Unveiling the tartrazine binding mode with ds-DNA by UV-visible spectroscopy, electrochemical, and QM/MM methods.

Here, we studied the interaction between the food colorant tartrazine (TZ) and double stranded DNA (dsDNA), using spectroscopic, electrochemical, and computational methods such as QM/MM combined with TD-DFT. Despite the UV-vis spectroscopy is widely used to study the interaction between molecules, for the case of TZ there are discrepancies in the analyses presented in the literature available, presenting both hyperchromic and hypochromic effects and consequently different rationalizations for their results. Herein we propose the combination of UV-vis experiments with the design of high-level computational models capable of reproducing the experimental behavior to finally define the proper binding mode at the molecular scale together with the rationalization of the experimental optical response due to the complex formation. To complement the UV-vis experiments, we propose the use of electrochemical measurements, to support the results obtained through UV-vis spectroscopy, as it has been successfully used for the determination of interaction modes between small molecules and biomolecules in any condition. Our UV-vis spectroscopy experiments showed only a hypochromic effect of the absorption spectra of TZ after interaction with DNA, indicative of TZ being deeply buried in the DNA structure. The effect of ionic strength in the experimental procedures led to the dissociation of TZ, thus indicating that the interaction mode was groove binding. On the other hand, the electrochemical studies showed an irreversible reduction peak of TZ, which after the interaction with DNA exhibited a positive shift in potential that can be attributed to groove binding. The binding constant for TZ-DNA was calculated as 4.45x104M-1 (UV-vis) and 5.75x104M-1 (electrochemistry), in line with other groove binder azo dyes. Finally, through the QM/MM calculations we found that the minor-groove binding mode interacting in zones rich in adenine and thymine was the model best suited to reproduce the experimental UV-vis response.

Nitro radical anion formation from nitrofuryl substituted 1,4-dihydropyridine derivatives in mixed and non-aqueous media.

Three new nitrofuryl substituted 1,4-dihydropyridine derivatives were electrochemically tested in the scope of newly found compounds useful as chemotherapeutic alternative to the Chagas' disease. All the compounds were capable to produce nitro radical anions sufficiently stabilized in the time window of the cyclic voltammetric experiment. In order to quantify the stability of the nitro radical anion we have calculated the decay constant, k2. Furthermore, from the voltammetric results, some parameters of biological significance as E7(1) (indicative of in vivo nitro radical anion formation) and KO2 (thermodynamic indicator of oxygen redox cycling) have been calculated. From the comparison of E7(1), KO2 and k2 values between the studied nitrofuryl 1,4-DHP derivatives and well-known current drugs an auspicious activity for one of the studied compounds i.e. FDHP2, can be expected.

Electrochemical, UV--visible and EPR studies on nitrofurantoin: nitro radical anion generation and its interaction with glutathione.

This paper deals with the reactivity of the nitro radical anion electrochemically generated from nitrofurantoin with glutathione. Cyclic voltammetry (CV) and controlled potential electrolysis were used to generate the nitro radical anion in situ and in bulk solution, respectively and cyclic voltammetry, UV--Visible and EPR spectroscopy were used to characterize the electrochemically formed radical and to study its interaction with GSH. By cyclic voltammetry on a hanging mercury drop electrode, the formation of the nitro radical anion was possible in mixed media (0.015M aqueous citrate/DMF, 40/60, pH 9) and in aprotic media. A second order decay of the radicals was determined, with a k2 value of 201 and 111 M(-1) s(-1), respectively. Controlled potential electrolysis generated the radical and its detection by cyclic voltammetry, UV--Visible and EPR spectroscopy was possible. When glutathione (GSH) was added to the solution, an unambiguous decay in the signals corresponding to a nitro radical anion were observed and using a spin trapping technique, a thiyl radical was detected. Electrochemical and spectroscopic data indicated that it is possible to generate the nitro radical anion from nitrofurantoin in solution and that GSH scavenged this reactive species, in contrast with other authors, which previously reported no interaction between them.

Reactivity of the one-electron reduction product from nifedipine with relevant biological targets.

