Peptide synthesis: Chemical or enzymatic Guzmán, Fanny; Barberis, Sonia Esther; Illanes, Andrés Peptides are molecules of paramount importance in the fields of health care and nutrition. Several technologies for their production are now available, among which chemical and enzymatic synthesis are especially relevant. The present review pretends to establish a non-biased appreciation of  the advantages, potentials, drawbacks and limitations of both technologies. Chemical synthesis is thoroughly reviewed and their potentials and limitations assessed, focusing on the different strategies and challenges for large-scale synthesis. Then, the enzymatic synthesis of peptides with   proteolytic   enzymes    is   reviewed    considering  medium, biocatalyst and substrate engineering, and recent advances and challenges in the field are analyzed. Even though chemical synthesis is the most mature technology for peptide synthesis, lack of specificity and environmental burden are severe drawbacks that can in principle be successfully overcame by enzyme biocatalysis. However, productivity of enzymatic synthesis is lower, costs of biocatalysts are usually high and no protocols exist for its validation and scale-up, representing challenges that are being actively confronted by intense research and development in this area. The combination of chemical and enzymatic  synthesis is probably the way to go, since the good properties of each technology can be synergistically used in the context of one process objective.

Further evidence for the absence of persistent polyproline II conformation in the XAO peptide Makowska, Joanna; Rodziewicz-Motowidlo, Sylwia; Baginska, Katarzyna; Makowski, Mariusz; Vila, Jorge Alberto; Liwo, Adam; Chmurzynski, Lech; Scheraga, Harold A. It has been suggested that the alanine-based peptide with sequence Ac-XX-[A]7-OO-NH2, termed XAO where X denotes diaminobutyric acid and O denotes ornithine, exists in a predominantly polyproline-helix (PII) conformation in aqueous solution. In our recent work, we demonstrated that this “polyproline conformation” should be regarded as a set of local conformational states rather than as the overall conformation of the molecule. In this work, we present further evidence to support this statement. Differential scanning calorimetry measurements showed only a very small peak in the heat capacity of an aqueous solution of XAO at 57°C, whereas the suggested transition to the PII structure should occur at ∼30°C. We also demonstrate that the temperature dependence of the 3JHNHα coupling constants of the alanine residues can be explained qualitatively in terms of Boltzmann averaging over all local conformational states; therefore, this temperature dependence proves that a conformational transition does not occur. Canonical MD simulations with the solvent represented by the generalized Born model, and with time-averaged NMR-derived restraints, demonstrate the presence of an ensemble of structures with a substantial amount of local PII conformational states but not with an overall PII conformation.

Resonance light scattering and derived techniques in analytical chemistry: past, present, and future Lu, Wei; Fernández Band, Beatriz Susana; Yu, Yu; Geng Li, Qin; Chuan Shang, Jing; Wang, Chi; Fang, Yan; Tian, Rui; Ping Zhou, Li; Li Sun, Li; Tang, Yu; Hua Jing, Shu; Huang, Wei; Ping Zhang, Ji From 1993 to 1995, with a conventional fluorescence spectrophotometer (CFS) (convenient) and working in a synchronous scan model (easy-to-use), Pasternack et al. proposed the resonance light-scattering (RLS) technique, to efficiently characterize self-assemblies or self-aggregations of chromophores with good electronic coupling. Incident wavelengths were specially considered within their absorption envelopes (rather unorthodox), and their amplified signals were observed (good sensitivity and selectivity). Due to these absorbing benefits, RLS technique, as a novel readout method, commenced on its exciting analytical tours soon after Liu et al. and especially Li et al., separately, set out their corresponding pioneering investigations from 1995 to 1997. From then on, it has received an increasing attention by analysts, as a consequence exhibiting more and more fascinating analytical applications. Moreover, various attractive RLS-derived techniques have been developed successively to improve it or to enlarge its possibilities. Later on, Liu et al. and Li et al., Tabak et al., Yguerabide et al., Huang et al., Lakowicz et al. and Fernández Band et al. have made their outstanding contributions. In this review, we concentrate on major achievements of RLS in analytical chemistry for over a decade, involving the developments and analytical applications of RLS derived techniques treated as an impacting progress of RLS technique in analytical chemistry. Finally, an indication of future directions of RLS technique in analytical chemistry is provided.

