Interaction of metal ions with biomolecular ligands: how accurate are calculated free energies associated with metal ion complexation?

To address fundamental questions in bioinorganic chemistry, such as metal ion selectivity, accurate computational protocols for both the gas-phase association of metal-ligand complexes and solvation/desolvation energies of the species involved are needed. In this work, we attempt to critically evaluate the performance of the ab initio and DFT electronic structure methods available and recent solvation models in calculations of the energetics associated with metal ion complexation. On the example of five model complexes ([M(II)(CH(3)S)(H(2)O)](+), [M(II)(H(2)O)(2)(H(2)S)(NH(3))](2+), [M(II)(CH(3)S)(NH(3))(H(2)O)(CH(3)COO)], [M(II)(H(2)O)(3)(SH)(CH(3)COO)(Im)], [M(II)(H(2)S)(H(2)O)(CH(3)COO)(PhOH)(Im)](+) in typical coordination geometries) and four metal ions (Fe(2+), Cu(2+), Zn(2+), and Cd(2+); representing open- and closed-shell and the first- and second-row transition metal elements), we provide reference values for the gas-phase complexation energies, as presumably obtained using the CCSD(T)/aug-cc-pVTZ method, and compare them with cheaper methods, such as DFT and RI-MP2, that can be used for large-scale calculations. We also discuss two possible definitions of interaction energies underlying the theoretically predicted metal-ion selectivity and the effect of geometry optimization on these values. Finally, popular solvation models, such as COSMO-RS and SMD, are used to demonstrate whether quantum chemical calculations can provide the overall free enthalpy (ΔG) changes in the range of the expected experimental values for the model complexes or match the experimental stability constants in the case of three complexes for which the experimental data exist. The data presented highlight several intricacies in the theoretical predictions of the experimental stability constants: the covalent character of some metal-ligand bonds (e.g., Cu(II)-thiolate) causing larger errors in the gas-phase complexation energies, inaccuracies in the treatment of solvation of the charged species, and difficulties in the definition of the reference state for Jahn-Teller unstable systems (e.g., [Cu(H(2)O)(6)](2+)). Although the agreement between the experimental (as derived from the stability constants) and calculated values is often within 5 kcal·mol(-1), in more complicated cases, it may exceed 15 kcal·mol(-1). Therefore, extreme caution must be exercised in assessing the subtle issues of metal ion selectivity quantitatively.     

Resonances in scattering by two magnetic fields at large separation and a complex scaling method

We study the quantum resonances in magnetic scattering in two dimensions. The scattering system consists of two obstacles by which the magnetic fields are completely shielded. The trajectories trapped between the two obstacles are shown to generate the resonances near the positive real axis, when the distance between the obstacles goes to infinity. The location is described in terms of the backward amplitudes for scattering by each obstacle. A difficulty arises from the fact that even if the supports of the magnetic fields are largely separated from each other, the corresponding vector potentials are not expected to be well separated. To overcome this, we make use of a gauge transformation and develop a new type of complex scaling method. We can cover the scattering by two solenoids at large separation as a special case. The obtained result heavily depends on the magnetic fluxes of the solenoids. This indicates that the Aharonov–Bohm effect influences the location of resonances.     

Variability of total and mobile element contents in ash derived from biomass combustion

Seven samples of the ash derived from biomass, representing both fly and bottom ash, were analysed for a wide spectrum of total and mobile contents of nutrient and potentially risk elements. Several techniques, X-ray fluorescence (XRF) spectrometry, instrumental neutron activation analysis (INAA), proton-induced gamma-ray emission (PIGE) and proton induced X-ray emission (PIXE), inductively coupled plasma-atomic emission spectrometry (ICP-OES), and flame atomic absorption spectrometry (F-AAS) were compared. The results showed fairly good agreement between the XRF and INAA results, where the correlation coefficients (r) varied between 0.96 and 0.98. Lower contents documenting insufficient dissolution of the ash samples in the applied acid mixture were observed for both ICP-OES and AAS. In this case, weaker correlation with the INAA results not exceeding r = 0.7 were obtained. Therefore, the sample decomposition step is a bottleneck of the accurate analysis of this type of materials. For the assessment of plant-available portions of the elements in the ash samples, the Mehlich III extraction procedure and the extraction with a 0.11 mol L^?1 solution of CH₃COOH were applied. The results showed relatively low mobility of the elements (especially micronutrients) in the ash samples regardless of their source and composition, suggesting limited immediate effect of direct ash application as a fertilizer.     

Towards the Development of a Selective Ruthenium‐Catalyzed Hydroformylation of Olefins

The ruthenium‐catalyzed hydroformylation of 1‐ and 2‐octene to give preferentially the corresponding linear aldehyde is reported. The catalyst system comprising of Ru₃(CO)₁₂ and an imidazole‐substituted monophosphine ligand allows for high chemo‐ and regioselectivity. The hydroformylation proceeds with unprecedented rates for a ruthenium‐based catalyst.     

Biosensor Based on Electrocodeposition of Carbon Nanotubes/Polypyrrole/Laccase for Neurotransmitter Detection

A new biosensor has been developed by a simple electrocodeposition of multiwalled carbon nanotubes (MWCNT), polypyrrole (PPy) and laccase (Lac) on the platinum (Pt) electrode surface. The neurotransmitter biosensor was applied to the detection of dopamine in urine samples using differential pulse voltammetry (DPV). The Pt/MWCNT/PPy/Lac biosensor exhibited a detection limit of 0.14 µM, which is an adequate level for monitoring dopamine in urine. Reproducibility and repeatability values of 2.9 % and 1.7 %, respectively, were obtained compared to the conventional procedure. The proposed biosensor was successfully applied in the determination of dopamine in urine, and the obtained results were in full agreement with those from the HPLC procedure.     

