Voltammetry of Osmium End‐Labeled Oligodeoxynucleotides at Carbon,Mercury, and Gold Electrodes

Voltammetric behavior of oligodeoxynucleotide (ODN) 5′‐T₄₀ (GAA)₇–3′ end‐labeled with osmium tetroxide,2,2‐bipyridine [Os(VIII)bipy] was compared with Os(VIII)bipy‐base‐ and with Os(VI)bipy‐sugar‐modified thymine ribosides. Cyclic voltammograms of Os(VIII)bipy‐modified ODN at mercury and carbon electrodes were similar but not identical to those of Os(VIII)bipy‐modified thymine riboside. Treatment of the ODN with Os(VI)bipy did not result in the ODN modification, in agreement with the known specificity of the reagent to the sugar cis‐diols. We show that in addition to mercury and carbon electrodes, the gold electrode can be used to detect Os(VIII)bipy‐labeled ODN. Comparison of voltammetric behavior of end‐labeled ODN using three types of electrodes most frequently used in DNA analysis may help to optimize electrochemical DNA sensors.     

Preparation and Characterization of Mixed‐Valent Osmium Hexacyanoferrate/Silicomolybdate Hybrid Films and Their Electrocatalytic Properties

A polynuclear mixed‐valent osmium hexacyanoferrate/silicomolybdate film electrode has been prepared using repetitive cyclic voltammetry. The cyclic voltammograms have been recorded for the deposition of a mixed‐valent osmium hexacyanoferrate/silicomolybdate hybrid film directly from the mixture of Os³⁺, Fe(CN₆)^3?, and SiMo₁₂O₄₀^4? ions from the acidic aqueous solutions. The polynuclear mixed‐valent osmium hexacyanoferrate/silicomolybdate film exhibited four redox couples. The electrocatalytic properties of the osmium hexacyanoferrate/silicomolybdate film electrode have been studied. The modified electrode has shown good electrocatalytic properties towards the oxidation of dopamine, ascorbic acid, epinephrine, norepinephrine, and reduction of IO₃^?, Fe³⁺.     

Metal‐Ion‐Binding Analogs of Ribonucleosides: Preparation and Formation of Ternary Pd2+ and Hg2+ Complexes with Natural Pyrimidine Nucleosides

Four metal‐ion‐binding nucleosides, viz. 2,6‐bis(1‐methylhydrazinyl)‐9‐(β‐D ‐ribofuranosyl)‐9H‐purine ( 2a ) and its N‐acetylated derivative, 2b , 2,4‐bis(3,5‐dimethyl‐1H‐pyrazol‐1‐yl)‐5‐(β‐D ‐ribofuranosyl)pyrimidine ( 3 ), and 2,4‐bis(1‐methylhydrazinyl)‐5‐(β‐D ‐ribofuranosyl)pyrimidine ( 4 ) have been synthesized. The ability of these nucleosides and the previously prepared 2,6‐bis(3,5‐dimethyl‐1H‐pyrazol‐1‐yl)‐9‐(β‐D ‐ribofuranosyl)‐9H‐purine to form Pd²⁺‐ and Hg²⁺‐mediated complexes with uridine has been studied by ¹H‐NMR spectroscopy. To obtain additional support for the interpretation of the NMR data, comparative measurements on the ternary‐complex formation between pyridine‐2,6‐dicarboxamide ( 5 ), pyrimidine nucleosides, and K₂PdCl₄ were carried out.     

Butylacrylate‐nucleobase Conjugates as Targets for Two‐step Redox Labeling of DNA with an Osmium Tetroxide Complex

Modification of nucleic acids with osmium tetroxide reagents (Os,L, such as OsO₄,2,2′‐bipyridine, Os,bpy) has been applied in redox DNA labeling, in probing DNA structure as well as in studies of DNA interactions with other molecules. In natural DNA, primarily thymine residues form adducts with the Os,bpy in a structure selective manner. In this paper we introduce a new two‐step technique of DNA modification with the electroactive Os,bpy, consisting in enzymatic construction of DNA bearing butyl acrylate (BA) moieties attached to uracil at C5 or to 7‐deaza adenine at C7, followed by chemical modification of a reactive C=C double bond in the acrylate residue. We demonstrate a facile modification of the BA conjugates in both single‐ and double‐stranded (ds) DNA under conditions when modification within the nucleobase rings in ds DNA is hindered. Various DNA−Os,bpy adducts can easily be analyzed electrochemically and distinguished by different redox potentials. The two‐step procedure appears to be applicable in osmium redox labelling of ds DNA.     