Successive projections algorithm improving the multivariate simultaneous direct spectrophotometric determination of five phenolic compounds in sea water Di Nezio, Maria Susana; Pistonesi, Marcelo Fabian; Fragoso, Wallace D.; Pontes, Márcio J.C.; Goicoechea, Hector Casimiro; Araujo, Mário C.U.; Fernández Band, Beatriz Susana This paper proposes an analytical method to determine directly and simultaneously five phenolic compounds (4-nitrophenol, 2-nitrophenol, phenol, 2,4,6-trichlorophenol and 4-chlorophenol) in sea water (Ria de Bahía Blanca, Argentine). The advantages of this method is that only requires spectrophotometric measurements (separation steps and derivatization reagents are avoided) and chemometric modelling (PLS and MLR-SPA). The statistical comparison between PLS - a well established multivariate method - and MLR-SPA - a recently presented chemometric modelling - demonstrated better analytical performance for the later one. This fact is indicative of the potentiality of MLR-SPA for solving complex analytical problems.

Design of catalyst systems for the one-pot synthesis of menthols from citral Trasarti, Andres Fernando; Marchi, Alberto Julio; Apesteguia, Carlos Rodolfo Stable, active, and highly selective bifunctional Ni/Al-MCM-41 catalysts were developed for the one-pot synthesis of menthols from citral. The liquid-phase hydrogenation of citral to citronellal was studied on silica-supported noble (Pt, Pd, Ir) and nonnoble (Ni, Co, Cu) metals. It was found that citronellal is selectively formed at the beginning of the reaction only on Pd and Ni catalysts. The consecutive ene-cyclization of citronellal to isopulegols was investigated on solid acids containing exclusively Lewis (ZnO/SiO2) or strong Brönsted (CsHPA) acid sites, and also on catalysts containing both Lewis and Brönsted acid sites of either strong (zeolites, SiO2-Al2O3) or moderate (Al-MCM-41) strength. The isopulegol formation rate was higher on samples exhibiting dual Lewis/Brönsted acidity, such as SiO2-Al2O3, Al-MCM-41, and zeolite HBEA. Based on these previous results, bifunctional catalysts containing Pd or Ni supported on SiO2-Al2O3, Al-MCM-41, or zeolite HBEA were prepared and tested for citral conversion to menthols. The catalyst stability and the effect of hydrogen pressure and metal loading on menthol productivity were also investigated. The best catalyst was Ni(8%)/Al-MCM-41, which yielded more than 90% menthols at 2026.0 kPa and showed no significant deactivation after two consecutive catalytic tests.

The origin of the enhanced activity of Pt/zeolites for combustion of C2-C4 alkanes Garetto, Teresita Francisca; Rincon, Eduardo Ricardo; Apesteguia, Carlos Rodolfo The deep oxidations of ethane, propane and butane were studied on Pt supported on MgO, alumina, and zeolites KL, HY, ZSM5, and Beta. The catalyst activities were evaluated through both conversion versus temperature (light-off curves) and conversion versus time catalytic tests. The Pt oxidation activity for the three lower alkanes was drastically increased when supported on zeolites as compared to Pt/Al2O3 or Pt/MgO. C2-C4 alkane oxidation turnover rates were about two (ethane, propane) and one (butane) orders of magnitude higher on Pt/acid zeolites than on Pt/Al2O3, but also weakly acid Pt/KL zeolite was significantly more active as compared to Pt/Al2O3 (more than one order of magnitude for ethane and propane). This latter result showed that the support acidity is not a major contributing factor for lower alkane combustion. Promotion of the alkane oxidation activity on Pt/zeolites was explained by considering the drastic increase observed for the density of alkane adsorbed species on zeolite supports; it was found, in fact, that the alkane uptake per m2 was about one order of magnitude higher on Pt/zeolites than on Pt/Al2O3. This alkane confinement in zeolite pores would enhance the Pt oxidation rate because the reaction is positive order with respect to the hydrocarbon and probably also because would promote an additional oxidation pathway in the metal-oxide interfacial region.

