Yiannis Deligiannakis

Electron spin echo envelope modulation (ESEEM) spectroscopy as a tool to investigate the coordination environment of metal centers

Abstract The applications of electron spin echo envelope modulation (ESEEM) spectroscopy to study paramagnetic metal centers in metalloproteins and bioinorganic complexes are reviewed, with special attention to the novel spectroscopic techniques applied and the structural information obtained. We summarize the physical principles and experimental techniques of ESEEM, the spectral shapes and the methods for their analysis. The physical meaning of the spin Hamiltonian parameters is highlighted in conjunction with their utilization for obtaining structural information.

Interaction of Cu2+ with His–Val–His and of Zn2+ with His–Val–Gly–Asp, two peptides surrounding metal ions in Cu,Zn-superoxide dismutase enzyme

His-Val-His and His-Val-Gly-Asp are two naturally occurring peptide sequences, present at the active site of Cu,Zn-superoxide dismutase (Cu,Zn-SOD). The interactions of His-Val-His=A (copper binding site) with Cu(II) and of His-Val-Gly-Asp=B (zinc binding site) with Zn(II) have been studied by using both potentiometric and spectroscopic methods (visible, EPR, NMR). The stoichiometry, stability constants and solution structure of the complexes formed have been determined. The binding modes of the species [CuAH](2+) and [CuA](+) were characterized by histamine type of coordination. [CuA](+) is further stabilized by the formation of a macrochelate with the involvement of the imidazole of the C-terminal histidine. The existence of macrochelate results in a slight distortion of the coordination geometry providing good base for the development of enzyme models. The enhanced stability of the macrochelate suppresses the formation of bis-complexes as well as the amide deprotonation. This process, however, takes place at higher pH resulting in the formation of the 4 N(-) coordinated [NH(2),N(-),N(-),N(im)] species [CuAH(2-)](-). On the other hand, in the case of the Zn(II)-His-Val-Gly-Asp system, coordination takes place at the terminal carboxylate in species [ZnBH(2)](2+). Monodentate binding occurs via the N-terminal imidazole in [ZnBH](+) while histamine type of coordination is possible in [ZnB], [ZnB(2)H](-) and [ZnB(2)](2-) species. Amide deprotonation does not take place in the case of Zn(2+), hydroxo-complexes are formed instead.

Metal ion–drug interactions. Preparation and properties of manganese (II), cobalt (II) and nickel (II) complexes of diclofenac with potentially interesting anti-inflammatory activity: Behavior in the oxidation of 3,5-di-tert-butyl-o-catechol

Abstract Some new complexes of diclofenac with potentially interesting biological activity are described. The complexes of diclofenac [MnL 2 (H 2 O)], [CoL 2 (H 2 O) 2 ] · 0.5H 2 O, [CoL 2 (H 2 O)], [NiL 2 (H 2 O) 2 ] · 2H 2 O, and [NiL 2 ], were prepared by the reaction of the sodium salt, of a potent anti-inflammatory drug with MnCl 2 , CoCl 2 and NiCl 2 ·6H 2 O. Optical, EPR, infrared and electrochemical properties of these new complexes are reported. Both five and six-coordinated species were isolated in the solid state for Co(II). Both four and six-coordinated species were isolated in the solid state for Ni(II), while in DMF or MeOH solution predominant formation of six-coordinated species is observed for Co(II) and Ni(II) complexes. The ability of the complexes to catalyze the oxidation of 3,5-di- tert- butyl- o- catechol to 3,5-di- tert- butyl- o- quinone was studied by following the appearance of quinone spectrophotometrically. Correlation of the catalytic activity of these complexes to the reduction potential is reported.

