1,721,090 research outputs found
Synergic Antioxidant Effects of the Essential Oil Component γ-Terpinene on High-Temperature Oil Oxidation
Full text linkThe synergic antioxidant activity of γ-terpinene with α-tocopherol, its synthetic analogue 2,2,5,7,8-pentamethyl-6-
chromanol (PMHC), BHT, TBHQ, and catechol was studied by measuring the O2 uptake and the hydroperoxide formation in
stripped sunflower oil at 130 °C. Although γ-terpinene was inactive when used alone, it prolonged, in a concentration-dependent
manner, the protecting activity of α-tocopherol and of PMHC, while it had no effect on BHT, TBHQ, and catechol. Mechanistic
studies performed with alkylnitroxides hydroxy-TEMPO and acetamido-TEMPO, used as molecular probes, suggested that γ-
terpinene generates hydroperoxyl (HOO•) radicals, that are responsible for the reduction of the tocopheroxyl radical by a H atom
transfer. Experiments on commercial sunflower oil showed that γ-terpinene was able to prolong the induction time due to
endogenous tocopherols, demonstrating that this terpene is a promising natural antioxidant for food applications at high
temperature
Methods to Measure the Antioxidant Activity of Phytochemicals and Plant Extracts
Full text linkMeasurement of antioxidant properties in plant-derived compounds requires appropriate methods that address the mechanism of antioxidant activity and focus on the kinetics of the reactions involving the antioxidants. Methods based on inhibited autoxidations are the most suited for chain-breaking antioxidants and for termination-enhancing antioxidants, while different specific studies are needed for preventive antioxidants. A selection of chemical testing methods is critically reviewed, highlighting their advantages and limitations and discussing their usefulness to investigate both pure molecules and raw extracts. The influence of the reaction medium on antioxidants' performance is also addressed
Methods to Determine Chain-Breaking Antioxidant Activity of Nanomaterials beyond DPPH<sup>•</sup>. A Review
Full text linkThis review highlights the progress made in recent years in understanding the mechanism of action of nanomaterials with antioxidant activity and in the chemical methods used to evaluate their activity. Nanomaterials represent one of the most recent frontiers in the research for improved antioxidants, but further development is hampered by a poor characterization of the ‘‘antioxidant activity’’ property and by using oversimplified chemical methods. Inhibited autoxidation experiments provide valuable information about the interaction with the most important radicals involved in the lipid oxidation, namely alkylperoxyl and hydroperoxyl radicals, and demonstrate unambiguously the ability to stop the oxidation of organic materials. It is proposed that autoxidation methods should always complement (and possibly replace) the use of assays based on the quenching of stable radicals (such as DPPH• and ABTS•+). The mechanisms leading to the inhibition of the autoxidation (sacrificial and catalytic radical trapping antioxidant activity) are described in the context of nanoantioxidants. Guidelines for the selection of the appropriate testing conditions and of meaningful kinetic analysis are also given
ROS and Phenolic Compounds
No full textReactive oxygen and nitrogen species (ROS and RNS) are detrimental to human health because
they initiate free radical–catalyzed oxidations of fundamental biomolecules such as DNA, proteins,
lipids in low-density lipoprotein (LDL) and cell membranes, polysaccharides, etc. Molecular oxygen
in its triplet ground state (the oxygen we breathe), 3O2, is the oxidant species in these processes
called autoxidation or peroxidation. Cells are equipped with defensive systems able to quench most
of the radicals responsible for initiating or propagating autoxidation in organic matter. Enzymes
(superoxide dismutases, catalases, glutathione peroxidases, and peroxiredoxins) destroy radicals
such as O2·− or non-radical species such as H2O2 and ROOH. Other small molecules, mainly phenols,
present in the diet are able to react with radicals and hence may cooperate with the enzymes
in keeping “oxidative stress” at bay. However, the physiological concentrations of phenols are
frequently low, and this has cast doubt on their effectiveness in vivo. On the other hand, there is