Electrochemical Characteristics of 4‐oxo‐4H‐pyran‐dicarboxylic Acid (Chelidonic Acid) and some of its Metal Complexes

A few novel metal complexes of chelidonic acid (chelH₂), namely [Ca(chel)(H₂O)₃] ( 1 ), [Cu(chel)(H₂O)₅] ⋅ 2H₂O ( 2 ) and [VO(chel)(H₂O)₃] ⋅ 2H₂O ( 3 ) were prepared, identified by elemental analysis and characterized by electrochemical methods. IR‐spectra and thermal stability in solid state are discussed as well. The electrochemical characteristics of the free chelidonic acid and its complexes 1 – 3 were studied by (cyclic) square‐wave voltammetry, on static mercury drop electrode (SMDE) and paraffin‐impregnated graphite electrode (PIGE), in aqueous media over a wide pH range. The reduction of chelidonic acid on SMDE is a kinetically controlled electrode reaction, occurring with the transfer of one electron and two protons for 1<pH<6, whereas in very alkaline media the electron transfer is pH independent, i.e . the mechanism of electro‐reduction of chelH₂ is proposed. The experimental parameters of the electroanalytical procedure were optimized and the method was applied for the investigation of the metal ion coordination preferences toward chelidonic acid. For the direct determination of solid complexes 1 – 3 , SW voltammetry of microparticles was used.     

Square Wave Stripping Voltammetry of Unlabeled Single‐ and Double‐Stranded DNAs

We show that, in difference to previously applied electrochemical methods working with stationary electrodes, square wave voltammetry produces well‐developed peaks II_SW (specific for dsDNA) and III_SW yielded by ssDNA at hanging mercury drop electrode (HMDE) and solid amalgam electrodes (SAEs). Using these peaks various kinds of DNA structural transitions can be studied, including unwinding of dsDNA at negatively charged electrode surfaces. The sensitivity of the DNA analysis is much better than that obtained with guanine oxidation signals at carbon electrodes. Both carbon electrodes and SAEs appear attractive as transducers in label‐free RNA and DNA sensors.     

Light Harvesting and Directional Energy Transfer in Long‐Lived Homo‐ and Heterotrimetallic Complexes of FeII,RuII, and OsII

A new family of trimetallic complexes of the form [(bpy)₂M(phen‐Hbzim‐tpy)M′(tpy‐Hbzim‐phen)M(bpy)₂]⁶⁺ (M=Ru^II, Os; M′=Fe^II, Ru^II, Os; bpy=2,2′‐bipyridine) derived from heteroditopic phenanthroline–terpyridine bridge 2‐{4‐[2,6‐di(pyridin‐2‐yl) pyridine‐4‐yl]phenyl}‐1H‐imidazole[4,5‐f][1,10]phenanthroline (phen‐Hbzim‐tpy) were prepared and fully characterized. Zn²⁺ was used to prepare mixed‐metal trimetallic complexes in situ by coordinating with the free tpy site of the monometallic precursors. The complexes show intense absorptions throughout the UV/Vis region and also exhibit luminescence at room temperature. The redox behavior of the compounds is characterized by several metal‐centered reversible oxidation and ligand‐centered reduction processes. Steady‐state and time‐resolved luminescence data show that the potentially luminescent Ru^II‐ and Os^II‐based triplet metal‐to‐ligand charge‐transfer (³MLCT) excited states in the triads are quantitatively quenched, most likely by intercomponent energy transfer to the lower lying ³MLCT (for Ru and Os) or triplet metal‐centered (³MC) excited states of the Fe^II subunit (nonluminescent). Interestingly, iron did not adversely affect the photophysics of the respective systems. This suggests that the multicomponent molecular‐wire‐like complexes investigated here can behave as efficient light‐harvesting antennas, because all the light absorbed by the various subunits is efficiently channeled to the subunit(s) in which the lowest‐energy excited states are located.     

