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1.
Degradation products of ultraviolet (UV-C, 254 nm) treated ascorbic acid (AA) are reported. Analysis by high-performance liquid chromatography-mass spectroscopy (HPLC-MS) conducted in a 0.5% malic acid model juice system (pH 3.3) demonstrated increased degradation of AA above untreated controls with concomitant increases in dehydroascorbic acid (DHA) and 2,3-diketogulonic acid (DKGA) levels. Electron spin resonance (ESR) spectroscopy studies, conducted in phosphate buffer (pH 7.0) to increase detection sensitivity, demonstrated that ascorbyl radical (AA?) formation occurs simultaneously with AA degradation. Consistent with a previous study in which UV treatments were shown to accelerate dark storage degradation, AA? radicals continued to form for up to 200 min after an initial UV treatment. Results from this study suggest that the mechanism for UV-induced degradation is the same as the general mechanism for metal-catalyzed oxidation of AA in juice.  相似文献   

2.
An electron spin resonance (ESR)-based method was used for evaluating the levels of radical formation during mashing and in sweet wort. The method included the addition of 5% (v/v) ethanol together with the spin trap alpha-4-pyridyl(1-oxide)- N- tert-butylnitrone (POBN) to wort, followed by monitoring the rate of formation of POBN spin adducts during aerobic heating of the wort. The presence of ethanol makes the spin trapping method more selective and sensitive for the detection of highly reactive radicals such as hydroxyl and alkoxyl radicals. Samples of wort that were collected during the early stages of the mashing process gave higher rates of spin adduct formation than wort samples collected during the later stages. The lower oxidative stability of the early wort samples was confirmed by measuring the rate of oxygen consumption during heating of the wort. The addition of Fe(II) to the wort samples increased the rate of spin adduct formation, whereas the addition of Fe(II) during the mashing had no effect on the oxidative stability of the wort samples. Analysis of the iron content in the sweet wort samples demonstrated that iron added during the mashing had no effect on the iron level in the wort. The moderate temperatures during the early steps of mashing allow the endogenous malt enzymes to be active. The potential antioxidative effects of different redox-active enzymes during mashing were tested by measuring the rate of spin adduct formation in samples of wort. Surprisingly, a high catalase dosage caused a significant, 20% reduction of the initial rate of radical formation, whereas superoxide dismutase had no effect on the oxidation rates. This suggests that hydrogen peroxide and superoxide are not the only intermediates that play a role in the oxidative reactions occurring during aerobic oxidation of sweet wort.  相似文献   

3.
The electron spin resonance (ESR) spin trapping technique was investigated as an analytical approach to follow lipid oxidation of rapeseed oil, sunflower oil, and fish oil during storage at 40 degrees C. Unlike previous investigations, alpha-phenyl-N-tert-butylnitrone (PBN), used as spin trap, was added to the fresh oils and formation of radicals was monitored during storage. Results were compared with the development in peroxide value (PV) and the thiobarbituric acid index (TBA). Increasing radical development was detected during the initial stages of oxidation, during which no significant changes in PV and TBA were observed. Evidence of spin adduct depletion was found during prolonged storage, suggesting that although spin trapping of radicals may be used to follow early events in lipid oxidation, it is not a suitable parameter for long periods of time. Addition of the spin trap after sequential samplings is recommended for getting an insight of oxidative changes during storage. Further, the influence of the spin trap (PBN) on lipid oxidation was studied in detail by application of PV and TBA and by following the depletion of naturally occurring tocopherol. PBN was found to possess a profound inhibiting effect on lipid oxidation. Such an effect was found to be dependent on the nature of the oil, and it was observed that the lower the oxidative stability, the larger the effect of PBN on lipid oxidation. This effect was interpreted in terms of the capability of PBN to react with peroxyl radicals, which in turn depends on the initial tocopherol content of the oils.  相似文献   

4.
Chemical interaction between polyphenols and thiols was investigated under radical oxidation conditions using a model cysteinyl thiol derivative, N-benzoylcysteine methyl ester. The radical oxidation was carried out with a stoichiometric amount of 2,2-diphenyl-1-picrylhydrazyl (DPPH), and the decreases in the amounts of polyphenols and the thiol were measured by HPLC analysis. Cross-coupling products between various polyphenols and the thiol were examined by LC-MS in reactions that showed decreases in both the polyphenols and the thiol. The LC-MS results indicated that three phenolic acid esters (methyl caffeate, methyl dihydrocaffeate, and methyl protocatechuate) and six flavonoids (kaempferol, myricetin, luteolin, morin, taxifolin, and catechin) gave corresponding thiol adducts, whereas three polyphenols (methyl ferulate, methyl sinapate, and quercetin) gave only dimers or simple oxidation products without thiol substituents. Thiol adducts of the structurally related compounds methyl caffeate and methyl dihydrocaffeate were isolated, and their chemical structures were determined by NMR analysis. The mechanism for the thiol addition was discussed on the basis of the structures of the products.  相似文献   

