Raffaele P. Bonomo

In vitro scavenger activity of some flavonoids and melanins against O2−dot

The scavenger activity against O2-. of some flavonoids and melanins (synthetic melanins and melanins isolated from animal tissues, vegetable seeds, and mushroom spores) has been studied by ESR spectrometry. All these substances, except flavon and flavanone, diminish the signal of O2-. generated in vitro by a system containing H2O2 and acetone in an alkaline medium. It is shown that the presence of hydroxyl groups in the B ring of flavonoids is essential for their scavenger activity. Moreover, the presence of a hydroxyl at C-3 enhances the scavenger ability of flavonoids. Generally, aglycons are more active than their glycosides. It seems plausible that the antioxidant property of these substances comes from their scavenger activity against O2-(.). It is also pointed out that the scavenger activity shown by melanins, is strictly correlated with their nature of stable free radical.

Copper(II) binding modes in the prion octapeptide PHGGGWGQ: a spectroscopic and voltammetric study.

The N-terminal octapeptide repeat region of human prion protein (PrPc) is known to bind Cu(II). To investigate the binding modes of copper in PrPc, an octapeptide Ac-PHGGGWGQ-NH2 (1), which corresponds to an octa-repeat sequence, and a tetrapeptide Ac-HGGG-NH2 (2) have been synthesised. The copper(II) complexes formed with 1 and 2 have been studied by circular dichroism (CD) and electron spin resonance (ESR) spectroscopy. Both peptides form 1:1 complexes with Cu(II) at neutral and basic pH. CD, ESR and visible absorption spectra suggest a similar co-ordination sphere of the metal ion in both peptides, which at neutral pH consists of a square pyramidal geometry with three peptidic nitrogens and the imidazole nitrogen as donor atoms. Cyclic voltammetric measurements were used to confirm the geometrical features of these copper(II) complexes: the observation of negative redox potentials are in good agreement with the inferred geometry. All these results taken together suggest that peptide 1 provides a single metal binding site to which copper(II) binds strongly at neutral and basic pH and that the binding of the metal induces the formation of a stiffened structure in the HGGG peptide fragment.

Structural and kinetic characterization of native laccases from Pleurotus ostreatus, Rigidoporus lignosus, and Trametes trogii.

A comparative study has been performed on five native laccases purified from the three basidiomycete fungi Pleurotus ostreatus, Rigidoporus lignosus, and Trametes trogii to relate their different catalytic capacities to their structural properties. Spectroscopic absorption features and EPR spectra at various pH values of the five enzymes are very similar and typical of the blue oxidases. The analysis of the dependence of kinetic parameters on pH suggested that a histidine residue is involved in the binding of nonphenolic substrates, whereas both a histidine and an acidic residue may be involved in the binding of phenolic compounds. His and an Asp residue are indeed found at the bottom of a cavity which may be regarded as a suitable substrate channel for approaching to type 1 copper in the 3D homology models of the two laccases from Pleuorotus ostreatus (POXC and POXA1b) whose sequences are known.

Re-examination of the formation of dinitrosyl–iron complexes during reaction of S-nitrosothiols with Fe(II)

The reaction of S-nitrosothiol compounds with ferrous ions in solution has been investigated and the generated dinitrosyl–iron complexes have been characterized. During the reaction of S-nitrosocysteamine with Fe(II) in water solution in the presence of a twofold excess (with respect to iron) of cysteamine hydrochloride (CSH), an EPR-silent dinuclear iron complex (complex A of formula [Fe2(RS)2(NO)4]) was formed as the major species and was characterized by FAB MS + , UV–Vis, NMR and IR spectroscopies. In the presence of a large excess of CSH (CSH/Fe(II)= 20:1), a green paramagnetic mononuclear complex (complex B of formula [Fe(RS)2(NO)2] − ) was formed. From EPR and UV–Vis data, and also on the basis of the few crystallographic structures known for similar complexes, complex B is proposed to display a distorted tetrahedral geometry (C2), approaching a trigonal bipyramid with a missing ligand, with the unpaired electron mainly localized on the d z 2 orbital of the iron characterized by a d 9 electronic configuration.

Potentiometric, spectroscopic and antioxidant activity studies of SOD mimics containing carnosine

Stability constant values and bonding details of the copper(II) complexes of the β-cyclodextrin functionalized with the carnosine dipeptide (β-alanyl-L-histidine) at its narrow (CDAH6) or at its wide (CDAH3) rim were determined in aqueous solution. The potentiometric and spectroscopic data (UV-vis, CD and EPR) show that the involvement of a secondary OH group induces drastic differences in the coordination properties of CDAH3, in comparison with those of CDAH6. Direct and indirect assays were carried out showing that the copper(II) complexes with the two cyclodextrin derivatives are SOD-mimics with high catalytic activity. In addition the complex species are scavenger compounds towards ˙OH radicals, giving rise to a particular kind of copper(II) complexes with a combined activity against two toxic radical species, O2˙− and ˙OH. The cyclodextrin moiety contributes to the scavenger activity, without damaging the cellular membranes of neuronal and red blood cells.