The SCR of NO with CH4 over Co-, Co,Pt-, and H-mordenite catalysts Lónyi, F.; Valyon, J.; Gutierrez, Laura Beatriz; Ulla, Maria Alicia del H.; Lombardo, Eduardo Agustin The catalytic reduction of NO with methane was studied operando in the presence and absence of oxygen in the reaction mixture over Co-, Co,Pt-, and H-mordenite (CoM, CoPtM, HM) catalysts using the coupled methods of diffuse reflectance infrared Fourier-transform spectroscopy and mass spectroscopy (Operando-DRIFTS-MS). The reaction temperature was in the 573-773 K range, and the GHSV for nitric oxide was varied between 6000 and 60,000 h-1. In general, the Co-containing mordenite catalysts were more active than the HM. Over each catalyst two major surface intermediates were identified such as nitrosonium ion (NO+) and ammonia. The NO+ cations were shown to balance the negative charge on the zeolite framework. The NH3 molecules were bound either coordinately to Lewis-acid site Co2+ ions or were protonated on Brønsted acid sites and retained by the zeolite as NH4 + cations. The prevailing route of N2 formation was found to involve the reaction of NO+ and NH3 or NH4 + surface species. It was shown that the surface concentration of the intermediate ammonia governed the rate of NO conversion. If O2 was present in the feed gas it competed with the NO in the oxidation of methane and the surface intermediate ammonia. As a result, the steady-state concentration of the ammonia intermediate and the overall NO conversion to N2 decreased. Additional routes of N2 formation were revealed over the CoM and CoPtM catalysts. The cobalt facilitated the formation of NO+ and surface nitrate (NO3 -). Also nitrile (CN) and isocyanate (NCO) surface species were detected. The NO3 - could be rapidly reduced by methane to intermediate NH3 and, thereby, it increased the rate of NO conversion significantly. Nitrogen forming reactions can pass also through CN and NCO intermediates; however, the share of these reaction routes in the overall N2 generation process was minor.

Aprostocetus (Ootetrastichus) infulatus (Hymenoptera: Eulophidae): description of the male, new distribution and first host records Luft Albarracin, Erica Beatriz; Triapitsyn, Serguei V. The eulophid Aprostocetus (Ootetrastichus) infulatus (De Santis) has been previously known only from a single female holotype from Buenos Aires Province in Argentina. We obtained a series of specimens of this species using sentinel eggs of different species of Cicadellidae and Delphacidae, exposed in cornfields near El Manantial and San Miguel de Tucumán, in Tucumán Province, Argentina. The previously unknown male of A. (O.) infulatus is described and illustrated. For the first time, information on the host associations of this species is provided while its known distribution range is expanded to northern Argentina. This eulophid is an economically significant species in Argentina, where its hosts include two well-known vectors of corn diseases, the leafhopper Dalbulus maidis (DeLong & Wolcott) and the planthopper Peregrinus maidis (Ashmead).

The activated glucocorticoid receptor inhibits the transcription factor T-bet by direct protein-protein interaction Liberman, Ana Clara; Refojo, Damian; Druker, Jimena; Toscano, Marta Alicia; Rein, Theo; Holsboer, Florian; Arzt, Eduardo Simon Glucocorticoids (GCs) immunosuppression acts via regulation of several transcription factors (TF), including AP-1, NFkB and NFAT. GCs inhibit Th1 cytokines and promote a shift towards Th2 differentiation. Th1 phenotype depends on TF T-bet. In this study we examined GC regulation of T-bet. We found that GCs inhibit T-bet transcriptional activity. We show that glucocorticoid receptor (GR) physically interacts with T-bet both in transfected cell lines and in primary splenocyte cultures with endogenous GR and T-bet. This interaction also blocks GR-dependent transcription. We show both in vitro and in vivo at endogenous binding sites that the mechanism underlying T-bet inhibition further involves reduction of T-bet binding to DNA. Using specific mutations of GR, we demonstrate that the first zinc finger region of GR is required for T-bet inhibition. GCs additionally inhibit T-bet both at mRNA and protein expression levels, revealing another layer of GR action on T-bet. Finally, we examined the functional consequences of GR/T-bet interaction on interferon-gamma, showing that GCs inhibit transcriptional activity of T-bet on its promoter. In view of the crucial role of T-bet in T cell differentiation and inflammation, we propose that GR inhibitory interaction with T-bet may be an important mechanism underlying the immunosuppressive properties of GCs.

How disorder can diminish avalanche risks: effect of size distribution Oger, Luc; Ippolito, Irene Paula; Vidales, Ana Maria We study the behavior of disk assemblies with a variable disorder distribution. The packing is first consolidated and then continuously tilted very slowly. The amount of displaced disks for each tilted angle is recorded. Large displacements of the disks can occur due to some local or global mechanical instabilities. The definition of neighboring disks is based on radical (extension of Voronoï) tessellation rules to decompose, in a unique and perfectly defined manner, the two-dimensional space for polydisperse disks. In this way, by comparing the characteristics of stability for one disk to the neighboring ones for local ordered cluster, we can predict the global amount of displaced disks. Some tilting cycles have been performed to check the correlation between the instability of the packing structure (collective displacements) with micro and macro order parameters.

