C. Yáñez

Cyclodextrin Inclusion Complex to Improve Physicochemical Properties of Herbicide Bentazon: Exploring Better Formulations

The knowledge of the host-guest complexes using cyclodextrins (CDs) has prompted an increase in the development of new formulations. The capacity of these organic host structures of including guest within their hydrophobic cavities, improves physicochemical properties of the guest. In the case of pesticides, several inclusion complexes with cyclodextrins have been reported. However, in order to explore rationally new pesticide formulations, it is essential to know the effect of cyclodextrins on the properties of guest molecules. In this study, the inclusion complexes of bentazon (Btz) with native βCD and two derivatives, 2-hydroxypropyl-β-cyclodextrin (HPCD) and sulfobutylether-β-cyclodextrin (SBECD), were prepared by two methods: kneading and freeze-drying, and their characterization was investigated with different analytical techniques including Fourier transform infrared spectroscopy (FT-IR), differential thermal analysis (DTA), X-ray diffractometry (XRD) and differential pulse voltammetry (DPV). All these approaches indicate that Btz forms inclusion complexes with CDs in solution and in solid state, with a stoichiometry of 1∶1, although some of them are obtained in mixtures with free Btz. The calculated association constant of the Btz/HPCD complex by DPV was 244±19 M−1 being an intermediate value compared with those obtained with βCD and SBECD. The use of CDs significantly increases Btz photostability, and depending on the CDs, decreases the surface tension. The results indicated that bentazon forms inclusion complexes with CDs showing improved physicochemical properties compared to free bentazon indicating that CDs may serve as excipient in herbicide formulations.

Study of the Interaction between Progesterone and β-Cyclodextrin by Electrochemical Techniques and Steered Molecular Dynamics

The interaction of progesterone with beta-cyclodextrin (beta-CD) was studied by differential pulse polarography. The aim of the present work was to study the effect of beta-CD on the electrochemical behavior of progesterone in aqueous solution and also to analyze the molecular interactions involved in formation of the inclusion complex. The complex with stoichiometry of 1:1 was thermodynamically characterized. In addition, steered molecular dynamics (SMD) was used to investigate the energetic properties of formation of the inclusion complex along four different pathways (reaction coordinates), considering two possible orientations. From multiple trajectories along these pathways, the potentials of mean force for formation of the beta-CD progesterone inclusion complex were calculated. The energy analysis was in good agreement with the experimental results. In the beta-CD progesterone inclusion complex, a large portion of the steroid skeleton is included in the beta-CD cavity. The lowest energy was found when the D-ring of the guest molecule is located near the secondary hydroxyls of the beta-CD cavity. In the most probable orientation, one intermolecular hydrogen bond is formed between the O of the C-20 keto group of the progesterone and a secondary hydroxyl of the beta-CD.

Voltammetric Behavior of a 4-Nitroimidazole Derivative Nitro Radical Anion Formation and Stability

A new synthesized compound, l-methyl-4-nitro-2-hydroxymethylimidazole (4-MNImOH), was electrochemically reduced at the mercury electrode in aqueous, mixed, and aprotic media. In an aqueous medium, only one voltammetric peak was observed because of the four-electron, four-proton reduction of the nitro group to the hydroxylamine derivative in the 2-12 pH range. For the mixed and nonaqueous media, it was possible to observe a reversible couple due to the first one-electron reduction step of the nitro group to the nitro radical anion.The nitro radical anion decays by a disproportionation reaction in mixed media and by dimerization in a nonaqueous medium. Both disproportionation and dimerization rate constants, k 2 , were determined according to Olmsteads approach, obtaining a value of 1460 ′ 110 M - 1 s - 1 in aprotic medium. In mixed media, the values were dependent both on pH and on the nature of the cosolvent.After comparison of 4-MNImOH with the parent compound, 4-nitroimidazole, we concluded that the substitution with 1-methyl and 2-hydroxymethyl produces a more easily reducible nitro compound and a less stable nitro radical anion than the unsubstituted 4-nitroimidazole. According to the electrochemical results, the 4-MNImOH derivative would be more suitable for enzymatic reduction and less toxic to the host than 4-nitroimidazole.