The reactivity of the electrochemically generated nitro radical anion from nifedipine, a nitro aryl 1,4-dihydropyridine derivative, with relevant endobiotics and thiol-containing xenobiotics, was quantitatively assessed by cyclic voltammetry. The method was based on the decrease in the return-to-forward peak current ratio after the addition of compounds. A quantitative procedure to calculate the respective interaction constants between the radicals and the xeno/endobiotics is also provided. In the optimal selected conditions, i.e. mixed media (0.015 M aqueous citrate/DMF: 40/60, 0.3 M KCl, 0.1 TBAI) at pH 9.0 the following order of reactivity was obtained: glutathione > uracil > adenine and cysteamine > N-acetylcysteine > captopril > penicillamine. In all cases, the interaction rate constants for these derivatives were greater than the natural decay constant of the radical. Studies on the reactivity at pH 7.4 were also conducted. Results from these experiments indicate a significant reactivity between the radical and the endo/xenobiotics. The increase in the stability of the radical anion by increasing the pH of the mixed media resulted in a decreased reaction with the endo/xenobiotics tested. Computerized simulation with DIGISIM 2.0 of the proposed mechanisms fitted very well with the experimental results for both the natural decay of the radical and its reaction with the tested compounds.

Electrochemical generation and reactivity of free radical redox intermediates from ortho- and meta-nitro substituted 1,4-dihydropyridines.

This paper reports a comprehensive study by cyclic voltammetry on the electrochemical characteristics and the reactivity of the one-electron reduction product from a series of nitro aryl 1,4-dihydropyridines in mixed and aprotic media. In addition, the effects of 1,4-DHP on the oxygen consumption of T. cruzi epimastigotes are reported. One-electron reduction products from 1,4-DHP derivatives significantly reacted with both thiol compounds and the nuclei acid bases, adenine and uracil. This reactivity was significantly higher than the natural decay of the radicals in mixed media. Based on these results the following tentative order of reactivity towards the xeno/endobiotics is as follows: cysteamine > glutathione > adenineuracil. Both the stability and the reactivity of the nitro radical anions electrochemically generated from 1,4-DHP showed a linear dependence with pH. The sensitivity to pH of the radicals derived from o-nitro substituted derivatives was significantly higher than m-nitro substituted derivatives. On the other hand, in all cases an increase of pH produced a significant decrease in the interaction rate constant. Interaction studies carried out in aprotic media did not show any reactivity of the radicals towards both thiol compounds and the nuclei acid bases, adenine and uracil. Therefore, we concluded that the interaction process requires certain proton activity in the media. All the tested 1,4-dihydropyridines inhibited the oxygen consumption by T. cruzi epimastigotes, Tulahuén strain. The drugs with higher electron-affinity produced greater inhibition than those with lower electron-affinity (i.e. nicardipine vs nifedipine).

An electrochemical evidence of free radicals formation from flutamide and its reactivity with endo/xenobiotics of pharmacological relevance.

This paper reports the feasibility of free radicals formation from flutamide by using cyclic voltammetry. The electrochemical characteristics and the reactivity of the one-electron reduction product from flutamide in mixed media with thiol compounds and the nuclei acid bases are characterized. Results from this paper show the thermodynamic feasibility of free radical formation expressed for both the cathodic peak potential and the second-order rate constant values. The reactivity of the radical towards thiol compounds (glutathione, cysteamine, N-acetylcysteine) and the nuclei acid base, adenine, thymine and uracil were quantitatively assessed through the calculation of the respective interaction rate constants. Based on these results, the following tentative order of reactivity towards the xeno/endobiotics is as follows: cysteamine > uracil > glutathione > adenine > N-acetylcysteine > thymine. The stability of the nitro radical anion electrochemically generated from flutamide showed a linear dependence with pH.

Comparative study of the electrochemical behavior and analytical applications of (bio)sensing platforms based on the use of multi-walled carbon nanotubes dispersed in different polymers.

This review present a critical comparison of the electrochemical behavior and analytical performance of glassy carbon electrodes (GCE) modified with carbon nanotubes (CNTs) dispersed in different polymers: polyethylenimine (PEI), PEI functionalized with dopamine (PEI-Do), polyhistidine (Polyhis), polylysine (Polylys), glucose oxidase (GOx) and double stranded calf-thymus DNA (dsDNA). The comparison is focused on the analysis of the influence of the sonication time, solvent, polymer/CNT ratio, and nature of the polymer on the efficiency of the dispersions and on the electrochemical behavior of the resulting modified electrodes. The results allow to conclude that an adequate selection of the polymers makes possible not only an efficient dispersion of CNTs but also, and even more important, the building of successful analytical platforms for the detection of different bioanalytes like NADH, glucose, DNA and dopamine.

Voltammetric behaviour of bromhexine and its determination in pharmaceuticals.