evidence that vitamin E is an effective peroxyl radical (ROO·) scavenger both in vitro and in vivo
with rate constants of ~106 M−1 s−1. Its effectiveness in vivo against other radicals and non-radical
oxidative species (HO·, R·, NO2 ·, RS·, 1O2, O3, and HOCl) is however uncertain. Vitamin C and
ubiquinol-10 are able to regenerate vitamin E from its radicals
Mode of Antioxidant Action of Essential Oils
No full textThere is a growing need for innovative food preservation systems. Reduction of food waste is recognised as a priority target by both FAO (2016) and the European Commission (2016). Antioxidants represent an important class of food preservatives because they are able to slow down the oxidation of unsaturated lipids (LH) contained in food, preventing therefore the development of rancidity (Schaich, 2005). Atmospheric oxygen (together with microorganisms) is the main oxidant species responsible for the decay of the organoleptic qualities of food. The reaction involved (called lipid autoxidation or peroxidation) is mediated by free radicals, which catalyse the formation of lipid hydroperoxides (LOOH) with a radical‐chain mechanism (Schaich, 2005). Butylated hydroxyanisole (BHA, E320), butylated hydroxytoluene (BHT, E321), propyl gallate (PG, E310) and tert‐butylhydroquinone (TBHQ, E319) are synthetic antioxidants commonly added to food to inhibit lipid peroxidation and the consequent rancidification. The impact of these petroleum‐derived antioxidants on human health is still under debate and generally natural alternatives are increasingly preferred (Williams et al ., 1999; Carocho et al ., 2014)
MODULATION OF BIORELEVANT RADICAL REACTIONS BY NON‐COVALENT INTERACTIONS
No full textRadical reactions are involved in a multitude of relevant
processes and their importance is increasingly recognized
in different fields ranging from biochemistry to materials
science.
For instance, radical polymerization accounts for
approximately 45% of world polymer production [1]; on the
other hand, the involvement of transient radical species in
enzyme regulated processes in living organisms has stimulated,
in recent years, enormous research efforts to rationalize
their role in key physiological processes like
mitochondrial respiration and aging [2, 3] or in the pathogenesis
of several diseases [4].
Perhaps the best known example of radical reaction is
hydrocarbon autoxidation, a chain process that affects any
organic material, from food to petrol‐derived chemicals to
human beings, existing under an oxygen‐rich atmosphere [5].
As a consequence, antioxidants and radical‐chain inhibitors
are among the most important compounds used to control
key radical reactions [5]. They act by trapping chain‐carrying
radical species, thereby competing with chain propagation,
as illustrated in Scheme 20.1. Their rational design and use
need to be strictly based on their reactivity with radical
species. Indeed, the understanding of radical reactions and
their role in biology, along with their use and control in
synthetic chemistry or in medicine, requires detailed
knowledge of their kinetics and how it is influenced by the
reaction medium.
In this chapter it will be explained how non‐covalent
interactions control the rates and the products of reactions
that are typical of the radicals involved in autoxidation,
namely, peroxyl (ROO∙), phenoxyl (PhO∙), and alkoxyl
(RO∙) radicals. These radicals are not only of biological
interest, but they are also key intermediates in green synthesis
protocols, which aim at obtaining fine chemicals from
renewable materials under mild conditions. For this reason,
we will illustrate some selected examples of biomimetic radical
reactions that are made possible by the control of these
reactive intermediates by non‐covalent interaction
From catechol-tocopherol to catechol-hydroquinone polyphenolic antioxidant hybrids
No full textMultidefence antioxidants represent a valuable solution for the protection against oxidative stress. From the planned synthesis of a catechol-tocopherol hybrid, we isolated a catechol-hydroquinone hybrid through a BBr3-mediated benzochromene-fluoren-1-ol transposition. The compound prepared showed a remarkable chain-breaking antioxidant in the catechol portion, while the very sensitive hydroquinone moiety revealed to be an efficient generator of hydroperoxyl radicals
Continuous fluorescence-based quantitative antioxidant assay using vegetable oil as an oxidizable substrate