Homogeneous and heterogenized copper(II) complexes as catechol oxidation catalysts

Abstract Two macroacyclic ligands represented as L1 and L2 with 3N2O and 5N donor atoms, respectively, have been synthesized by Schiff base condensation. They were subsequently grafted on a silica surface via covalent bonds. The organic ligands L1 and L2 as well as the heterogenized ligands L1·SiO2 and L2·SiO2 reacted with copper(II) leading to the formation of dinuclear copper(II) complexes. Catalytic oxidation of 3,5-di-t-butylcatechol (DTBC) by dioxygen was studied using as catalysts the homogeneous Cu2(L1) and Cu2(L2) and the heterogenized Cu2(L1)·SiO2 and Cu2(L2)·SiO2 complexes. These complexes were found to be very effective catalysts for DTBC oxidation producing mainly 3,5-di-t-butylquinone (DTBQ). During the catalytic process the formation of an o-semiquinone radical has also been confirmed. The immobilized on modified silica surface copper(II) complexes gave significantly higher DTBC conversion than the homogeneous copper(II) complexes.

Antioxidant and antiradical SiO2 nanoparticles covalently functionalized with gallic acid.

Gallic acid (GA) and its derivatives are natural polyphenolic substances widely used as antioxidants in nutrients, medicine and polymers. Here, nanoantioxidant materials are engineered by covalently grafting GA on SiO(2) nanoparticles (NPs). A proof-of-concept is provided herein, using four types of well-characterized SiO(2) NPs of specific surface area (SSA) 96-352 m(2)/g. All such hybrid SiO(2)-GA NPs had the same surface density of GA molecules (~1 GA per nm(2)). The radical-scavenging capacity (RSC) of the SiO(2)-GA NPs was quantified in comparison with pure GA based on the 2,2-diphenyl-1-picrylhydrazyl (DPPH(•)) radical method, using electron paramagnetic resonance (EPR) and UV-vis spectroscopy. The scavenging of DPPH radicals by these nanoantioxidant SiO(2)-GA NPs showed mixed-phase kinetics: An initial fast-phase (t(1/2) <1 min) corresponding to a H-Atom Transfer (HAT) mechanism, followed by a slow-phase attributed to secondary radical-radical reactions. The slow-reactions resulted in radical-induced NP agglomeration, that was more prominent for high-SSA NPs. After their interaction with DPPH radicals, the nanoantioxidant particles can be reused by simple washing with no impairment of their RSC.

A humic-acid-like polycondensate produced with no use of catalyst.

The synthesis and physicochemical characterization of a water soluble humic-acid-like polycondensate (HALP), which mimics fundamental physicochemical and spectroscopic properties of natural humic acids is presented. The polymer can be synthesized under ambient O(2) by oxidative co-polymerization of gallic (GA) and protocatechuic (PA) acid at pH >9, with no need for a catalyst. Solid state (13)C NMR spectra for GA-PA-HALP show that ring-opening reactions of GA, PA monomers are involved in GA-PA-HALP formation. EPR spectroscopy shows that GA-PA-HALP contains stable phenol-based radicals, with pH-dependent concentration. The H-binding profile of GA-PA-HALP follows the NICA-Donnan model, with two sets of distributed pK(a) values in the range 4-6 corresponding to carboxyl groups, and 7-9 corresponding to phenolic groups, as in natural HA. GA-PA-HALP bears a permanent negative charge Q(0)=-2.3 equiv kg(-1) which is higher than the literature Q(0) values of -1 to -2 equiv kg(-1) observed for natural HAs. A critical comparison of the present data with literature data is provided.

The effect of charged axial ligands on the EPR parameters in oxovanadium(IV) compounds: an unusual reduction of the Az (51V) values.

