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

Studi di reattività dell'enzima tirosinasi e dei suoi sistemi modello

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Reversible stabilization of transition-metal-binding DNA G-quadruplexes.

You can't top the CopperTop: Tetramolecular G-quadruplexes modified with terminal pyridine ligands exhibit metal-triggered stabilization as monitored by thermal denaturation studies, circular dichroism, and nondenaturing gel electrophoresis. Formation of the square-planar CuII(pyridine)4 complex was confirmed by EPR measurements. The metal complexation is fully reversible by removal of the transition metal with ethylenediaminetetraacetic acid (edta)

The mode of α-synuclein binding to membranes depends on lipid composition and lipid to protein ratio.

AbstractInteractions of the presynaptic protein α-synuclein with membranes are involved in its physiological action as well as in the pathological misfolding and aggregation related to Parkinsons’s disease. We studied the conformation and orientation of α-synuclein bound to model vesicular membranes using multiparametric response polarity-sensitive fluorescent probes together with CD and EPR measurements. At low lipid to α-synuclein ratio the protein binds membranes through its N-terminal domain. When lipids are in excess, the α-helical content and the role of the C-terminus in binding increase. Highly rigid membranes also induce a greater α-helical content and a lower polarity of the protein microenvironment

Tyrosinase catalyzes asymmetric sulfoxidation

Mushroom tyrosinase was found to catalyze the oxidation of organic sulfides to sulfoxides in the presence of a catechol as cosubstrate, in a reaction which is unprecedented for this enzyme and resembles those performed by external monooxygenases. Only the oxy form of the enzyme is in fact capable of oxidizing the sulfide in a two-electron process, while the resulting met form can only be recycled by reduction with catechol. The cosubstrate competes with the sulfide also in the reaction with oxy-tyrosinase. For this reason, the sulfoxidation of thioanisole in the presence of L-3,4-dihydroxyphenyilalanine (L-dopa) occurs with moderate yields (~20%) but high enantioselectivity (~85% e,e.), and favors (S)-methyl phenyl sulfoxide. The enantioselectivity can be further increased to >90% when excess ascorbic acid is added to the reaction to limit enzyme inactivation by the quinones produced by L-dopa oxidation. An experiment using 18O2 showed that 18-O incorporation into methyl phenyl sulfoxide was above 95%, confirming that the mechanism of the sulfoxidation involves oxygen transfer from oxy-tyrosinase to the sulfide

Copper(II) compounds with NNO tridentate Schiff base ligands: effect of subtle variations in ligands on complex formation, structures and magnetic properties

The formation and the magnetic properties of the copper(II) compounds [Cu(L1)(py)](ClO4) (1a) and [Cu(L2)(py)](ClO4) (2a), bearing the NNO tridentate Schiff base ligand L1 = (E)-2-((3-aminoethylimino)methyl)phenolate or L2 = (E)-2-((3-aminopropylimino)methyl)phenolate (obtained by monocondensation of salicylaldehyde, salH, and ethylenediamine, en, or 1,3-propylenediamine, tn, respectively) and pyridine (py) are presented. These complexes are converted into new mono-, di- and trinuclear derivatives, whose nature depends on the length of the diamine used and hence on the size of the corresponding metallacycle. Pyridine can be substituted by a molecule of N,N-dimethylformamide (DMF) in compound 2a, leading to the mononuclear [Cu(L2)(dmf)](ClO4) (2b), while 1a undergoes only decomposition under similar conditions. Pyrazine does not act as bridging ligand between two copper centres by substitution of py, but its reaction with 2a yields either the dinuclear compound [Cu2(L2)2(ClO4)2] (2c), with exclusion of pyridine, or the trinuclear [Cu3(L2)3(l3-OH)](ClO4)2 (2d), where pyrazine acts as base generating OH, and it does not appear in the product. Reaction of 1a with pyrazine yields only the trinuclear [Cu3(L1)3(l3-OH)](ClO4)2 (1d). Also with 2,20-bipyridine (2,20-bpy) dinuclear complexes are not formed, but bpy acts as bidentate ligand to copper yielding the pentacoordinated mononuclear compounds [Cu(L1)(2,20-bpy)](ClO4) (1e) and [Cu(L2)(2,20-bpy)](ClO4) (2e). The crystal structures of compounds 2b, 2c and 1e have been solved and are reported. The magnetic susceptibilities vM(T) of 1a and 2a have been studied, showing the absence of any measurable Cu–Cu interaction for 1a (en, five-membered ring), while a weak but interesting intermolecular Cu–Cu ferromagnetic coupling (J = +0.96(3) cm1) through the short dimeric CuO contacts is detected for 2a (tn, six-membered ring). The X-band EPR spectrum of 2a in a frozen methanol solution at 70 K shows the hyperfine coupling of mononuclear copper with the three coordinated 14N atoms, yielding seven narrow lines

Unraveling unidirectional threading of α-cyclodextrin in a [2]rotaxane through spin labeling approach.