El Círculo Médico Argentino y su papel en la configuración del pensamiento médico clínico (Buenos Aires, 1875-1883) Souza, Pablo Andres Recientes trabajos en historia de la medicina argentina de la segunda mitad del siglo XIX han estudiado su profesionalización en la Buenos Aires posrosista, echando luz sobre un período y problemáticas poco conocidas...

Ácidos aril-2-propiónicos o profenos Igarza, Leticia Manuela; Soraci, Alejandro Luis Los ácidos aril-2-propiónicos o profenos constituyen un grupo de medicamentos antiinflamatorios que tienen como característica estructural un carbono asimétrico que les permite existir bajo la forma de dos enantiómeros R-(-) y S-(+). Los enantiómeros pueden diferir ampliamente en sus propiedades farmacodinámicas y farmacocinéticas. La enantioselectividad es un aspecto importante en su acción inhibitoria sobre la ciclooxigenasa, pues el enantiómero-S es el único activo. Dependiendo del ácido aril-2-propiónico, el enantiómero-R inducido puede servir de sustrato al proceso de inversión quiral y a otras vías metabólicas alternativas, formar acilglucurónidos potencialmente reactivos, formar tioésteres con coenzima A y ser incorporados en glicerolípidos interfeririendo con el metabolismo lipídico y/o procesos de la membrana biológica, apareciendo así como potenciales vías toxicológicas. El proceso de inversión quiral, permite la transformación de un enantiómero en otro, proceso que repercute desde el punto de vista terapéutico. La tendencia actual es investigar las implicanciones biológicas de cada enantiómero, con el objetivo de ejercer un uso terapéutico racional de la forma enantiomérica activa o del racemato y de evitar consecuencias toxicológicas.

Oxfordian ramp system (La Manga Formation) in the Bardas Blancas area (Mendoza Province) Neuquén Basin, Argentina: Facies and depositional sequences Palma, Ricardo Manuel; López Gómez, José; Piethé, Ricardo D. The outcrops of the Oxfordian La Manga Formation at Bardas Blancas, Neuquén Basin, west-central Argentina, allow the recognition of six different depositional facies (A to F) on the basis of sedimentological analysis, taphonomic attributes and microfacies studies. These depositional facies correspond to outer ramp (A), middle ramp (B), inner ramp- oolitic shoal (C), inner ramp margin (patch reef) (D), lagoon deposits (E), and a paleokarst surface (F). Outer ramp deposits which are not completely represented, consist of greyish carbonate beds, where the fabric of the shell beds (gryphaeids) reflects the action of waves and currents. Middle ramp deposits consist of a packstone-grainstone lithofacies indicating the importance of storm processes and is dominated by ooids, intraclasts, pelecypods, echinoderms and gastropods which accumulated on a middle-ramp storm-dominated shoreface. Trace fossils belong to the Skolithos and Cruziana Ichnofacies characterizing the upper, lower and middle shoreface setting respectively. The inner ramp deposits consist of oolitic grainstones and subordinate packstones shoal with a small sponge bioherm at the base. Different types of ooids, peloids and coated grains are abundant, as well as skeletal fragments of molluscs, echinoderms and corals. Lithofacies and microfacies studies suggest a high energy and shallow-water depositional setting. The inner ramp margin deposits consist of reef core facies, fore and back reef facies characterized by a scleractinian community of relatively low generic diversity. The rich associated fauna consists of bivalves, echinoids, serpulids, bryozoans, dasycladacean algae and cyanophytes, as well as foraminifers and ostracods. The growth forms of the corals are indicative of shallow well illuminated water. Both the back and fore reef deposits suggest intensive reworking by storm waves or currents. The lagoon deposits consist of bioclastic and peloidal wackestones as well as bioclastic-intraclastic packstones which accumulated on a lagoon under intermittently agitated water in a shallow subtidal to intertidal settings. A stratiform breccia with both matrix and clasts supported fabrics is interpreted as paleokarst. The clasts are derived from the rocks of oolitic shoal and inner ramp margin (patch reef). The six depositional facies are included into a major organizational framework of three third-order depositional sequences (DS-1, DS-2, DS-3) mainly represented by transgressive and highstand systems tracts stages with sequence boundaries of regional importance. The general depositional evolution is here related to the slow subsidence experienced during the Oxfordian-earliest Kimmeridgian time related to tectonic inversion in the Neuquén Basin. A four step (architectural and sedimentary) schematic model of the response of the platform to sea-level changes is proposed.

