Browsing by Author "Contreras, David"

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    Chemiluminescence emission in fenton reaction driven by 1,2-dihydroxybenzenes: Mechanistic approaches using 4-substituted ligands
    (2021) Romero, Romina; Marquéz, Katherine; Benítez Olivares, Francisca Javiera; Toro Labbé, Alejandro Miguel; Cornejo Ponce, Lorena; Melín, Victoria; Contreras, David
    Fenton (F) and Fenton-like (FL) reactions can be amplified by dihydroxybenzenes (DHBs). These compounds chelate and reduce Fe(III), promoting the hydroxyl radical production ((OH)-O-center dot). The products or intermediaries of F and FL reactions driven by DHBs can produce chemiluminescence (CL) with different profiles, depending on the type of DHB involved. In this work, CL produced by F and FL systems driven by different -para substituted DHBs was measured and compared with the reactivity of each system and with the structural parameters of each DHB. CL emission was not related to the reactivity of each studied system but was favored by DHBs substituents with -NHR and -OH groups combined in the branching (NHR-DHBs). PLS multivariate regression models were constructed using computational parameters for each DHB, quinone (Q) and semiquinone (SQ(center dot)) to find the influence of structural and electronic parameters over CL emission. Analysis showed that in NHR-DHBs, the higher CL exhibited could be explained by cycling ability of these compounds. In DHBs with an electron-donor group (EDG) the CL emission would depend only on the stability of the intermediary species generated by DHB and (OH)-O-center dot reaction. While DHBs with electron-withdrawing groups (EWG) showed that CL will increase depending on the stability of the intermediaries by resonance, and by the acidity of the hydroxyl protons of the ring. PLS-SQ(center dot) showed that spin densities were strongly correlated with an increase in CL emission. DHBs with substituents that favor the delocalization of charge in the SQ(center dot) to the ramification would enhance CL emission. Meanwhile, when the delocalization is promoted over the DHB-ring, these systems become more reactive, and the CL emission is disadvantaged by quinone formation.
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    Highly efficient hydrogen evolution reaction, plasmon-enhanced by AuNP-l-TiO2NP photocatalysts
    (2020) Castillo-Rodriguez, Judith ; Ortiz, Pedro D. ; Isaacs, Mauricio ; Martinez, Natalia P. ; O’Shea, James N. ; Hart, Jack ; Temperton, Robert ; Zarate, Ximena ; Contreras, David ; Schott, Eduardo
    A set of AuNPs-l-TiO2NP nanoaggregates which showed efficient covering of the semiconductor's surface by AuNPs, as well as appropriate AuNP sizes for effective sensibilization were used as photocatalysts for the hydrogen evolution reaction (HER). Three aliphatic short-chain linkers: 3-mercaptopropionic acid (MPA), thioglycolic acid (TGA) and thiolactic acid (TLA) were used as stabilizing agents. The slight structure variations of the linkers did not produce differences in the AuNP size and morphology. However, it was interesting to show how the photocatalytic performance of the nanoaggregates is dependent on the linker present, as well as to determine the influence of the Au/TiO(2)ratio. It was found that TGA gave the best performance at a longer irradiation time, though high amounts of H(2)were also obtained for the other two linkers. Furthermore, for all samples large amounts of hydrogen were obtained, which are significantly higher than that usually obtained with plasmon-sensitized TiO(2)nanostructures. In addition, high amounts of H(2)were obtained after five catalytic cycles for all samples, showing the suitability of these nanoaggregates for the photoinduced HER.
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    Organic radicals stabilization in natural rubber: Discerning the influence of thermo-oxidation using chemically modified or unmodified lignin as antioxidant
    (2024) Troncoso-Ortega, Eduardo; Castano-Rivera, Patricia; Romero, Romina; Henriquez, Adolfo; Mendez, Camila; Schott, Eduardo; Contreras, David
    We have implemented a lignin compatibilization strategy through silylation and elaborated NR compounds with lignin and silylated lignin. Then, the lignin's contribution to the preservation of physico-mechanical properties and stability of the materials obtained against thermo-oxidation was studied. Thus, our work considered the indirect study of the antioxidant activity on NR compounds by determining the physico-mechanical properties before and after accelerated thermo-oxidative aging, which allowed us to determine that the increase in crosslinking density during the initial hours of accelerated aging was the main observed cause associated with a higher tensile strength retention rate (TSRR: 13.4 %) in NR compound with 5 phr of lignin (CL5) versus a TSRR: 4.7 % for the compound with 5 phr of silylated lignin (CL5Si1). On the other hand, the direct study of the antioxidant capacity by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) using oxygen atmosphere indicated greater thermal stability in NR compounds when lignin is not silylated. The maximum degradation temperature for the CL5 compound was 341.6 degrees C, while for the CL5Si1 compound it was 328.6 degrees C. The oxidation induction temperature (OITtemp) in the CL5 compound was 190.9 degrees C with a peak at 221.0 degrees C, while for CL5Si1 it was 170.3 degrees C with a peak at 194.1 degrees C, indicating again that unsilylated lignin is more suitable as an antioxidant. Finally, through Electron Paramagnetic Resonance (EPR), we determined that the radical quenching effect due to the presence of lignin was significant as the lignin content increases, with a reduction in radical presence close to 50 % compared to a reference compound without lignin.