Two series of octahedral oxovanadium(IV) compounds, containing charged or neutral axial ligands, with the tetradentate amidate molecules Hcapca and H2capcah of the general formulae trans-[V(IV)OX(capca)]0/+ (where X = Cl- (1.CH2Cl2), SCN- (2), N3 (3), CH3COO- (4), PhCOO- (5), imidazole (6. CH3NO2), and eta-nBuNH2 (7)) and cis-[V(VI)OX(Hcapcah)]0/+ (where X = Cl- (8.0.5CH2Cl2), SCN (9), N3 (10.2CH3OH), and imidazole (11)), were synthesized and characterized by X-ray crystallography (1.CH3OH,8.CHCl3, 9.2CH3CN, 10.CH3CN and cis-[VO(imidazole)(Hcapcah)+) and continuous-wave electron paramagnetic resonance (cw EPR) spectroscopy. In addition to the synthesis, crystallographic and EPR studies, the optical, infrared and magnetic properties (room temperature) of these compounds are reported. Ab initio calculations were also carried out on compound 8 CHCl3 and revealed that this isomer is more stable than the trans isomer, in good agreement with the experimental data. The cw EPR studies of compounds 1-5, that is, the V(IV)O2+ species containing monoanionic axial ligands, revealed a novel phenomenon of the reduction of their A, components by about 10% relative to the N4 reference compounds ([V(IV)O-(imidazole)4]2+ and [V(IV)O(2,2-bipyridine)2]2+). In marked contrast, such a reduction is not observed in compounds 6. CH3NO2-11, which contain neutral axial ligands. Based on the spin-Hamiltonian formalism a theoretical explanation is put forward according to which the observed reduction of Az is due to a reduction of the electron - nuclear dipolar coupling (P). The present findings bear strong relevance to cw EPR studies of oxovanadium(IV) in vanadoproteins, V(IV)O2+-substituted proteins, and in V(IV)O2+ model compounds, since the hyperfine coupling constant, Az, has been extensively used as a benchmark for identification of equatorial-donor-atom sets in oxovanadium(IV) complexes.

Humic acid-inspired hybrid materials as heavy metal absorbents

Three SiO(2)-based materials were prepared via covalent immobilization of carboxyl groups (COOH), phenolic groups (GA), or humic acid on an SiO(2) surface. Their sorbing properties were evaluated for removal of heavy metals (Pb(2+), Cd(2+), Cu(2+), Zn(2+), and Mg(2+)) from aqueous solution. The data show a significant improvement for metal uptake, compared to unmodified silica that can be attributed to the adsorption of metals to the deprotonated form of functional groups (COOH, GA, HA). The metal-uptake capacity of the SiO(2)-HA material was 10 times higher that those of the other two materials. The present data provide direct experimental proof that HA can be viewed and modeled as a combination of -COO and R-OH functional groups.

Effect of metal ions on the indigenous radicals of humic acids: high field electron paramagnetic resonance study.

The interaction of indigenous radicals of humic acid (HA) with metal cations has been studied using high magnetic field (10.5T-285 GHz) electron paramagnetic resonance (HFEPR) spectroscopy. Strong [HA]-[metal] interaction was observed in the case of heavy metals, Cd(2+), Pb(2+), and Sr(2+), leading to formation of covalent bonds with the radicals of HA. On the contrary, alkaline earth metal ions, such as Mg(2+), generate only electrostatic interaction. The two types of indigenous radicals that exist in all HAs are influenced by the metal cations in a unified manner. This provides evidence that the two types of indigenous radicals in HAs originate from a unique, phenolic, moiety in HA. Mg(2+) ions dramatically changed the pH profile of the two radical types of HA, downshifting their interconversion pK(a) by ca. 3 pH units. This is the first experimental observation of the effect of metals on the H-dissociation of the radical centers in HAs.

Effects of acetate on cation exchange capacity of a Zn-containing montmorillonite: physicochemical significance and metal uptake.

Fundamental properties such as cation exchange capacity (CEC), permanent charge, pH(PZC), and metal uptake of a Zn-containing montmorillonite are modified, in a predictable manner, by a mild chemical treatment using acetate. Acetate treatment allows a controllable increase of the CEC of montmorillonite up to 180 mequiv/100 g. The CEC of the clay is increasing for decreasing Zn content, with a slope of Delta[Zn]/Delta[CEC] approximately -2. X-ray powder diffraction analysis shows that the lamellar structure of the clay remains unaltered by the acetate treatment, while XPS substantiates the removal of Zn. H(+) uptake data show that the intrinsic protonation pK values and concentration of the variable charge sites ( identical with SOH) are not modified by the acetate treatment. In contrast, the concentration of the permanent charge sites ( identical with X(-)) increased linearly with Zn removal by acetate, leading to a significant H(+) and Cd(2+) uptake enhancement. A physical model is suggested where acetate removes Zn ions strongly bound in the clay, and this in turn modulates the permanent charge and the CEC of the clay.