5.
Free radical scavenging reactions of green tea polyphenols (GTP) were investigated with electron spin resonance (ESR) spectroscopy in the phospholipid bilayer of liposomes, using 1, 1-diphenyl-2-picrylhydrazyl (DPPH) radical as a model. The results showed that (1) GTP reacts with DPPH radicals in the bilayer of liposomes of both 1-hexadecanoyl-2-[(cis,cis,cis,cis,cis,cis)-4,7,10, 13,16,19-docosahexaenoyl]-sn-glycero-3-phosphocholine (DHAPC) and 1, 2-di[cis-9-hexadecenoyl]-sn-glycero-3-phosphocholine) (DPPC); and (2) GTP protects DHAPC liposomes effectively from the oxidation initiated by DPPH radicals. These results provide direct evidence that GTP reacts with free radicals in the model membrane and support the hypothesis that GTP protects unsaturated phospholipids from oxidation by reacting directly with the radicals.  相似文献   

6.
Reduction of free radicals generated by Fe(II)/cumene-hydroperoxide (CumOOH) by amino acids (Gly, Cys, Met, His, and Trp) and proteins (bovine serum albumin (BSA), beta-lactoglobulin, and lactoferrin) was followed by electron spin resonance spectroscopy using alpha-phenyl-N-tert-butylnitrone (PBN), 2-methyl-2-nitrosopropane (MNP), and 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as spin traps. The radical species detected were mostly carbon-centered radicals from CumOOH fragmentation (methyl/*H3 and ethyl/*H2CH3), although carbon-centered radicals originated from amino acids could be formed in the presence of Cys, Met, His, or Trp. All proteins and amino acids, except Cys, were effective at inhibiting generation of radicals from the Fe(II)/CumOOH system. Trp was the amino acid with the highest antiradical activity, followed by His > Gly approximately Met. Lactoferrin was the protein showing the most efficient inhibition of radical formation from the Fe(II)/CumOOH system, and BSA and beta-lactoglobulin were not significantly different in their antiradical activities. These results suggest that proteins with higher inhibitory activity on lipid oxidation promoted by transition metal catalytic decomposition of hydroperoxides should be those with elevated metal-chelating and radical-scavenging properties as well as low concentration and accessibility of reducing groups from amino acids capable of activating metals, such as sulfhydryl groups.  相似文献   

7.
The antioxidant properties of components of leaf extracts of the evergreen plant, Sempervivum tectorum (ST), have been evaluated using UV irradiated liposomal systems containing the spin trap 5-(diethoxyphosphoryl)-5-methyl-pyrroline-N-oxide. Decreases in free radical activity in the liposomal systems as measured by electron paramagnetic resonance (EPR) spectroscopy demonstrate that the lipophilic ST juice components, kaempferol (KA) and kaempferol-3-glucoside (KG) contribute significantly to the antioxidant properties of the juice. EPR spectral simulation established the presence of oxygen and carbon centered free radical adducts. The mixtures with low pH, citric and malic acid, and ST juice reveal increased EPR signals from oxygen centered radicals in comparison to the control, pointing to the important role of pH in oxygen radical formation. Parallel assays that measured thiobarbituric acid related substances confirm the antioxidant effects of KA and KG and explain the results of spin trapping experiments complicated by low pH's.  相似文献   

8.
Influence of malt roasting on the oxidative stability of sweet wort was evaluated based on radical intensity, volatile profile, content of transition metals (Fe and Cu) and thiols. Malt roasting had a large influence on the oxidative stability of sweet wort. Light sweet worts were more stable with low radical intensity, low Fe content, and ability to retain volatile compounds when heated. At mild roasting, the Fe content in the wort increased but remained close to constant with further roasting. Dark sweet worts were less stable with high radical intensities, high Fe content, and a decreased ability to retain volatiles. Results suggested that the Maillard reaction compounds formed during the roasting of malt are prooxidants in sweet wort. A thiol-removing capacity was observed in sweet wort, and it was gradually inhibited by malt roasting. It is possibly caused by thiol oxidizing enzymes present in the fresh malt.  相似文献   

