Maciej Witwicki

Influence of Pb(II) ions on the EPR properties of the semiquinone radicals of humic acids and model compounds: high field EPR and relativistic DFT studies.

X-band (9.76 GHz) and high field (416.00 GHz) electron paramagnetic resonance spectroscopy (EPR) was used to study the interactions between Pb(II) ions and semiquinone radicals of natural humic acids and their simple models. The EPR experiments were performed on powder samples. The formation of Pb(II) complexes with the radicals was accompanied by a significant decrease of g parameters as compared to those observed for parent radicals. Two types of complexes were identified depending on the initial concentration of Pb(II) ions. For one of them the anisotropic hyperfine coupling with the (207)Pb nucleus was observed. Systematic DFT calculations were carried out for complexes with different forms of radical ligands (L(2)(-*), HL(-*), and H(2)L*) derived from 3,4-dihydroxybenzoic acid representing different ligation schemes. The g parameters calculated for the structure characterized by a significant accumulation of spin density on the Pb atom are strongly deviated from the values observed experimentally. Moreover, a decrease of the spin population on all oxygen atoms as a result of complexation of Pb(II) via carboxyl oxygens and protonation of hydroxyl oxygens is required to reproduce the experimental g parameters.

Effects of solvents, ligand aromaticity, and coordination sphere on the g tensor of anionic o-semiquinone radicals complexed by Mg2+ ions: DFT studies.

Density functional theory (DFT) was employed to study the impact of Mg(2+) ions on the o-semiquinone radical anions of different aromaticity in protic and aprotic solvents. After the geometry optimization of ligands and complexes, their g tensors were computed at the UBP86/TZVP and UB3LYP/TZVP theory levels. The suitability of various model systems, assuming continuum dielectric approaches, different Mg(2+) coordination spheres (completed by solvent molecules), and inclusion of additional solvent molecules H-bonded to the ligands, was tested in terms of correlation between the experimental and calculated g-shifts. The effects of complexation, ligands aromaticity, and solvents on the electron spin density for o-semiquinones are discussed. To recognize clearly the changes in the nature of the g tensor components, the contributions from particular excited states were analyzed. A structural characterization of the tested complexes is expected to be helpful in investigations on the complicated biosystems in which the similar paramagnetic units are present.

New copper(II) complexes of the anti-inflammatory drug mefenamic acid: a concerted study including synthesis, physicochemical characterization and their biological evaluation

Reaction of hydrated copper(II) mefenamate in the presence of diverse N-donor ligands such as N,N,N′,N′-tetramethylethylenediamine (temed), ethylenediamine (en), β-picoline (β-pic) in a methanol:water mixture (4:1, v/v) yielded crystalline monomeric copper(II) complexes [Cu(temed) (mefenamato)2], 1, [Cu(en)2(H2O)2](mefenamato)2, 2 and [Cu(β-pic)2(mefenamato)2]·H2O, 3. The newly synthesized complexes have been characterized by elemental analysis, spectroscopic methods (FT-IR, UV-Vis and EPR), thermogravimetric analyses and single-crystal X-ray structure determination in the case of complexes 2 and 3. The ground-state geometry optimization of complex 1 was performed by DFT calculations. In order to verify the complexes capability to get bound and possibly transported by the albumin towards their biological targets (cells and/or tissues), the interaction with bovine (BSA) and human serum albumin (HSA) was studied by fluorescence emission spectroscopy. The interaction of complexes 1–3 with calf-thymus DNA (CT DNA) was monitored by UV-Vis spectroscopy, cyclic voltammetry, viscosity measurements and via the ethidium bromide (EB) displacement from the EB–DNA conjugate performed by fluorescence emission spectroscopy, as a preliminary approach to evaluate their potential biological activity.