Adsorptive Square‐Wave Voltammetry for the Analysis and Speciation of Complexes of Cd(II)‐Ferron

The theory of adsorptive stripping square‐wave voltammetry (SWV) for relatively low ligand concentrations is employed to determine the reduction mechanism of Cd(II)‐ferron complexes accumulated on a static mercury drop electrode at different pH values. The electrochemical behavior of ferron molecules indicated that the adsorptive concentration of Cd(II) is possible in solutions with 3.5<pH<11, providing a wide pH range where the interference of other ligands present in real samples would be not so critical. Cyclic voltammetry experiments were also performed for the purpose of comparison. Fitting between experimental and theoretical square‐wave voltammograms shows that the prevailing species at the reaction layer coincide with the equilibrium bulk distribution. The simulation procedure indicated that the electrochemical rate constants of Cd(II)‐ferron complexes varied from (6±1) s^?1 to (0.17±0.01) s^?1 for solutions analyzed at pH 3.9 and 10.8, respectively. Changes at the surface concentrations are discussed considering the ligand to complex ratios at the electrode surface and at the solution bulk. From this analysis it is possible to infer that the oxidized metal species are produced in the electrolytic solution instead of on the electrode surface.     

Voltammetric Detection of Thrombin by Labeling with Osmium Tetroxide Bipyridine and Binding with Aptamers on a Gold Electrode

This communication reports on electrochemical detection of thrombin based on labeling with osmium tetroxide bipyridine [OsO₄(bipy)]. Tryptophan amino acids can be labeled at the C−C‐double bond, and at least some tryptophan moieties are accessible for labeling in thrombin. Using the catalytic hydrogen signal from adsorptive stripping voltammetry performed on hanging mercury drop electrode, we could detect as little as 1.47 nM [OsO₄(bipy)]‐modified thrombin. We also tested the binding of [OsO₄(bipy)]‐modified thrombin with the classic thrombin binding aptamer (TBA) on gold electrodes. This preliminary study revealed that even after modification, a major part of the affinity was conserved, and that the aptamer self‐assembled monolayer (SAM) could be regenerated several times. Molecular simulations confirm that [OsO₄(bipy)]‐modified thrombin largely preserves the high binding affinity also of the alternative HD22 aptamer to thrombin, albeit at slightly reduced affinities due to steric hindrance when tryptophans 96 and 237 are labelled. Based on these simulations, compensatory modifications in the aptamer should result in significantly improved binding with labelled thrombin. This combined experimental‐computational approach lays the groundwork for the rational design of improved aptamer sensors for analytical applications.     

Hybridization Detection of Osmium Tetroxide Bipyridine‐Labeled DNA and RNA on Heated Gold Wire Electrodes

We report about hybridization detection of different nucleic acids on capture probe‐modified heated gold wire electrodes. We have compared three kinds of nucleic acid targets: DNA, uracil‐conjugated DNA, and RNA. All three sorts of nucleic acids targets could be labeled with osmium tetroxide bipyridine, hybridized with immobilized DNA capture probes and then detected by square‐wave voltammetry. Heating the gold electrode instead of the entire bulk hybridization solution leads to improved hybridization efficiency in most cases. The reason could be found in a thermal micro‐stirring effect around the heated wire electrode. Also selectivity was improved. Mismatches could be discriminated for DNA and uracil‐conjugated DNA targets. Mismatches in RNA strands, however, are more difficult to detect due to relatively stable secondary structures.     