9.
In the anode side of a two-chamber electrolyzer, electrolysis of a NaCl solution generates acidic electrolyzed oxidizing (EO) water, which exhibits bactericidal effects against a large number of pathogens. This study was undertaken to investigate whether OH radical species are present in EO water or are formed when EO water reacts with iron ions. Electron spin resonance spectroscopy (ESR) coupled with the spin trapping technique was used for the detection of free radicals. Samples of EO water were collected at 0.5, 1, 2, 3, and 5 min of electrolysis and immediately mixed with the spin trapping agent 5,5-dimethyl-1-pyrroline-N-oxide (DMPO). The 5,5-dimethyl-2-hydroxypyrrolidine-N-oxyl (DMPO-OH) spin adduct, characteristic of OH radicals, was not observed. Starting with 2-min electrolysis, a seven-line spectrum characteristic of 5,5-dimethyl-2-pyrrolidone-N-oxyl (DMPOX) was formed. The reactions of EO water with Fe3+ and Fe2+ in the presence of DMPO yielded the spin adduct DMPO-OH. However, the addition of OH radical scavengers (ethanol and methanol) did not generate the characteristic DMPO-alkyl spin adducts. This indicated that the DMPO-OH spectrum was due to a nucleophilic addition of water to DMPO and not to trapping of OH radicals.  相似文献   

10.
Myoglobin-induced lipid oxidation. A review   总被引：1，自引：0，他引：1  
An overview of myoglobin-initiated lipid oxidation in simple model systems, muscle, and muscle-based foods is presented. The potential role of myoglobin spin and redox states in initiating lipid oxidation is reviewed. Proposed mechanisms for myoglobin-initiated lipid oxidation in muscle tissue (pH 7.4) and meat (pH 5.5) are evaluated with the purpose of putting forward general mechanisms explaining present observations regarding the catalytic events.  相似文献   

11.
Free radicals, antioxidants, and soil organic matter recalcitrance   总被引：1，自引：0，他引：1  
Highly reactive, and potentially damaging, free radicals are readily generated in our oxygen‐rich environment, and are ubiquitous in biological systems. However, plants and animals have evolved protection against them with a range of antioxidant molecules, such as vitamins C and E, many of which are phenolic compounds. These stop the destructive chain reaction of free radical formation by being transformed into unreactive, stable free radicals. The biodegradation of food involves oxidation by free radicals, and is retarded by antioxidants. Similarly, the biodegradation of plant residues in soils involves free radicals; so the questions arise: (i) do soils have antioxidants, and (ii) what function might they have? The evidence suggests that they probably do have antioxidants. First, plant and animal remains added to soils will contain antioxidants. These are likely to persist for a time, particularly tannins, which are polyphenolic compounds with known antioxidant properties and which are relatively resistant to degradation. Second, studies using electron spin resonance spectroscopy have shown that humic materials contain stable semiquinone free radicals, and that their concentration increases as humification progresses. These semiquinone species are most likely to be derived from the reaction of phenolic compounds with reactive radicals. If this is the case, the phenolics are acting as antioxidants, because they are scavenging the reactive free radicals and terminating the oxidative chain reaction responsible for soil organic matter (SOM) degradation. Thus the soil's antioxidant capacity could control the rate of breakdown of organic matter in the more labile pools and could provide a chemical mechanism for the recalcitrance of SOM. Current available evidence for the nature of the recalcitrant pool in SOM is discussed in the light of this hypothesis, and the experimental approaches necessary for testing it are outlined.  相似文献   

12.
This study is the second of two publications that investigate the interactions between volatile and nonvolatile components in coffee brew. The purpose here was to shed some light into the chemical mechanisms responsible for the decrease of volatile thiols when in contact with coffee nonvolatiles. A mixture of volatile thiols covering a large range of physicochemical properties was monitored over time in the presence of a coffee brew model. The binding potential was estimated by SPME-GC-MS. Additives inhibiting specific reaction pathways were preincubated with the coffee brew 1 h prior to addition of the volatile compounds. Degradation kinetics of the volatile thiols were characterized by their rate constants k(obs). The effect of individual additives was shown by calculating k(rel), the relative rate constant as compared to the reference without additive. The conclusion was that thiols, mainly responsible for the "roasty" and "burnt" notes, disappear via two main chemical mechanisms. The results suggest that nucleophilic addition is the major pathway for thiol degradation. Addition occurs on oxidized species generated in the matrix in the presence of air. This mechanism prevails for aliphatic thiols (e.g., ethanethiol, methanethiol). Benzylic thiols (such as 2-furfurylthiol) can react in parallel via another pathway that is slowed in the absence of oxygen and in the presence of a radical scavenger. This points to a radical mechanism, but further work is needed to support this hypothesis. A direct correlation between thiol hydrophobicity and the magnitude of the interactions was shown as well. Therefore, weak physical interactions or hydrophobic assistance accelerating chemical reactions cannot be excluded at this point of the study.  相似文献   