We present here the results of a CW-ESR investigation of a double spin labeled α-cyclodextrin-based [2]rotaxane that is characterized by the presence of nitroxide labels both at the wheel and at the dumbbell. This was accomplished by synthesizing a spin labeled α-CD (the wheel) that was mechanically blocked on a thread containing a nitroxide unit by a Cu(I) catalyzed azide–alkyne cycloaddition (CuAAC). Both ESI-MS analysis and NMR spectroscopy were used to provide evidence of the threading processes. Because of the unsymmetrical structure of both the wheel and the axle, two different geometrical isomers could be predicted on the basis of the orientation of the CD along the thread. By combining molecular dynamic calculations and information extracted from the CW-ESR spectra, we were able to determine the geometrical nature of the isomer that was isolated as the only species. The ESR spectra showed J-coupling between the two mechanically connected nitroxide units and were analyzed by a model assuming three main molecular states. The intramolecular noncovalent nature of spin exchange was confirmed by reversibly switching the magnetic interaction on–off by changing the pH of the solution in the presence of a competing macrocyclic host

Synthesis, crystal structures and magnetic properties of dinuclear copper(II) compounds with NNO tridentate Schiff base ligands and bridging aliphatic diamine and aromatic diimine linkers

The synthesis and the characterization of new dinuclear copper(II) compounds of general formula [(La–d)2Cu2(m-N–N)](ClO4)2 (1–6) with either neutral aliphatic diamine (N–N = piperazine, pip) or aromatic diimine (N–N = 4,4¢-bipyridine, 4,4¢-bipy) linker are reported. The copper ligands L- (La- = (E)-2-((2-aminoethylimino)methyl)phenolate, Lb- = (E)-2-((2-aminopropylimino)methyl)-phenolate, Lc- = (E)-2-((2-aminoethylimino)methyl)4-nitrophenolate, Ld- = (E)-2-((2-aminoethylimino)methyl)4-methoxyphenolate) are NNO tridentate Schiff bases derived from the monocondensation of a substituted salicylaldehyde 5-G-salH (G = NO2, H, OMe) with ethylenediamine, en, or 1,3-propylenediamine, tn. The crystal structures of compounds [(La)2Cu2(MeOH)2(m-4,4¢-bipy)](ClO4)2 (1·2MeOH), [(Lb)2Cu2(MeOH)2(m-4,4¢-bipy)](ClO4)2 (2·2MeOH), [(Ld)2Cu2(m-4,4¢-bipy)](ClO4)2 (4), [(La)2Cu2(m-pip)](ClO4)2 (5) and [(Lb)2Cu2(m-pip)](ClO4)2 (6) have been determined, revealing the preferred (e-e)-chair conformation of the bridging piperazine in compounds 5 and 6. The presence of hydrogen-bond-mediated intermolecular interactions, that involve the methanol molecules, yields dimers of dinuclear units for 1·2MeOH, and infinite zig-zag chains for 2·2MeOH. The temperature dependences of the magnetic susceptibilities cM(T) for all compounds were measured, indicating the presence of antiferromagnetic Cu–Cu exchange. For the compounds 2–4 with 4,4¢-bipy, the coupling constants J are around -1 cm-1, while in compound 1 no interaction could be detected. The compounds 5 and 6 with piperazine display higher Cu–Cu magnetic interactions through the s-bonding backbone of the bridging molecule, with J around -8 cm-1, and the coupling is favoured by the (e-e)-chair conformation of the diamine ring. The non-aromatic, but shorter, linker piperazine gives rise to stronger Cu–Cu antiferromagnetic couplings than the aromatic, but longer, 4,4¢-bipyridine. In the latter case, the rotation along the C–C bond between the two pyridyl rings and the consequent non co-planarity of the two copper coordination planes play an important role in determining the magnetic communication. EPR studies reveal that the dinuclear species are not stable in solution, yielding the solvated [(L)Cu(MeOH)]+ and the mononuclear [(L)Cu(N–N)]+ species; it appears that the limited solubility of the dinuclear compounds is responsible for their isolation in the solid state