Novel 3‐Hydroxy‐2‐naphthoic hydrazone and Ni(II), Co(II) and Cu(II) Complexes: Synthesis,Spectroscopic Characterization,Antimicrobial, DNA Cleavage and Computational Studies

A novel hydrazone ligand derived from condensation reaction of 3‐hydroxy‐2‐naphthoic hydrazide with dehydroacetic acid, and its Ni(II), Cu(II) and Co(II) complexes were synthesized, characterized by spectroscopic, elemental analyses, magnetic susceptibility and conductivity methods, and screened for antimicrobial, DNA binding and cleavage properties. Spectroscopic analysis and elemental analyses indicated the formula, [MLCl₂], for the complexes; square planar geometry for the nickel, and tetrahedral geometry for copper and cobalt complexes. The non‐electrolytic natures of the complexes in Dimethyl Sulphoxide (DMSO) were confirmed by their molar conductance values in the range of 6.11–14.01 Ω^?1cm²mol^?1. The copper complex had the best antibacterial activity against Staphylococcus aureus (ATCC 29213). DNA cleavage activities of the compounds, evaluated on pBR322 DNA, by agarose gel electrophoresis, in the presence and absence of oxidant (H₂O₂) and free radical scavenger (DMSO), indicated no activity for the ligand, and moderate activity for the complexes, with the copper complex cleaving pBR322 DNA more efficiently in the presence of H₂O₂. When the complexes were evaluated for antibacterial and A‐DNA activity using Molecular docking technique, the copper complex was found to be most effective against Gram‐positive (S. aureus) bacteria. [CuLCl₂] showed good hydrogen bonding interaction with the major‐groove (C₂^.G₁₃ base pair) of A‐DNA. Density functional theory (DFT) calculations of the structural and electronic properties of the complexes revealed that [CuLCl₂] had a smaller HOMO‐LUMO gap, suggesting a higher tendency to donate electrons to electron‐accepting species of biological targets.     

Electrocapillary Activity and Adsorptive Accumulation of U(VI)‐Cupferron and U(VI)‐Chloranilic Acid Complexes on Mercury Electrode

Interfacial activity of uranium(VI)‐cupferron and uranium(VI)‐chloranilic acid (CAA) complexes (in 0.1 M acetate buffer pH 4.6 or 0.1 M NaClO₄ respectively) on polarized mercury electrode at 110 mV, 10 mV or ?240 mV respectively vs. saturated calomel electrode (SCE), and under conditions of the application of adsorptive stripping voltammetric techniques was studied. It revealed a competitive effect of interfacial activity of the mentioned complexes consisting in a nonmonotonous effect of the bulk concentration of U(VI) on the adsorption of the mentioned complexing reagents at their constant concentrations. At concentrations lower than 5×10^?5 mol L^?1 the complexes U(VI)‐cupferron or U(VI)‐CAA exhibited a relatively strong electrosorption providing the adsorption coefficients β of the order 10⁴ L mol^?1, the maximum surface excess Γ_m ≈ 5 to 10 μmol m^?2 and average Frumkin interaction coefficients reaching their absolute values 2 to 2.6.     

Mixed‐Ligand Complexes of Copper(II) with α‐Hydroxycarboxylic Acids and 1,10‐Phenanthroline

Eight new two‐ligand complexes of copper(II) with 1,10‐phenanthroline and one of four different α‐hydroxy‐carboxylic acids (glycolic, lactic, mandelic and benzylic) were prepared. The complexes of general formula [Cu(HL)₂(phen)] · nH₂O (HL = monodeprotonated acid) ( 1 – 4 ) were characterized by elemental analysis, IR, electronic and EPR spectroscopy, magnetic measurements and thermo‐gravimetric analysis. The complexes of general formulae [Cu(HL)(phen)₂](HL) · H₂L · nSolv [ 1 a (HL = HGLYO^–, n = 1, Solv = MeCN) and 3 a (HL = HMANO^–, n = 0)] and [Cu(L)(phen)(OH₂)] · nH₂O [ 2 a (L = LACO^2–, n = 4) and 4 a (L = BENO^2–, n = 2)] were characterized by X‐ray diffractometry. In all these latter a pentacoordinated copper atom has a basically square pyramidal coordination polyhedron, the distortion of which towards a trigonal bipyramidal configuration has been evaluated in terms of the parameter τ. In 1 a and 3 a there are three forms of α‐hydroxycarboxylic acid: a monodentate monoanion, a monoanionic counterion, and a neutral molecule lying in the outer coordination sphere; in 2 a and 4 a the α‐hydroxycarboxylic acid is a bidentate dianion coordinating through carboxyl and hydroxyl oxygens.     

