Laura Santagostini

Inhibition of the catecholase activity of biomimetic dinuclear copper complexes by kojic acid

Abstract The inhibition of the catechol oxidase activity exhibited by three dinuclear copper(II) complexes, derived from different diaminotetrabenzimidazole ligands, by kojic acid [5-hydroxy-2-(hydroxymethyl)-γ-pyrone] has been studied. The catalytic mechanism of the catecholase reaction proceeds in two steps and for both of these inhibition by kojic acid is of competitive type. The inhibitor binds strongly to the dicopper(II) complex in the first step and to the dicopper-dioxygen adduct in the second step, preventing in both cases the binding of the catechol substrate. Binding studies of kojic acid to the dinuclear copper(II) complexes and a series of mononuclear analogs, carried out spectrophotometrically and by NMR, enable us to propose that the inhibitor acts as a bridging ligand between the metal centers in the dicopper(II) catalysts.

Covalently modified microperoxidases as heme-peptide models for peroxidases

Microperoxidase-8 (MP8) and microperoxidase-9 (MP9) have been covalently modified by attachment of proline-containing residues to the amino terminal peptide chain in order to obtain new peroxidase model systems. The catalytic activities of these derivatives in the oxidation of p-cresol by hydrogen peroxide have been compared to that of MP8. The presence of steric hindrance above the heme reduces the formation rate of the catalytically active species, while the reactivity is increased when the amino group of a proline residue is close to the iron. The modification of the catalyst affects the rate of degradation processes undergone by the heme group during catalysis. A bulky aromatic group on the distal side decreases the stability of the complex because it reduces the mobility of a phenoxy radical species formed during catalysis, while the presence of proline residues increases the number of turnovers of the heme catalysts before degradation. The complex Pro2-MP8 obtained by addition of two proline residues to MP8 exhibits the best catalytic performance in terms of activity and chemical stability.

Nutritional Characterization and Phenolic Profiling of Moringa oleifera Leaves Grown in Chad, Sahrawi Refugee Camps, and Haiti

Moringa oleifera is a plant that grows in tropical and subtropical areas of the world. Its leaves are rich of nutrients and bioactive compounds. However, several differences are reported in the literature. In this article we performed a nutritional characterization and a phenolic profiling of M. oleifera leaves grown in Chad, Sahrawi refugee camps, and Haiti. In addition, we investigated the presence of salicylic and ferulic acids, two phenolic acids with pharmacological activity, whose presence in M. oleifera leaves has been scarcely investigated so far. Several differences were observed among the samples. Nevertheless, the leaves were rich in protein, minerals, and β-carotene. Quercetin and kaempferol glycosides were the main phenolic compounds identified in the methanolic extracts. Finally, salicylic and ferulic acids were found in a concentration range of 0.14–0.33 and 6.61–9.69 mg/100 g, respectively. In conclusion, we observed some differences in terms of nutrients and phenolic compounds in M. oleifera leaves grown in different countries. Nevertheless, these leaves are a good and economical source of nutrients for tropical and sub-tropical countries. Furthermore, M. oleifera leaves are a source of flavonoids and phenolic acids, among which salicylic and ferulic acids, and therefore they could be used as nutraceutical and functional ingredients.

Stereoselective catalytic oxidations of biomimetic copper complexes with a chiral trinucleating ligand derived from 1,1-binaphthalene