A complete electrochemical study and a novel electroanalytical procedure for bromhexine quantitation are described. Bromhexine in methanol/0.1molL(-1) Britton-Robinson buffer solution (2.5/97.5) shows an anodic response on glassy carbon electrode between pH 2 and 7.5. By DPV and CV, both peak potential and current peak values were pH-dependent in all the pH range studied. A break at pH 5.5 in E(P) versus pH plot revealing a protonation-deprotonation (pK(a)) equilibrium of bromhexine was observed. Spectrophotometrically, an apparent pK(a) value of 4.3 was also determined. An electrodic mechanism involving the oxidation of bromhexine via two-electrons and two-protons was proposed. Controlled potential electrolysis followed by HPLC-UV and GC-MS permitted the identification of three oxidation products: N-methylcyclohexanamine, 2-amino-3,5-dibromobenzaldehyde and 2,4,8,10-tetrabromo dibenzo[b,f][1,5] diazocine. DPV at pH 2 was selected as optimal pH for analytical purposes. Repeatability, reproducibility and selectivity parameters were adequate to quantify bromhexine in pharmaceutical forms. The recovery was 94.50+/-2.03% and the detection and quantitation limits were 1.4x10(-5) and 1.6x10(-5)molL(-1), respectively. Furthermore, the DPV method was applied successfully to individual tablet assay in order to verify the uniformity content of bromhexine. No special treatment of sample were required due to excipients do not interfered with the analytical signal. Finally the method was not time-consuming and less expensive than the HPLC one.

Electrochemical and spectroelectrochemical behavior of the main photodegradation product of nifedipine: the nitrosopyridine derivative.

PURPOSE: To characterize the electrochemical behavior of the photodegradation product of nifedipine, i.e., 2,6-dimethyl-4-(2-nitrosophenyl)-3,5-pyridine-carboxylic acid dimethyl ester (NPD) in different electrolytic media. We also evaluated the interaction between free radicals generated from NPD and xeno/endobiotics. METHODS: Tast polarography, differential pulse polarography, and cyclic voltammetry were used for the characterization. Controlled potential electrolysis and ultraviolet-visible spectroscopy were used to generate and to detect the nitroso radical anion. RESULTS: In protic media, the NPD derivative gave a reversible well-defined peak either on Hg or glassy carbon electrodes in a reaction involving two electrons and two protons to give the hydroxylamine derivative. In mixed aqueous-organic media (pH 9) and in aprotic media, nitroso radical anion was isolated and characterized, exhibiting second-order dimerization rate constant (k2) values of 11,300 +/- 210 [Ms](-1) and 8,820 +/- 78 [Ms](-1), respectively. Reactivity of the nitroso radical anion with relevant pharmacologic targets revealed a significant interaction with the tested endo/xenobiotics (cysteamine, GSH, N-acetylcysteine, and adenine). CONCLUSIONS: Both in mixed and aprotic media, NPD generated free-radical species, the nitroso radical anion. Taking into account their respective interaction rate constants, the following tentative rank order of reactivity can be established as follows: cysteamine > N-acetylcysteine > GSH > adenine.

Ultratrace measurements of nucleic acids by baseline-corrected adsorptive stripping square-wave voltammetry.

The direct and reliable electrochemical detection of DNA is of paramount importance to the development of modern DNA hybridization chips, for the detection of nucleic acids following their electrophoretic separations, or for the sensing of DNA damage and interactions. Such solid electrode voltammetric measurements of nucleic acids have been traditionally hampered by the large solvent decomposition background current that obscures the oxidation signals of the purine nucleobases. This paper reports on the use of adsorptive stripping square-wave voltammetry, in connection with the "moving average baseline correction" approach, for monitoring ultratrace levels of DNA and RNA. Compared to other baseline-fitted or background-subtraction protocols, the moving average baseline scheme is particularly effective in isolating the small purine nucleobase peaks, which appear as small shoulders on the steep background discharge contribution. The remarkably flat baseline thus obtained (up to extreme potentials) leads to a dramatic lowering of the detection limits to the femtomole level and to a performance that compares favorably with that of computerized chronopotentiometric measurements of nucleic acids. Combined with the speed of square-wave voltammetric measurements, such developments should expand the role of voltammetry in DNA diagnostics and nucleic acid research.

Effects of 3-chloro-phenyl-1,4-dihydropyridine derivatives on Trypanosome cruzi epimastigotes.

A series of 3-chloro-phenyl-1,4-dihydropyridine derivatives produced different degrees of inhibition of parasite growth and respiration on clone Brener, LQ and Tulahuen strains of Trypanosome cruzi epimastigotes. Respiratory chain inhibition appears to be a posible determinant of the trypanosomicidal activity of this compounds. No difference in the action of these derivatives was found among the different parasite strains. For comparative purposes, the inhibitory effects of felodipine and nicardipine are also reported. A good correlation between toxic effects and the easiness of oxidation of the dihydripyridine ring was found. The presence of a fused ring on the dihydropyridine moiety significantly diminished the inhibitory effects.