13.
An increasing body of evidence on the association between adherence to the Mediterranean diet and healthy status is being accumulated. Floral buds of Capparis spinosa L. are commonly used in the Mediterranean cuisine as flavoring for meat and other foods. The present study evaluated bioactive components and antioxidant activity of Sicilian capers stabilized in salt. Whereas alpha-tocopherol was absent, low levels of gamma-tocopherol and vitamin C were measured. With reference to one serving size (8.6 g of capers), rutin was 13.76 mg, isothiocyanates, recently acknowledged as anticarcinogen phytochemicals, were 42.14 micromol, total phenols were 4.19 mg of gallic acid equivalents (GAE), and the total antioxidant potential measured using the [2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid)] diammonium salt (ABTS) cation radical decolorization assay was 25.8 micromol of Trolox equivalents. The antioxidative activity of a caper hydrophilic extract was assessed in a number of assays. The extract at 3.5 and 7.0 microM GAE exhibited a dose-dependent peroxyl radical scavenging activity in a methyl linoleate methanol solution oxidized by azo initiator, and reduced hypervalent iron myoglobin species formed from met-Mb an H 2O 2, at 180 microM GAE. The hydrophilic extract, at 70-280 microM GAE, caused a dose-dependent inhibition of lipid autoxidation in heated red meat, incubated with simulated gastric fluid for 180 min. In the same model rutin tested at a concentration corresponding to its content in the extract was ineffective, and alpha-tocopherol at 25 microM was poorly effective. The hydrophilic extract (70 microM GAE) prevented the consumption of the co-incubated alpha-tocopherol, whereas lipid oxidation was inhibited for the experimental time, suggesting cooperative interactions between extract components and the vitamin. The findings encourage the use of caper with foods that contribute oxidizable lipids in view of the association between dietary oxidized lipids and risk of oxidative stress-based diseases.  相似文献   

14.
A model system was used to study the effect of nonheme iron on myoglobin oxidation at pH 5.6 and pH 7.2 at 23 degrees C. The addition of ferrous iron significantly (p < 0.05) increased the rate of myoglobin oxidation in the absence of lipid, demonstrating that iron promoted myoglobin oxidation independent of the effect of lipid oxidation. The addition of the type II, iron chelating antioxidants sodium tripolyphosphate (at pH 7.2) or milk mineral (at pH 5.6) negated the effect of added iron, slowing oxidation of myoglobin. A clear concentration dependence was seen for iron-stimulated myoglobin oxidation, based on both spectral and visual evidence. Further investigation is needed to determine the possible role for nonheme ferrous iron on myoglobin oxidation in vivo or in meat.  相似文献   

15.
Pepsin proteolysis at pH approximately 4 resulted in a lowering of the (pseudo)peroxidase activity of metmyoglobin both at physiological pH and at meat pH, as measured by a peroxidase assay with H(2)O(2) and ABTS as substrates. In contrast, the mildly proteolyzed myoglobin had a strongly enhanced prooxidative effect on lipid oxidation in an oil in water methyl linoleate emulsion compared to native metmyoglobin, as evidenced by rates of oxygen depletion. More severe proteolysis of metmyoglobin at lower pH values near the optimum for pepsin did not result in a similar enhancement of prooxidative activity. The mildly proteolyzed metmyoglobin had spectral characteristics in agreement with a relative stabilization of the iron(II) state. On the basis of the observed effects of metal chelators, of lipophilic and hydrophilic peroxides and of radical scavengers on oxygen depletion rates, it is suggested that the increased prooxidative effect is due to radicals formed by cleavage of lipid peroxides by iron(II)/iron(III) cycling of a heme pigment with affinity for the lipid/water interface.  相似文献   

16.
Sulfur speciation in low molecular weight (LMW) subunits of glutenin after reoxidation with potassium iodate and potassium bromate at different pH values, aged subunits of glutenin as well as gluten, and gliadin have been investigated in situ by S K-edge X-ray absorption near-edge structure (XANES) spectroscopy. XANES spectra were analyzed quantitatively using a least-squares fitting routine to provide relative percentage contribution of different sulfur species occurring in the samples. Using potassium iodate and potassium bromate for reoxidation of reduced LMW subunits of glutenin led not only to disulfide states but also to higher oxidation states (sulfoxide state, sulfonic acid state). Strongest oxidation occurred at low pH values. Higher oxidation states were also predominantly detected in the aged subunits of glutenin, whereas the disulfide state was the main sulfur species in gluten and gliadin samples. The results showed that the oxidation state of sulfur prior to oxidation (thiol, disulfide) strongly influences sulfur speciation after oxidation. The choice of the oxidizing reagent seems to be of minor importance.  相似文献   