Abstract The new octadentate ligand R-(−)-N,N′-dimethyl-N,N′-bis{3-[bis(1-methyl-2-benzimidazolyl)amino]propyl}1,1′-binaphthalenyl-2,2′-diamine (L) was employed for the synthesis of dinuclear and trinuclear copper(II) complexes. Two terminal binding sites with tridentate aminobis(benzimidazole) linkages (A sites) and one central binding site with the bidentate diamino-binaphthalenyl residue (B site) are used by the ligand to bind divalent metal centres in the trinuclear complex [Cu3L][ClO4]6. Spectroscopic measurements suggest that in the dinuclear complex [Cu2L][ClO4]4 the copper ions are five-coordinated, with ligation by the aminobis(benzimidazole) residues, one of the tertiary amine donors of the diamino-binaphthalenyl moiety, and one water molecule. The complexes bind azide in the μ-1,3 fashion at low concentration and in the terminal mode at high concentration. The copper(II) complexes derived from L are catalytically active in the oxidation of 3,5-di-tert-butylcatechol (DTBC) by dioxygen. The oxidations are biphasic, with a fast initial stoichiometric phase corresponding to reduction of a pair of copper(II) centres and oxidation of DTBC to quinone, followed by the catalytic reaction, that follows substrate saturation behaviour. The complexes act as stereoselective catalysts in the biomimetic oxidations of the optically active catechol derivatives l - and d -Dopa and their methyl esters. In all the cases, the preferred enantiomeric substrate has the L configuration. This preference is dictated by the chirality of the binaphthalenyl residue.

A new chiral, poly-imidazole N8-ligand and the related di- and tri-copper(II) complexes: synthesis, theoretical modelling, spectroscopic properties, and biomimetic stereoselective oxidations

The new poly-imidazole N(8) ligand (S)-2-piperazinemethanamine-1,4-bis[2-((N-(1-acetoxy-3-(1-methyl-1H-imidazol-4-yl))-2-(S)-propyl)-(N-(1-methyl-1H-imidazol-2-ylmethyl)))ethyl]-N-(phenylmethyl)-N-(acetoxy), also named (S)-Pz-(C2-(HisIm))(2) (L), containing three chiral (S) centers, was obtained by a multi-step synthesis and used to prepare dinuclear [Cu(2)(L)](4+) and trinuclear [Cu(3)(L)](6+) copper(II) complexes. Low-temperature EPR experiments performed on [Cu(2)(L)](4+) demonstrated that the two S = ½ centers behaved as independent paramagnetic units, while the EPR spectra used to study the trinuclear copper complex, [Cu(3)(L)](6+), were consistent with a weakly coupled three-spin ½ system. Theoretical models for the two complexes were obtained by DFT/RI-BP86/TZVP geometry optimization, where the structural and electronic characteristics nicely supported the EPR experimental findings. In addition, the theoretical analysis unveiled that the conformational flexibility encoded in both [Cu(2)(L)](4+) and [Cu(3)(L)](6+) arises not only from the presence of several σ-bonds and the bulky residues attached to the (S)-Pz-(C2-(HisIm))(2) ligand scaffold, but also from the poor coordination ability of the tertiary amino groups located in the ligand side-chains containing the imidazole units towards the copper(II) ions. Both the dinuclear and trinuclear complexes are efficient catalysts in the stereoselective oxidation of several catechols and flavonoid compounds, yielding the corresponding quinones. The structural features of the substrate-catalyst adduct intermediates were assessed by searching the conformational space of the molecule through MMFF94/Monte Carlo (MMFF94/MC) methods. The conformational flexibility of the bound ligand in the complexes proves to be beneficial for substrate binding and recognition. For the dinuclear complex, chiral recognition of the optically active substrates derives from weak electrostatic interactions between bound substrates and folded regions of the ligand scaffold. For the trinuclear complex, in the case of L/D-Dopa, the chiral recognition has a remarkable stereoselectivity index of 75%, the highest so far reported for this type of reaction. Here the dominant contribution to stereoselectivity arises from the direct interaction between a donor group (the Dopa carboxylate) far from the substrate reaction site (the catechol ring) with the additional (third) copper center not involved in the oxidative catalysis. On the other hand, in the case of bulky substrates, such as L/D-catechin, the observed poor substrate recognition is associated with much weaker interactions between the chiral regions of the complex and the chiral part of the substrate.

