are supported by computational calculations.

Cd hyperfine interactions in DNA bases and DNA of mouse strains infected with Trypanosoma cruzi investigated by perturbed angular correlation spectroscopy and ab initio calculations.

In this work, perturbed angular correlation (PAC) spectroscopy is used to study differences in the nuclear quadrupole interactions of Cd probes in DNA molecules of mice infected with the Y-strain of Trypanosoma cruzi. The possibility of investigating the local genetic alterations in DNA, which occur along generations of mice infected with T. cruzi, using hyperfine interactions obtained from PAC measurements and density functional theory (DFT) calculations in DNA bases is discussed. A comparison of DFT calculations with PAC measurements could determine the type of Cd coordination in the studied molecules. To the best of our knowledge, this is the first attempt to use DFT calculations and PAC measurements to investigate the local environment of Cd ions bound to DNA bases in mice infected with Chagas disease. The obtained results also allowed the detection of local changes occurring in the DNA molecules of different generations of mice infected with T. cruzi, opening the possibility of using this technique as a complementary tool in the characterization of complicated biological systems.

Comparative studies of oxindolimine-metal complexes as inhibitors of human DNA topoisomerase IB

Abstract Copper(II) and zinc(II) complexes with oxindolimine ligands, obtained by a condensation reaction between isatin (1H-indole-2,3-dione) and 2-(2-aminoethyl)pyridine, have been previously shown to activate apoptosis very efficiently in different cancer cell lines. Here, we show that these compounds inhibit human DNA topoisomerase IB (Topo I) activity, and this characteristic may be implicated in its pro-apoptotic and potential antitumor properties. The studied metal complexes prevent DNA relaxation and cleavage reaction, whilst they do not have any effect on the religation process. The protein inhibition occurs in two different ways since the copper compound does not permit the enzyme-DNA complex formation, while the zinc analogue permits this complex formation but inhibits the catalytic cleavage reaction. Other related copper and zinc complexes, one with an asymmetric imine ligand derived from isatin, 1,3-diaminopropane, and salicyladehyde, and another one previously reported, derived from isatin and 1,3-diaminopropane, had their corresponding inhibition results towards Topo I compared. These data demonstrated that such complexes can act as good catalytic inhibitors of Topo I, in a process modulated by the ligand features and the nature of the metal ion. Computational studies complemented and supported experimental data, showing three different characteristics of the metal complexes that influence its interaction and consequent inhibition of Topo I. The ligand planarity when bound to the protein increases the occurring interactions in different binding sites, the total charge of the complex modulates the preferential region of interaction, and the copper(II) complexes are expected to be more efficient inhibitors compared to analogous zinc(II), as verified experimentally.

Metal coordination study at Ag and Cd sites in crown thioether complexes through DFT calculations and hyperfine parameters

Structural and electronic properties of [C12H24S6X], [C13H26S6OX], and [C14H28S6OX] (X: Ag+, Cd2+) crown thioether complexes were investigated within the framework of the density functional theory (DFT) using the projector augmented wave (PAW) method. The theoretical results were compared with time-differential perturbed γ–γ angular correlations (TDPAC) experiments reported in the literature using the 111Ag→111Cd probe. In the case of X=Ag+, a refinement of the structure was performed and the predicted equilibrium structures compared with available X-ray diffraction experimental data. Structural distortions induced by replacing Ag+ with Cd2+ were investigated as well as the electric-field gradient (EFG) tensor at the Cd2+ sites. Our results suggest that the EFG at Cd2+ sites corresponds to the Ag+ coordination sphere structure, i.e., before the structural relaxations of the molecule with X=Cd2+ are completed. The results are discussed in terms of the characteristics of the TDPAC 111Ag→111Cd probe and the time window of the measurement, and provide an interesting tool with which to probe molecular relaxations.