Lanthanide sorbent based on magnetite nanoparticles functionalized with organophosphorus extractants

Abstract In this work, an adsorbent was prepared based on the attachment of organophosphorus acid extractants, namely, D2EHPA, CYANEX 272, and CYANEX 301, to the surface of superparamagnetic magnetite (Fe3O4) nanoparticles. The synthesized nanoparticles were coated with oleic acid, first by a chemisorption mechanism and later by the respective extractant via physical adsorption. The obtained core–shell functionalized magnetite nanoparticle composites were characterized by dynamic light scattering, scanning electron microscopy, transmission electron microscopy, thermogravimetry, infrared absorption and vibrating sample magnetometry. All the prepared nanoparticles exhibited a high saturation magnetization capacity that varied between 72 and 46 emu g−1 and decreased as the magnetite nanoparticle was coated with oleic acid and functionalized. The scope of this study also included adsorption tests for lanthanum, cerium, praseodymium, and neodymium and the corresponding analysis of their results. Sorption tests indicated that the functionalized nanoparticles were able to extract the four studied lanthanide metal ions, although the best extraction performance was observed when the sorbent was functionalized with CYANEX 272, which resulted in a loading capacity of approximately 12–14 mgLa/gMNP. The magnetization of the synthesized nanoparticles was verified during the separation of the lanthanide-loaded sorbent from the raffinate by using a conventional magnet.

Determination of Nitrendipine with β‐Cyclodextrin Modified Carbon Paste Electrode

Carbon paste electrodes modified with -cyclodextrin have been investigated for voltammetric determination of nitrendipine. The immobilization of the -cyclodextrin on the carbon paste leads to a modification of the electrode surface that causes a significant increase in the peak current of the nitrendipine reduction, probably due to formation of an inclusion complex between -cyclodextrin and nitrendipine. This property was used with analytical purposes by developing a stripping differential pulse voltammetric (SDPV) method to determine nitrendipine.

VOLTAMMETRIC DETERMINATION OF NITROIMIDAZOPYRAN DRUG CANDIDATE FOR THE TREATMENT OF TUBERCULOSIS

PA-824 (2-nitro-6-(4-trifluoromethoxy-benzyloxy)-6,7-dihydro-5H-imidazo [2,1-b][1,3]oxazine) a nitroimidazopyran drug candidate for the treatment of tuberculosis was electrochemically studied with the aims of to propose a new electroanalytical method and to investigate the potential of an electrochemical system to mimic phase I reduction metabo lism in PA-824. The electrochemical behavior was studied using tast polarography, differential pulse polarography, differential pulse voltammetry and cyclic voltammetry. PA-824 was electrochemically reducible showing one reduction peak due to the four-electronreduction of the nitro group to form the hydroxylamine derivative. A method for the determination of PA-824 by differential pulse polarography and differential pulse voltammetry at pH 7 were proposed. The differential pulse polarography method has adequate accuracy and precise with a mean recovery of 101.11% and a standard relative deviation of 2.27%. The detection limit was 2.23 × 10−7 M. Furthermore, for comparative purposes an alternative differential pulse voltammetry and UV spectrophotometric methods were developed. On the other hand, from the electrochemical point of view, PA-824 shows similar reduction capabilities that metronidazole supporting the hypothesis that both drugs could share a similar reductive metabolism.

Voltammetric Study and Direct Analytical Determination of the Antiparkinson Drug Benserazide

For the first time, a simple differential pulse voltammetry methodology for direct determination of benserazide in presence of levodopa in tablets was developed without any redox mediator, modified electrodes, or the aplication of mathematic deconvolution of signals. Benserazide was studied by differential pulse voltammetry using glassy carbon electrode in aqueous media. The drug exhibited a main well-defined oxidation signal in a broad pH range (2–10), and two poorly resolved signals at higher potentials. We have found that levodopa does not interfere on the electrochemical response of benserazide at pH 6.0. Thus, at this pH value, the developed analytical method exhibited adequate repeatability and reproducibility (RSD < 2%), recoveries >98.5%, which permitted its successful application to both the assay and the uniformity content of benserazide. Also, hydrolytic degradation studies of benserazide were carried out by differential pulse voltammetry.