Enantio-differentiating catalytic oxidation by a biomimetic trinuclear copper complex containing L-histidine residues

The trinuclear complex [Cu3PHI]6+, derived from a ligand containing two chiral L-histidine residues, performs the catalytic oxidation of L- and D-Dopa with remarkable enantio-differentiation; this depends on the anchoring effect provided by the copper center which is not participating in the catalytic reaction and recognizes the chirality of the substrate.

Inhibitor binding studies on ascorbate oxidase

Abstract The characteristic features of the plant multicopper enzyme ascorbate oxidase are described, together with the current knowledge about its catalytic mechanism and substrate specificity. A variety of small anionic inhibitors have been used as spectroscopic probes for the enzyme metal sites, but recently interest has arisen for a new type of phenolic inhibitors which act competitively against ascorbate. These simple phenolic compounds can bind to the enzyme in the same pocket near type 1 copper as the substrate ascorbate binds, as shown by docking and molecular mechanics computations.

Isolation of the met-derivative intermediate in the catalase-like activity of deoxygenated Octopus vulgaris hemocyanin

Abstract The deoxygenated form [Cu(I)Cu(I)] of molluscan hemocyanin exhibits a catalase-like activity. The initial formation of the met-derivative [Cu(II)Cu(II)] is followed by reaction of a second molecule of hydrogen peroxide, leading to oxy-hemocyanin. Sodium azide, a ligand that is also able to coordinate to the binuclear cupric site of met-hemocyanin, shows competitive inhibition of the regeneration reaction by hydrogen peroxide. Therefore, in the presence of an excess of azide the reduction of met-hemocyanin by hydrogen peroxide is prevented and the met-hemocyanin azide complex becomes the main reaction product. After removal of excess reactants, the derivative obtained exhibits the characteristic features of met-hemocyanin. The preparation of this derivative by the present method requires a shorter time and is carried out under milder chemical conditions than those used in other methods previously reported in the literature. Furthermore, this new method is based on trapping of a reaction intermediate and not on the chemical modification of the protein after the labilization of the active site.

Erratum to: “Inherently chiral” thiophene-based electrodes at work: a screening of enantioselection ability toward a series of pharmaceutically relevant phenolic or catecholic amino acids, amino esters, and amine

“Inherently chiral” thiophene-based electroactive oligomer films have recently been shown to exhibit outstanding chirality manifestations. One of the most exciting among them is an unprecedented enantioselection ability as electrode surfaces. In fact, in preliminary chiral voltammetry experiments, the new electrodes have been shown to both discriminate the enantiomers of chiral probes (either enantiopure or in a mixture, in terms of large differences in peak potentials) and quantify them (in terms of linear dynamic ranges in peak currents), without the need for preliminary separation steps. Such ability has now been tested on a series of chiral DOPA-related molecules, from phenolic amino acid tyrosine (together with its methyl ester) to catecholic amino acid DOPA (together with its methyl ester), to catecholamine epinephrine (adrenaline). The wide-range enantioselectivity of the new inherently chiral electrode surfaces is fully confirmed, as large peak potential differences are obtained for probe enantiomers of the whole series working in common aqueous buffers. Moreover, interesting modulating effects on enantiodiscrimination can be observed as a function of both molecular structure and pH.

Biomimetic modeling of copper complexes: a study of enantioselective catalytic oxidation on d-(+)-catechin and L-( - )-epicatechin with copper complexes.

The biomimetic catalytic oxidations of the dinuclear and trinuclear copper(II) complexes versus two catechols, namely, D-(+)-catechin and L-( − )-epicatechin to give the corresponding quinones are reported. The unstable quinones were trapped by the nucleophilic reagent, 3-methyl-2-benzothiazolinone hydrazone (MBTH), and have been calculated the molar absorptivities of the different quinones. The catalytic efficiency is moderate, as inferred by kinetic constants, but the complexes exhibit significant enantio-differentiating ability towards the catechols, albeit for the dinuclear complexes, this enantio-differentiating ability is lower. In all cases, the preferred enantiomeric substrate is D-(+)-catechin to respect the other catechol, because of the spatial disposition of this substrate.