Alessandro Dessì

EPR and potentiometric reinvestigation of copper(II) complexation with simple oligopeptides and related compounds

Abstract The coordination modes of Cu(II) to Di-, Tri-, and Tetra-glycine and related ligands were investigated, within the entire measurable pH range and over a wide range of ligand excess, by means of electron paramagnetic resonance, electron spectroscopy, and in some cases, pH potentiometry. The results, besides allowing the identification of the complex species involved and the attribution of distinctive spectral data set to the various structures, provide significant insight in relation to: i) the coordination ability of the zwitterionic AH form of simple oligopeptides to yield COO − - or 2COO − -complexes or species with mixed [(NH 2 , CO)(COO − )] or [(NH 2 , N − , COO − )(COO − )] coordination; ii) the influence of the size of the (NH 2 , CO), (NH 2 , N − ), and (N − , COO − ) chelated rings on the stability and structure of the complex species; iii) the bis-complex formation processes taking place with Gly-β-ala, β-Alagly, Tri-, and Tetraglycine.

Copper(II) complexation by D-glucosamine. Spectroscopic and potentiometric studies

The Cu(II) complex formation equilibria of D- glucosamine were studied in aqueous solution by potentiometric and spectroscopic (ESR, CD, absorption spectra) techniques. All data agree that two major species are formed in the pH region 6–9 involving two D-glucosamine ligand molecules bound to the cupric ion via NH2(CuL2) or NH2 and O− (CuH−2L2). In the latter case deprotonated hydroxyls were found to be very effective coordination sites for Cu(II) giving rise to chelate complexes. On the contrary, no complex formation was observed for the Cu(II) N-acetyl-D-glucosamine system.

Copper(II), nickel(II), zinc(II), and molybdenum(VI) complexes of desferrioxamine B in aqueous solution

Abstract Based on pH-metric, spectrophotometric, and EPR measurements, stability constants and bonding modes are reported for the complexes formed in aqueous solutions of the copper(II)-, nickel(II)-, zinc(II)-, and molybdenum(VI)-desferrioxamine B (DFA) systems. Besides the totally deprotonated species, several protonated mononuclear complexes were found in the copper(II)-, nickel(II)-, and zinc(II)-DFA systems, and also the dinuclear species [Cu 2 AH] 2+ . All three hydroxamate groups are able to coordinate to nickel(II) and zinc(II), but only two of them to the copper(II). Molybdenum(VI) yields only one complex species, [MoO 2 (H 2 DFA)] + . This species, which exists below pH 7, involves two hydroxamate groups coordinated to the metal ion. DFA completely prevents the formation of polyoxomolybdates below pH 7, but MoO 4 2− and free DFA exist above this pH.

Chromium adsorption by plant roots and formation of long-lived Cr(V) species: An ecological hazard?

Abstract The adsorption of either chromium(III) or chromate(VI) by plants has been investigated by means of ESR spectroscopy. Cr(III) absorption yielded complexes, in part immobilized, of 0-donor ligands in the roots. The adsorption of chromium(VI) produced first mobile Cr(V) species, where the metal ion was bound to low-molecular weight ligands, and then also Cr(III) species. The intensity ratio between the ESR signals of Cr(V) and Cr(III) species became almost constant with elapsing contact time, but decreased as the Cr(VI) concentration in solution was lowered. Noticeably, Cr(V) species were detected even at chromium concentration in solution as low as 1 ppm. The formation of stable Cr(V) species in plants, could produce dangerous effects to ecological cycles, because, according to literature data, Cr(V) may be involved in the mutagenic and carcinogenic properties of the element.

Design of Novel Bioisosteres of β-Diketo Acid Inhibitors of HIV-1 Integrase

HIV-1 integrase (IN) is an attractive and validated target for the development of novel therapeutics against AIDS. Significant efforts have been devoted to the identification of IN inhibitors using various methods. In this context, through virtual screening of the NCI database and structure-based drug design strategies, we identified several pharmacophoric fragments and incorporated them on various aromatic or heteroaromatic rings. In addition, we designed and synthesized a series of 5-aryl(heteroaryl)-isoxazole-3-carboxylic acids as biological isosteric analogues of β-diketo acid containing inhibitors of HIV-1 IN and their derivatives. Further computational docking studies were performed to investigate the mode of interactions of the most active ligands with the IN active site. Results suggested that some of the tested compounds could be considered as lead compounds and suitable for further optimization.