Reduced Graphene Oxides: Influence of the Reduction Method on the Electrocatalytic Effect towards Nucleic Acid Oxidation

For the first time a critical analysis of the influence that four different graphene oxide reduction methods have on the electrochemical properties of the resulting reduced graphene oxides (RGOs) is reported. Starting from the same graphene oxide, chemical (CRGO), hydrothermal (hTRGO), electrochemical (ERGO), and thermal (TRGO) reduced graphene oxide were produced. The materials were fully characterized and the topography and electroactivity of the resulting glassy carbon modified electrodes were also evaluated. An oligonucleotide molecule was used as a model of DNA electrochemical biosensing. The results allow for the conclusion that TRGO produced the RGOs with the best electrochemical performance for oligonucleotide electroanalysis. A clear shift in the guanine oxidation peak potential to lower values (~0.100 V) and an almost two-fold increase in the current intensity were observed compared with the other RGOs. The electrocatalytic effect has a multifactorial explanation because the TRGO was the material that presented a higher polydispersity and lower sheet size, thus exposing a larger quantity of defects to the electrode surface, which produces larger physical and electrochemical areas.

Electrochemistry and XPS of 2,7-dinitro-9-fluorenone immobilized on multi-walled carbon nanotubes

We report that glassy carbon electrodes (GCE) modified with multi-walled carbon nanotubes (MWCNTs) can be derivatized with 2,7-dinitro-9-fluorenone (2,7-NFN). The derivatization procedure involves simple immersion of the MWCNT-modified electrode in a solution containing 2,7-NFN. SEM images indicate that the MWCNTs form a twisted, three-dimensional array that remains attached to the GCE surface. Both electrochemical and spectroscopic measurements (XPS) indicate that 2,7-NFN is immobilized on the electrode, most probably by being trapped within the pockets of the mentioned three-dimensional array. The electrode with the immobilized 2,7-NFN is sufficiently stable to resist washing but allows both its manipulation and reduction to form the hydroxylamine derivative. This derivative can be oxidized to form a nitroso compound. Both the nitroso and hydroxylamine derivatives are also trapped within the MWCNT surface pockets. Furthermore, depending on the selected working potential, the nature of the encapsulated compound, i.e., nitro, nitroso, or hydroxylamine derivative and mixtures thereof, can be selected. All these redox pathways were verified by cyclic voltammetry and XPS.

Electrolytic Oxidation of C4-Nitrofuryl 1,4-Dihydropyridines in Nonaqueous Medium

A study of the electrolytic oxidation of three new C-4 nitrofuryl 1,4-dihydropyridines in nonaqueous aprotic medium is presented. Controlled-potential electrolysis (CPE) in dimethylformamide +0.1 M tetrabutyl-ammonium hexafluorophosphate (TBAPF 6 ) was followed by UV-visible spectroscopy, high-performance liquid chromatography (HPLC)-photodiode array (PDA), and gas chromatography-Mossbauer spectroscopy (GC-MS) chromatography, and electron spin resonance and electrochemical techniques. Carbon-centered radical intermediates produced in the electrochemical oxidation of C-4 nitrofuryl substituted 1,4-dihydropyridines were trapped with N-benzylidene-tert-butylamine-N-oxide (PBN) and their splitting constants were calculated. The neutral pyridine derivatives were identified by GC-MS techniques as final oxidation products. HPLC-PDA and GC-MS chromatographic techniques were used to follow the time-course of CPE of both parent 1,4-DHP derivatives and their respective oxidation products. Also, an overall oxidation mechanism of C-4 nitrofuryl-1,4-DHP derivatives is presented.