Inhibition of Candida albicans CC biofilms formation in polystyrene plate surfaces by biosurfactant produced by Trichosporon montevideense CLOA72

This study evaluated the effects of glycolipid-type biosurfactant produced by Trichosporon montevideense CLOA72 in the formation of biofilms in polystyrene plate surfaces by Candida albicans CC isolated from the apical tooth canal. Biofilm formation was reduced up to 87.4% with use of biosurfactant at 16 mg/ml concentration. It has been suggested that the interaction with the cell or polystyrene plate surface could ultimately be responsible for these actions. Therefore, the interaction of C. albicans CC cells with the biosurfactant, as well as the corresponding thermodynamic parameters, have been determined by isothermal titration calorimetry and zeta potential measurements. This process is endothermic (((int)H°=+1284±5 cal/mg OD(600)) occurring with a high increase of entropy (T((int)S°=+10635 cal/mg OD(600)). The caloric energy rate data released during the titulation indicates saturation of the cell-biosurfactant at 1.28 mg/ml OD(600). Also, the zeta potential of the cell surface was monitored as a function of the biosurfactant concentration added to cell suspension showing partial neutralization of net surface charge, since the value of zeta potential ranged from -16 mV to -6 mV during the titration. The changes of cell surface characteristics can contribute to the inhibition of initial adherence of cells of C. albicans in surface. The CMC of the purified biosurfactant produced from T. montevideense CLOA72 is 2.2 mg/ml, as determined both by ITC dilution experiments and by surface tension measurements. This biomolecule did not presented any cytotoxic effect in HEK 293A cell line at concentrations of 0.25-1 mg/ml. This study suggests a possible application of the referred biosurfactant in inhibiting the formation of biofilms on plastic surfaces by C. albicans.     

Native red electrophoresis--a new method suitable for separation of native proteins

A new type of native electrophoresis was developed to separate and characterize proteins. In this modification of the native blue electrophoresis, the dye Ponceau Red S is used instead of Coomassie Brilliant Blue to impose uniform negative charge on proteins to enable their electrophoretic separation according to their relative molecular masses. As Ponceau Red S binds less tightly to proteins, in comparison with Coomassie Blue, it can be easily removed after the electrophoretic separation and a further investigation of protein properties is made possible (e.g. an enzyme detection or electroblotting). The tested proteins also kept their native properties (enzyme activity or aggregation state).     

Structural diversification of the coordination mode of divalent metals with 1,3-propanediaminetetraacetate (1,3-pdta): The missing crystal structure of the s-block metal complex [Sr2(1,3-pdta)(H2O)6]·H2O

This paper reports the synthesis and X-ray characteristics of the missing homonuclear s-block metal complex {[Sr₂(1,3-pdta)(H₂O)₆]·H₂O}_n. In the title compound, the hexadentate 1,3-propanediaminetetraacetate (1,3-pdta) ligand joins to two Sr(II) centers via the diamine chain. Moreover, each Sr(II) is bridged through two carboxylate O atoms and a water molecule to two neighboring Sr(II) ions. The coordination sphere around each Sr(II) ion consists of one diamine nitrogen, four carboxylate oxygens and four water molecules. Comparison with the previously reported M(II)-1,3-pdta complexes reveals that increasing of the ion size results in the incorporation of water molecules into its first coordination sphere and consequent increase of the coordination number (C.N.) from six to seven or eight, while keeping the hexadentate coordination mode of the ligand. Further increase of the metal ion size leads to the loss of the chelating properties of the diamine and formation of a bis-tridentate complex. Associated with it is the change in the binding mode of the carboxylate groups. This forms the basis for classification of divalent metal 1,3-pdta complexes into five distinct structural classes. Additionally, in the present study X-ray powder diffraction and IR spectroscopy were used to distinguish the different structural types of M(II)-1,3-pdta complexes, including Ba[Ba(1,3-pdta)]·2H₂O which has been used for their preparation.     

Photochemical approach to naphthoxazoles and fused heterobenzoxazoles from 5-(phenyl/heteroarylethenyl)oxazoles

A new synthetic approach is presented for the synthesis of naphthoxazoles and fused heterobenzoxazoles. The starting 5-(aryl/furyl/thienyl/pyridyl ethenyl)oxazoles are prepared from the corresponding α,β-unsaturated aldehydes using Van Leusen reagent in very good yields and are transformed into naphthoxazoles and fused heterobenzoxazoles on irradiation under aerobic conditions and in the presence of iodine.     

A Spectrophotometric Study of Streptomycin Effect on the Clinical Urea Determination

The results of studying inhibitory effect of streptomycin on the modified Berthelot reaction were presented in this paper and a new kinetic method for determining streptomycin in pharmaceutical preparations and human urine was developed on the basis of the obtained results. The rates of catalytic and catalytic‐inhibitory reaction were monitored at 700 nm (t=25±0.1°C) using UV/vis spectrophotometer. By analyzing the spectra and experimental dependences of the catalytic and catalytic‐inhibitory reaction rates on the reactant concentrations, it was noticed that streptomycin attacked nitroprusside and hypochlorite causing the inhibition of the production of 2,2′‐dicarboxylindophenol. According to this effect, an analytical decrease for determination of urea by modified Berthelot reaction appeared in the presence of small amounts of streptomycin. Beer's law was obeyed in the interval of streptomycin sulfate concentration from 18.2 to 182 µg·mL^?1. The detection limit calculated by two methods was obtained at 11.75 µg·mL^?1 and 8.54 µg·mL^?1. The relative standard deviation of 0.55%–8.83% and the recovery of 109.10% were determined. The obtained results were validated using the referent HPLC method.