DFT insight into o-semiquinone radicals and Ca2+ ion interaction: structure, g tensor, and stability

AbstractDensity functional theory methods were employed to elucidate the interactions between calcium ions and various o-semiquinone radicals mimicking the interactions occurring in biochemical systems. Predicted changes in the molecular and electronic structures of the radicals on Ca2+ coordination were correlated with the changes of g tensor and compared with those exerted by Mg2+ ions (reported by us previously). In order to broaden the insight into the differences between the Mg2+ and Ca2+ complexes, their relative stability was estimated on the basis of theoretically predicted Gibbs energies for the process of the complex formation.

Understanding natural semiquinone radicals – Multifrequency EPR and relativistic DFT studies of the structure of Hg(II) complexes

Multifrequency EPR spectroscopy and DFT calculations were used to investigate Hg(II) complexes with semiquinone radical ligands formed in a direct reaction between the metal ions and tannic acid (a polyphenol closely related to tannins). Because of the intricate structure of tannic acid a vast array of substituted phenolic compounds were tested to find a structural model mimicking its ability to react with Hg(II) ions. The components of the g matrix (the g tensor) determined from the high field (208 GHz) EPR spectra of the Hg(II) complexes with the radical ligands derived from tannic acid and from the model compounds were analogous, indicating a similar coordination mode in all the studied Hg(II) complexes. Since catechol (1,2-dihydroxybenzene) was the simplest compound undergoing the reaction with Hg(II) it was selected for DFT studies which were aimed at providing an insight into the structural properties of the investigated complexes. Various coordination numbers and different conformations and protonation states of the ligands were included in the theoretical analyses. g Matrices were computed for all the DFT optimized geometries. A good agreement between the theoretical and experimental values was observed only for the model with the Hg(II) ion tetracoordinated by two ligands, one of the ligands being monoprotonated with the unpaired electron mainly localized on it.

pH-dependent formation of Hg(II)-semiquinone complexes from natural phenols.

The ability of various natural phenols to form Hg(II)-semiquinone complexes was tested in the pH range of 2.8-12. EPR experiments performed at 9.6 and 34 GHz (the X- and Q-band, respectively) revealed that the complexes formed at low and high pH values exhibit a significant dissimilarity between their g-matrices (g-tensors), strongly suggesting that the complexes differ structurally. Our previous investigation on the low pH complex (Chemosphere 2015, 119, 479-484) had shown the Hg(II) ion to be tetracoordinated by two ligands, one of the ligands being monoprotonated with the unpaired electron mainly located on it. In order to reveal the molecular structure of the high pH form a DFT-based theoretical analysis was carried out in this work. For all the optimized model structures the g-matrices were computed and compared with their experimental counterparts. Good agreement was observed only if the geometry of the model Hg(II) complex was planar and the coordination sphere was composed of one fully deprotonated radical ligand and hydroxyl anions.

Can Carbamates Undergo Radical Oxidation in the Soil Environment? A Case Study on Carbaryl and Carbofuran

Radical oxidation of carbamate insecticides, namely carbaryl and carbofuran, was investigated with spectroscopic (electron paramagnetic resonance [EPR] and UV-vis) and theoretical (density functional theory [DFT] and ab initio orbital-optimized spin-component scaled MP2 [OO-SCS-MP2]) methods. The two carbamates were subjected to reaction with •OH, persistent DPPH• and galvinoxyl radical, as well as indigenous radicals of humic acids. The influence of fulvic acids on carbamate oxidation was also tested. The results obtained with EPR and UV-vis spectroscopy indicate that carbamates can undergo direct reactions with various radical species, oxidizing themselves into radicals in the process. Hence, they are prone to participate in the prolongation step of the radical chain reactions occurring in the soil environment. Theoretical calculations revealed that from the thermodynamic point of view hydrogen atom transfer is the preferred mechanism in the reactions of the two carbamates with the radicals. The activity of carbofuran was determined experimentally (using pseudo-first-order kinetics) and theoretically to be noticeably higher in comparison with carbaryl and comparable with gallic acid. The findings of this study suggest that the radicals present in soil can play an important role in natural remediation mechanisms of carbamates.