No full textSeveral spectrophotometric assays, such as 1,1-diphenyl-2-picrylhydrazyl (DPPH) and oxygen radical absorbance capacity (ORAC), are commonly used to assess antioxidant activity. However, these methods often lack real-world relevance as they do not inhibit autoxidation in actual food substrates. Although direct measurement of oxygen consumption or peroxide formation during inhibited autoxidation offers certain advantages, it is labor intensive and requires specialized equipment. In this study, we introduce a small-volume inhibited autoxidation approach that utilizes a standard microplate reader and a food-derived oxidizable substrate, specifically stripped sunflower oil (SSO), and styrene-conjugated BODIPY (STY-BODIPY) chromophores that oxidizes with the substrate, enabling straightforward monitoring of the reaction progress without interfering with it. The rate of initiation (Ri) was controlled by using azobis(isobutyronitrile) (AIBN) at 30 °C (Ri = 8.6 ± 0.5 × 10−10 M s−1) to accurately determine the rate constant of antioxidant reaction with peroxyl radicals (kinh). The method was standardized using the synthetic α-tocopherol analogue 2,2,5,7,8-pentamethyl-6-chromanol (PMC) as a reference antioxidant and was successfully applied to evaluate its synergistic interactions with γ-terpinene, quercetin, and caffeic acid. The rate constant for the reaction of peroxyl radicals with STY-BODIPY was determined, kST = 890 ± 52 M−1 s−1. Induction time (τ) of PMC increased in a concentration-dependent manner by the synergistic interactions of PMC/γ-terpinene, PMC/quercetin, and PMC/caffeic acid. The kinh value for PMC in SSO at 30 °C remained constant at 1.5 × 106 M−1 s−1. The validity of this approach was further confirmed using isothermal calorimetry, demonstrating its potential as a reliable and accessible tool for antioxidant testing in food systems
Synergic Antioxidant Activity of γ-Terpinene with Phenols and Polyphenols Enabled by Hydroperoxyl Radicals
Full text linkAntioxidant interactions of γ-terpinene with α-tocopherol mimic 2,2,5,7,8-pentamethyl-6-chromanol (PMHC)
and caffeic acid phenethyl ester (CAPE), used as models, respectively, of mono- and poly-phenols were
demonstrated by differential oximetry during the inhibited autoxidation of model substrates: stripped sunflower
oil, squalene, and styrene. With all substrates, γ-terpinene acts synergistically regenerating the chain-breaking
antioxidants PMHC and CAPE from their radicals, via the formation of hydroperoxyl radicals. The inhibition
duration for mixtures PMHC/γ-terpinene and CAPE/γ-terpinene increased with γ-terpinene concentration, while
rate constants for radical-trapping were unchanged by γ-terpinene, being 3.1 × 106 and 4.8 × 105 M 1s 1 for
PMHC and CAPE in chlorobenzene (30 ◦C). Using 3,5-di-tert-butylcatechol and 3,5-di-tert-butyl-1,2-bezoquinone
we demonstrate that γ-terpinene can reduce quinones to catechols enabling their antioxidant activity. The
different synergy mechanism of γ-terpinene with mono- and poly-phenolic antioxidants is discussed and its
relevance is proven in homogenous lipids using natural α-tocopherol and hydroxytyrosol as antioxidants, calling
for further studies in heterogenous food products
Singlet oxygen quenching- and chain-breaking antioxidant-properties of a quercetin dimer able to prevent age-related macular degeneration
No full textA dimer of quercetin prepared through a Mannich reaction protects pyridinium bisretinoid A2E from photooxidation at 430 nm in aqueous medium at pH 7.4. In the presence of light and O2, A2E is quickly attacked by 1O2 produced in situ (by excited A2E) to give nonaoxirane and other oxygenated compounds which can be involved in diseases of the macula. Peroxyl radicals might have only a marginal role on the photooxidation of A2E. The dimer is actually a potent quencher of 1O2 with a rate constant kQ of 8.5 × 108 M−1 s−1 in methanol at room temperature. On the other hand, its antioxidant abilities against ROO· radicals are quite limited since kROO· = 7.3 × 105 M−1 s−1 (in buffer solution at pH 7.4), the value being essentially identical to that of quercetin. The quenching of 1O2 by the dimer is therefore the main reason for the A2E protection and prevention of age-related macular degeneration
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