Coordination properties of 6-deoxy-6-[1-(2-amino) ethylamino]-β-cyclodextrin and the ability of its copper(II) complex to recognize and separate amino acid enantiomeric pairs

The functionalized cyclodextrin 6-deoxy-6-[1-(2-amino)ethylamino]-β-cyclodextrin was synthesized, and an NMR, EPR, pH-metric, and calorimetric investigation was carried out in aqueous solution in order to ascertain its behaviour towards protonation and copper(II) complex formation. The thermodynamic parameters of the ternary complex formation with alanine, phenylalanine and tryptophan enantiomeric pairs were also determined (25° C andI=0.1 mol dm−3). No thermodynamic enantioselectivity was observed in these systems, while a chiral, though poor, discrimination was observed in LEC: c.d. spectra also show enantiomeric stereoselectivity. The results of the present investigation, compared with previously reported results, suggest the occurence of acis-complex ⇆trans-complex equilibrium in such systems.

A re-investigation of copper coordination in the octa-repeats region of the prion protein.

An aqueous solution spectroscopic (Vis and EPR) study of the copper(II) complexes with the Ac-HGGG-NH2 and Ac-PHGGGWGQ-NH2 polypeptides (generically designated as L) suggests square base pyramids ascribable to [Cu(L)H(-2)] complex species, which contain three nitrogen donor atoms, arising from imidazole and peptide groups, in the equatorial plane and for a pseudo-octahedral geometry in the case of [CuLH-3]- and [Cu(L)H-4]2- which have four nitrogen donor atoms in their equatorial plane. The coordination sphere of the copper complex in the [Cu(L)H(-2)] species, which is present at neutral pH values, is completed by two oxygen donor atoms. ESI-MS spectra ascertained that water molecules are not present in the coordination equatorial plane of this latter species, in comparison with other copper(II) complexes with ligands bearing nitrogen and oxygen donor atoms and surely having equatorial water molecules. This indicates the coordination of a carbonyl oxygen atom in the equatorial plane has to be invoked. However, no direct proof about the involvement of a carbonyl group oxygen donor atom apically linked to copper was obtained, due to the flexibility of these structures at room temperature. Additionally, the low A(ll) value leads one to consider another oxygen atom of a carbonyl group being involved in the apical bond to copper in a fast exchange fashion. This apical interaction, which may also involve a water molecule, is more pronounced in the Cu-Ac-HGGG-NH2 than in the analogous Cu-Ac-PHGGGWGQ-NH2 system, probably because of the presence of tryptophan and proline in the polypeptide sequence.

O2− scavenger properties of copper(II) complexes with diamino-diamide-type ligands

Abstract Copper(II) complexes with diamino-diamido-type ligands were tested as catalysts of O 2 − dismutation. ESR spectra of these systems showed that, in consequence of the O 2 − interaction, all the bis complexes of formula [Cu(L) 2 H −2 ] (where L = L-valylamide, L-phenylalanylamide, L-prolylamide) transformed into [Cu(L)H −1 (OH) 2 ] species. They also showed the highest scavenger as well as catalytic activity. The complex species formed after the interaction continued to show scavenger activity also. Stronger copper(II) complexes hardly interacted and showed scarce scavenger properties. In the series of [Cu(L′)H −2 ] complex species (where L′ = (S,S)N,N′-bis(phenylalanyl)-1,2-ethanediamine and -1,3-propanediamine, and (S,S)N,N′-bis(prolyl)-1,2-ethanediamine), the attempt to correlate catalytic activity with structural features revealed that a longer methylene chain could become a critical factor.

Lanthanide complexes with n-oxides. Complexes with pyridine 1-oxide, 2,2′-bipyridine 1,1′-dioxide and 2,2′,2″-terpyridine 1,1′, 1″-trioxide

Several complexes of lanthanide(III) thiocyanates, nitrates, perchlorates and trifluoromethanesulphonates with pyridine 1-oxide (pyO) 2,2′-bipyridine 1,1′-dioxide (bipyO2) and 2,2′,2″-terpyridine 1,1′, 1″-trioxide (terpyO3) have been prepared and characterized. The complexes achieve preferentially the eight-coordination; nine- and ten-coordination have been also observed. Complexation of lanthanide(II) cations with these ligands occurs also in aqueous solution. Information on the electron-donating power of the ligands and geometry of the complexes has been obtained from the absorption and emission f–f spectra. Correlation between Sinhas parameter and hypersensitivity has been observed. The intensity of the hypersensitive transitions increases in the order: pyO < terpyO3 < bipyO2 reaching very high and uncommon values for the bipyO2 complexes. The emission spectra of the octakis(pyO) complexes suggest a rather high symmetry for the electrostatic field surrounding the cation. A drop in site symmetry occurs on passing from the pyO to the bipyO2 and terpyO3 complexes. Exceptionally high values for the intensity ratios 5D0 → 7F2/5D0 → 7F1 in europium(III) complexes and 5D4 → 7F5/5D4 → 7F6 in terbium(III) complexes have been determined.

Low valence state of metal chelates. I. Complexes of iron(II) perchlorate with 1,10-phenanthroline, 4,7-dimethyl-1,10-phenanthroline and 4,7-diphenyl-1,10-phenanthroline

Abstract D. C. polarographic and cyclic voltammetric studies on the reduction of iron(II) complexes with bidentate imine ligands: 1,10-phenanthroline, 4,7-dimethyl 1,10-phenanthroline and 4,7-diphenyl-1,10-phenanthroline, have been carried out in acetonitrile solutions with 0,8 M tetraethyl ammonium perchlorate as supporting electrolyte. The iron(II) complexes gave a five-step reduction wave. The behavior of the first three steps was found to be diffusion controlled and these steps were to be considered as successive reduction of the iron(II) in the complex. The other steps were to be ascribed to the reduction of the ligand. All three complexes led to the formation of the species Fe(L) 3 − (where L is the ligand). The effect of substituent upon the reduction mechanism was also discussed.