Sobre la no convergencia del método de mínimos cuadrados en dimensión infinita Spies, Ruben Daniel; Temperini, Karina Guadalupe Un procedimiento muy utilizado en diversas aplicaciones para aproximarlas soluciones de un problema inverso infinito-dimensional de la formaAx=b, dondeAes un operador lineal y compacto sobre un cierto espacio de HilbertXybes eldato dado, consiste en encontrar una sucesi ́on{XN}de subespacios aproximantes finito-dimensionales deXcuya uni ́on es densa enXy construir la sucesi ́on{xN}de solucionesde m ́ınimos cuadrados del problema en cada subespacioXN. En [3], Seidman demostr ́oque si el problema es mal condicionado, entonces sin ninguna hip ́otesis adicional sobrela soluci ́on exacta o sobre la sucesi ́on de subespacios aproximantes{XN}, no se puedegarantizar que la sucesi ́on{xN}converger ́a a la soluci ́on exacta. En este art ́ıculo seextiende este resultado: se prueba que siXes separable, entonces para cualquierb∈X,b6= 0, y para cualquier funci ́on no negativa definida sobre los naturalesf: IN→IR+,existe un operador lineal, compacto e inyectivoAy una sucesi ́on creciente de subespaciosfinito-dimensionalesXN⊂Xtales que∥∥xN−A−1b∥∥≥f(N) para todoN∈IN, dondexNes la soluci ́on de m ́ınimos cuadrados del problemaAx=benXN.

Cranial sexual discrimination in hatchling broad-snouted caiman (Caiman latirostris); Discriminación de sexo en neonatos de yacaré overo (Caiman latirostris) Piña, Carlos Ignacio; Larriera, Alejandro; Siroski, Pablo Ariel; Verdade, Luciano Martins Los neonatos de yacaré overo (Caiman latirostris) presentan un dimorfismo sexual en la forma y tamaño de su cráneo. Los neonatos machos tienen cráneos más pequeños que las hembras. Mediante el uso de análisis estadísticos multivariados es posible discriminar el sexo de los neonatos de yacaré overo en función de su forma y tamaño con una razonable eficiencia. La comprensión del dimorfismo sexual de los neonatos de crocodilianos podría ser mejorada mediante un abordaje experimental de variables genéticas y fenotípicas, tales como la temperatura de incubación y el nido de origen.; Broad-snouted caiman (Caiman latirostris) hatchlings present a consistent sexual dimorphism in their cranium shape and size. Male hatchlings have smaller crania than females. Using multivariate statistical analyses it is possible to discriminate sex in broadsnouted caiman hatchlings by their cranial shape with a reasonable efficiency. The understanding of sexual dimorphism of crocodilian hatchlings might be possibly improved by experimental approach considering, genetic and phenotypic variables such as incubation temperature and clutch of origin.

Eocene paleomagnetic pole for South America: Northward continental motion in the Cenozoic, opening of Drake Passage and Caribbean convergence Somoza, Ruben A paleomagnetic study of Eocene volcanic rocks in Patagonia yields high unblocking temperature and high-coercivity magnetizations. Combining these results with those of a previous study on Patagonian Eocene basalts yields a high-precision, high-quality pole located at latitude 81°S, longitude 337.4°E, A95 = 5.7°. Critically, this paleopole is indistinguishable from that of the Late Cretaceous (circa 85-65 Ma) pole position of South America, indicating that the plate was essentially motionless with respect to the spin axis for a period of ∼45 m.y. The pole position places South America at higher (∼5°) than present-day latitudes during the Eocene, indicating that northward continental motion toward present-day latitudes must have been accomplished sometime since the late Eocene. Paleomagnetic and tectonic correlation admits the hypothesis that Cenozoic northward drift was associated with Oligocene-Miocene extension in the southern continental edge, leading to the opening of the Drake Passage, and it agrees with the timing of foredeep formation and development of fold-thrust belts in the northern continental edge. This positive correlation between the paleomagnetically predicted drift of a major continent with extension at its trailing edge and convergence at its leading edge during times for which seafloor tectonic fabric and the geological record are particularly well preserved illustrates the utility of paleomagnetism in constraining paleogeographic and tectonic reconstructions for pre-Cretaceous times.