In vitro interaction of mutagenic chromium(VI) with red blood cells

Glutathione; Chromium; ESR; (Red blood cell)

Reduction of chromium(VI) by D-galacturonic acid and formation of stable chromium(V) intermediates

Abstract The interaction of dichromate with D-galacturonic acid in aqueous solution, as a function of pH, is described. The reaction involves the reduction of Cr(VI) to Cr(III), but the reaction rate is remarkably dependent on pH. In fact, the reduction of Cr(VI) to Cr(III) proceeds rather quickly in strongly acidic solutions, while it is slow in neutral or moderately acidic media. In all cases, according to the ESR evidence, Cr(V) species are found as intermediates. The stability of the Cr(V) species increases with increasing pH, so that it may be suggested that the overall reaction rate is controlled by the Cr(V) to Cr(III) conversion.

Reduction of chromate ions by glutathione tripeptide in the presence of sugar ligands

Polarographic and ESR data obtained for the reduction of chromate in the presence of the tripeptide glutathione (GSH) and sugars are presented. The results indicate that in the binary GSH-Cr(VI) system, glutathione binds chromate forming a thioester species which can be reduced efficiently by free tripeptide molecules. In the systems containing chromate, sugars and glutathione, chromium(VI) interacts with the sugar ligand (or with both sugar and GSH) yielding esters which are easily reduced by GSH. The formed Cr(V) ions are then stabilized by the coordination to the sugar ligands. The sugars having pairs of cis hydroxyl groups available for binding to metal are the most effective in the formation, first, of Cr(VI) esters and then, of Cr(V) complexes.

Mutational Analysis of the Binding Pockets of the Diketo Acid Inhibitor L-742,001 in the Influenza Virus PA Endonuclease

ABSTRACT The influenza virus PA endonuclease, which cleaves capped host pre-mRNAs to initiate synthesis of viral mRNA, is a prime target for antiviral therapy. The diketo acid compound L-742,001 was previously identified as a potent inhibitor of the influenza virus endonuclease reaction, but information on its precise binding mode to PA or potential resistance profile is limited. Computer-assisted docking of L-742,001 into the crystal structure of inhibitor-free N-terminal PA (PA-Nter) indicated a binding orientation distinct from that seen in a recent crystallographic study with L-742,001-bound PA-Nter (R. M. DuBois et al., PLoS Pathog. 8:e1002830, 2012). A comprehensive mutational analysis was performed to determine which amino acid changes within the catalytic center of PA or its surrounding hydrophobic pockets alter the antiviral sensitivity to L-742,001 in cell culture. Marked (up to 20-fold) resistance to L-742,001 was observed for the H41A, I120T, and G81F/V/T mutant forms of PA. Two- to 3-fold resistance was seen for the T20A, L42T, and V122T mutants, and the R124Q and Y130A mutants were 3-fold more sensitive to L-742,001. Several mutations situated at noncatalytic sites in PA had no or only marginal impact on the enzymatic functionality of viral ribonucleoprotein complexes reconstituted in cell culture, consistent with the less conserved nature of these PA residues. Our data provide relevant insights into the binding mode of L-742,001 in the PA endonuclease active site. In addition, we predict some potential resistance sites that should be taken into account during optimization of PA endonuclease inhibitors toward tight binding in any of the hydrophobic pockets surrounding the catalytic center of the enzyme.

Oxovanadium(IV) complex formation by simple sugars in aqueous solution

The binding of oxovanadium(IV) to simple sugars in neutral or basic aqueous solution, as studied by EPR and electronic absorption spectroscopy, is reported. The complexation is favored in basic media and involves the coordination of the metal ion to couples of adjacent deprotonated hydroxyls of the sugar molecule. However, only the ligands provided with cis couples can adopt this chelating ligand behavior. The ability of the cis hydroxyl couples to yield chelated complexes has been related to the structural rearrangement (decrease of the O-C-C-O torsion angle in the five-membered chelated ring) needed to permit the oxovanadium(IV) coordination by the sugar molecule.