Osmium Complexes with Tridentate 6‐Pyrazol‐3‐yl 2,2′‐Bipyridine Ligands: Coarse Tuning of Phosphorescence from the Red to the Near‐Infrared Region

We have prepared and characterized a series of osmium complexes [Os₂(CO)₄(fpbpy)₂] ( 1 ), [Os(CO)(fpbpy)₂] ( 2 ), and [Os(fpbpy)₂] ( 3 ) with tridentate 6‐pyrazol‐3‐yl 2,2′‐bipyridine chelating ligands. Upon the transformation of complex 2 into 3 through the elimination of the CO ligand, an extremely large change in the phosphorescence wavelength from 655 to 935 nm was observed. The results are rationalized qualitatively by the strong π‐accepting character of CO, which lowers the energy of the osmium d_π orbital, in combination with the lower degree of π conjugation in 2 owing to the absence of one possible pyridine‐binding site. As a result, the energy gap for both intraligand π–π* charge transfer (ILCT) and metal‐to‐ligand charge transfer (MLCT) is significantly greater in 2 . Firm support for this explanation was also provided by the time‐dependent DFT approach, the results of which led to the conclusion that the S₀→T₁ transition mainly involves MLCT between the osmium center and bipyridine in combination with pyrazolate‐to‐bipyridine ³π–π* ILCT. The relatively weak near‐infrared emission can be rationalized tentatively by the energy‐gap law, according to which the radiationless deactivation may be governed by certain low‐frequency motions with a high density of states. The information provided should allow the successful design of other emissive tridentate metal complexes, the physical properties of which could be significantly different from those of complexes with only a bidentate chromophore.     

Complexes of Group 12 Metal Dihalides with 2‐Acetylpyridine‐N‐oxide 4N‐methylthiosemicarbazone (H4MLO). The Crystal Structures and Organization of [Zn(H4MLO)X2] (X = Br,I)

Reaction of group 12 metal dihalides with 2‐acetylpyridine‐N‐oxide ⁴N‐methylthiosemicarbazone (H4MLO) in ethanol afforded compounds [M(H4MLO)X₂] (M = Zn^II, Cd^II, Hg^II; X = Cl, Br, I), the structures of which were characterized by elemental analysis and by IR and ¹H and ¹³C NMR spectroscopy. In addition, the complexes of ZnBr₂ and ZnI₂ were analysed structurally by X‐ray diffractometry. In [Zn(H4MLO)Br₂] the ligand is O,N,S‐tridentate and the metal is pentacoordinated, while in [Zn(H4MLO)I₂] the thiosemicarbazone is S,O‐bis‐monodentate and the Zn^II cation has a distorted tetrahedral coordination polyhedron. In assays of antifungal activity against Aspergillus niger and Paecilomyces variotii, only the mercury compounds showed any activity, and only [Hg(H4MLO)Cl₂] and [Hg(H4MLO)I₂] were competitive with nystatin against A. niger.     

Osmium(III) and Osmium(V) Complexes Bearing a Macrocyclic Ligand: A Simple and Efficient Catalytic System for cis‐Dihydroxylation of Alkenes with Hydrogen Peroxide

A simple protocol that uses [Os^III(OH)(H₂O)(L ‐N₄Me₂)](PF₆)₂ ( 1 ; L ‐N₄Me₂=N,N′‐dimethyl‐2,11‐diaza[3.3](2,6)pyridinophane) as a catalyst and H₂O₂ as a terminal oxidant for efficient cis‐1,2‐dihydroxylation of alkenes is presented. Unfunctionalized (or aliphatic) alkenes and alkenes/styrenes containing electron‐withdrawing groups are selectively oxidized to the corresponding vicinal diols in good to excellent yields (46–99 %). In the catalytic reactions, the stoichiometry of alkene:H₂O₂ is 1:1, and thus the oxidant efficiency is very high. For the dihydroxylation of cyclohexene, the catalytic amount of 1 can be reduced to 0.01 mol % to achieve a very high turnover number of 5500. The active oxidant is identified as the Os^V(O)(OH) species ( 2 ), which is formed via the hydroperoxide adduct, an Os^III(OOH) species. The active oxidant 2 is successfully isolated and crystallographically characterized.