Effect of the location of cobalt species on NO adsorption and NOx-SCR over Co-mordenite Gutierrez, Laura Beatriz; Miro, Eduardo Ernesto; Ulla, Maria Alicia del H. A thorough characterization of Co-mordenite was carried out using NO-TPD, H2-TPR, Raman, DRX and FTIR with NO as probe molecule. Different cobalt-loading catalysts (8.85, 5.70, 2.91, 2.45 and 1.15 wt.%) were prepared by ion exchange over NH4-mordenite and treated with different procedures (heated on He, O2 and H2). The effect of adding water to the reaction stream at 500 °C on the location of the cobalt species was also analyzed. All catalysts were evaluated for the SCR of NOx with methane as a test reaction both under dry and wet conditions. The NO adsorption capacity depended not only on the Co/Al ratio and pretreatments but also on the presence of Co oxides. In fact, for similar Co/Al ratios, Co2.91MOR showed a lower NO/Co ratio than Co2.45MOR due to the higher Co3O4 concentration in the former. These species probably blocked the main mordenite channels and/or produced the formation of some Co island on the zeolite surface, lowering the amount of surface Co. The main NO surface species detected were dinitrosyl and mononitrosyl. Their stability and relative amount depended on the cobalt species and the pretreatments. Heating in He or H2 induced the stabilization of part of the exchanged Co in less gas-accessible sites, whereas the calcination with O2 involved the mobilization of Co to more external sites. However, those effects were less significant for samples containing Co oxides suggesting that the spinel hinders the Co migration. During the wet reaction treatment, the cobalt mobilization to hidden positions occurred along with the formation of Co-oxides, which negatively affected both the catalytic activity and the NO adsorption capacity.

Heterogeneous esterification of oil with high amount of free fatty acids Marchetti, Jorge Mario; Miguel, Victor Ubaldo; Errazu, Alberto Felipe Frying oils have become the newest raw material for the transesterification reaction for the production of biodiesel. However, these compounds usually come with a certain amount of free fatty acids. These impurities can be transformed into esters and the production of biodiesel could be increased. The use of basic resins to perform the esterification reaction into biodiesel is studied in this work. The effect of the most relevant variables of the process such as reaction temperature, molar ratio between alcohol and oil, amount of catalyst and amount of free fatty acids fed with the oil have been analyzed. For this purpose, an ideal frying oil using oleic acid and soybean oil was made. The alcohol used was ethanol. The esterification of free fatty acid using this heterogeneous catalyst appears as a great alternative to purify frying oil; in this case, the final conversion achieved was around 80%.

Genetically modified sunflower release: Opportunities and risks Cantamutto, Miguel Ángel; Poverene, María Mónica Sunflower (Helianthus annuus L.) is a crop native to North America for which there are no genetically modified commercial varieties. Some of the transgenic traits incorporated in other crops have already been subjected to research and experimentation in sunflower. Several new traits have also been noted, with the most relevant of these being the aim to increase latex production. GM sunflower release would modify crop management through improved mineral nutrition, weed control, insect and disease resistance, and product quality. In this research, the traits investigated were reviewed and analyzed in connection with main crop constraints. These characters could potentially influence agro-ecosystem components and produce a significant environmental impact. In regions where sunflower coexists with wild relatives this situation could affect germplasm resources, with this being especially important at the centre of origin and where Helianthus populations established in Africa, Asia, and Europe.

Thermal characterization of UHMWPE stabilized with natural antioxidants Peltzer, Mercedes Ana; Wagner, Jorge Ricardo; Jiménez, A. This work presents a study of the thermal degradation of ultra-high molecular mass polyethylene (UHMWPE) stabilized with natural (tocopherol and carvacrol) and synthetic antioxidants. Thermogravimetric analysis in dynamic mode was used to determine the apparent activation energies of different samples. The stabilization of UHMWPE with low concentrations (around 0.3%) of tocopherol is enough to obtain an efficient thermal performance of the polymer. Carvacrol is also a good stabilizer for UHMWPE, but at higher concentration than in the case of -tocopherol. The comparison of apparent activation energy between samples with natural and synthetic antioxidants in similar concentration shows that tocopherol is a better stabilizer in terms of their thermal performance.