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    Quantification of carbonate radical formation by the bicarbonate-dependent peroxidase activity of superoxide dismutase 1 using pyrogallol red bleaching
    (2019) Figueroa Alegría, Juan David; Fuentes Lemus, Eduardo Felipe; Dorta Pérez, Eva; Melin, Victoria; Cortés Ríos, Javiera Alejandra; Faúndez Cáceres, Mario; Contreras, David; Denicola, Ana; Álvarez, Beatriz; Davies, Michael J.; López Alarcón, Camilo Ignacio
    Carbonate radicals (CO3radical dot-) are generated by the bicarbonate-dependent peroxidase activity of cytosolic superoxide dismutase (Cu,Zn-SOD, SOD-1). The present work explored the use of bleaching of pyrogallol red (PGR) dye to quantify the rate of CO3radical dot- formation from bovine and human SOD-1 (bSOD-1 and hSOD-1, respectively). This approach was compared to previously reported methods using electron paramagnetic resonance spin trapping with DMPO, and the oxidation of ABTS (2,2-azino-bis(3-ethylbenzothiazoline)-6-sulfonic acid). The kinetics of PGR consumption elicited by CO3radical dot- was followed by visible spectrophotometry. Solutions containing PGR (5–200 μM), SOD-1 (0.3–3 μM), H2O2 (2 mM) in bicarbonate buffer (200 mM, pH 7.4) showed a rapid loss of the PGR absorption band centered at 540 nm. The initial consumption rate (Ri) gave values independent of the initial PGR concentration allowing an estimate to be made of the rate of CO3radical dot- release of 24.6 ± 4.3 μM min−1 for 3 μM bSOD-1. Both bSOD-1 and hSOD-1 showed a similar peroxidase activity, with enzymatic inactivation occurring over a period of 20 min. The single Trp residue (Trp32) present in hSOD-1 was rapidly consumed (initial consumption rate 1.2 ± 0.1 μM min−1) with this occurring more rapidly than hSOD-1 inactivation, suggesting that these processes are not directly related. Added free Trp was rapidly oxidized in competition with PGR. These data indicate that PGR reacts rapidly and efficiently with CO3radical dot- resulting from the peroxidase activity of SOD-1, and that PGR-bleaching is a simple, fast and cheap method to quantify CO3radical dot- release from bSOD-1 and hSOD-1 peroxidase activity.
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    The ferryl generation by fenton reaction driven by catechol
    (2023) Benítez Olivares, Francisca Javiera; Melin, Victoria; Pérez González, Gabriel; Henríquez, Adolfo; Zarate, Ximena; Schott Verdugo, Eduardo Enrique; Contreras, David
    The Fenton and Fenton-like reactions are based on the decomposition of hydrogen peroxide catalyzed by Fe(II), primarily producing highly oxidizing hydroxyl radicals (HO∙). While HO∙ is the main oxidizing species in these reactions, Fe(IV) (FeO2+) generation has been reported as one of the primary oxidants. FeO2+ has a longer lifetime than HO∙ and can remove two electrons from a substrate, making it a critical oxidant that may be more efficient than HO∙. It is widely accepted that the preferential generation of HO∙ or FeO2+ in the Fenton reaction depends on factors such as pH and Fe: H2O2 ratio. Reaction mechanisms have been proposed to generate FeO2+, which mainly depend on the radicals generated in the coordination sphere and the HO∙ radicals that diffuse out of the coordination sphere and react with Fe(III). As a result, some mechanisms are dependent on prior HO∙ radical production. Catechol-type ligands can induce and amplify the Fenton reaction by increasing the generation of oxidizing species. Previous studies have focused on the generation of HO∙ radicals in these systems, whereas this study investigates the generation of FeO2+ (using xylidine as a selective substrate). The findings revealed that FeO2+ production is increased compared to the classical Fenton reaction and that FeO2+ generation is mainly due to the reactivity of Fe(III) with HO∙ from outside the coordination sphere. It is proposed that the inhibition of FeO2+ generation via HO∙ generated from inside the coordination sphere is caused by the preferential reaction of HO∙ with semiquinone in the coordination sphere, favoring the formation of quinone and Fe(III) and inhibiting the generation of FeO2+ through this pathway.