17.
Aging, free radicals, and antioxidants in wheat seeds   总被引：1，自引：0，他引：1  
Free radical oxidative attack is considered a major cause of disruption and deteriorative changes observed in aged seeds. Antioxidant defense mechanisms may remove potentially damaging molecular species, and carotenoids may act as radical scavengers. The content of lutein, the major carotenoid in wheat seeds, was determined in the flours. It showed a rapid decrease during seed aging. In addition, the content of free radicals in glutens made from flours of wheat seeds after long-term storage was studied. The concentration of radicals appeared to be age dependent, because the highest content of radicals was detected between 13 and 15 years of aging over 36 years of storage. Specific spin labeling of the sulfhydryl groups of gluten proteins enabled comparative EPR studies of the rigidity of the protein chains. A progressive stiffening of polymeric gluten with seed storage was found.  相似文献   

18.
Humic substances contain significant quantities of free radicals which have important functions in many biological processes in soil involving electron transfer. Such radicals may be detected by electron spin resonance spectroscopy in both solid and solution state and are usually reported as giving a single unstructured peak. Humic acids from a range of soil types varying in parent material and pH have been examined by electron spin resonance spectroscopy of their solutions in 0.1 m KOH. Spectra with an effective g-value typical of an organic free radical were obtained. Observation of hyperfine structure was highly dependent on the instrument settings used. It was present at a modulation amplitude of 0.0142 mT, but not at 0.101 mT. Similarly, there was a reduction in resolution as microwave power was increased from 0.505–20 mW. This may explain the inability of some investigators to observe structure in soil humic acids. Resolution of hyperfine features was greatly enhanced by recording spectra in the second derivative mode. Using appropriate instrument settings all the humic acids showed hyperfine structure, although the intensity of these components varied by a factor of 100, being much greater in humic acid from highly acidic soils.  相似文献   

19.
The objectives of this study were to investigate the effects of dietary fat (6% soy oil or rapeseed oil or tallow) and alpha-tocopheryl acetate supplementation at two levels (30 or 200 ppm) on radical production, measured by ESR spectroscopy, and on lipid and protein oxidation in turkey muscle extracts oxidized by an enzymic system (NADPH, ADP, FeSO(4)/cytochrome P450 reductase). Two muscles were tested: pectoralis major (glycolytic) and sartorius (oxidative) muscles. Radical production measured by ESR was higher in pectoralis major muscle than in sartorius muscle, whereas lipid and protein oxidation was more important in sartorius muscle, showing the importance of the pro-/antioxidant ratio in oxidative processes in muscular cells and of the measurement methodology to appreciate the free radical production. Dietary fat had no effect on the level of ESR signals, whereas feeding of animals with soy oil induced higher oxidation of lipids. Protein oxidation was less sensitive to the nature of the dietary fat than lipid oxidation. Vitamin E supplementation significantly decreased radical production, as measured by ESR spectroscopy. Vitamin E also decreased lipid and protein oxidation, but the effect of vitamin E on protein oxidation was less pronounced than on lipid oxidation.  相似文献   

20.
Electron spin resonance (ESR) spectrometry has been adapted to accelerated (light and temperature) storage experiments with processed cheese comparing the spin trapping technique with 5, 5-dimethyl-1-pyrroline-N-oxide (DMPO) as spin trap added to the cheese and the spin labeling technique with 2,2,6, 6-tetramethylpiperidine-1-oxyl radical (TEMPO) as spin label added to the cheese. Both methods showed that light was the more important factor compared to temperature for early stages (up to 11 days) in the formation of radicals in processed cheese. For the spin labeling technique other, unidentified reactions interfered during the early stages of the accelerated storage, indicating that longer reaction times are required for the evaluation, and the spin trapping technique is recommended. As a third method, direct measurement of free radicals formed in processed cheese stored for 15 months and subsequently freeze-dried showed that for longer storage, temperature is more important for the steady-state concentration of radicals than light exposure. In agreement with this result, formation of secondary lipid oxidation products determined as thiobarbituric acid reactive substances was found to correlate with the direct ESR measurement.  相似文献   

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