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1.
The reactivity of purine derivatives (uric acid, xanthine, hypoxanthine, and purine) toward triplet-excited riboflavin in aqueous solution at pH 6.4 is described on the basis of kinetic (laser flash photolysis), electrochemical (square-wave voltammetry), and theoretical data (density functional theory, DFT). Direct deactivation of triplet-excited riboflavin in aqueous solution, pH 6.4 at 24 degrees C, in the presence of uric acid, xanthine, and hypoxanthine strongly suggests a direct electron transfer from the purine to the triplet-excited riboflavin with k = 2.9 x 10(9) M(-1) s(-1) (DeltaH(++) = 14.7 kJ mol(-1), DeltaS(++) = -15.6 J mol(-1) K(-1)), 1.2 x 10(9) M(-1) s(-1) (DeltaH(++) = 34.3 kJ mol(-1), DeltaS(++) = +45.3 J mol(-1) K(-1)), and 1.7 x10(8) M(-1) s(-1) (DeltaH(++) = 122 kJ mol(-1), DeltaS(++) = +319 J mol(-1) K(-1)), respectively. From the respective one-electron oxidation potentials collected in aqueous solution at pH 6.4 for uric acid (E = +0.686 vs normal hydrogen electrode, NHE), xanthine (E = +1.106 vs NHE), and hypoxanthine (E = +1.654 vs NHE), the overall free energy changes for electron transfer from the quencher to the triplet-excited riboflavin are as follows: uric acid (DeltaG(o) = -114 kJ mol(-1)), xanthine (DeltaG(o) = -73.5 kJ mol(-1)), hypoxanthine (DeltaG(o) = -20.6 kJ mol(-1)), and purine (DeltaG(o) > 0). The inertness observed for purine toward triplet-excited riboflavin corroborates with its electrochemical inactivity in the potential range from 0 up to 2 V vs NHE. These data are in agreement with the DFT results, which show that the energy of the purine highest occupied molecular orbital (HOMO) (-0.2685 arbitrary unit) is lower than the energy of the semioccupied molecular orbital (SOMO) (-0.2557 a.u.) of triplet-excited riboflavin, indicating an endergonic process for the electron-transfer process. The rate-determining step for deactivation by purine derivatives can be assigned to an electron transfer from the purine derivative to the SOMO orbital of the triplet-excited riboflavin. The results show that uric acid may compete with oxygen and other antioxidants to deactivate triplet-excited riboflavin in milk serum and other biological fluids leading to a free radical process.  相似文献   

2.
Isohumulones, dihydroisohumulones, tetrahydroisohumulones, and humulinones, important hop-derived bittering compounds in beer, were shown to give rise to reactive triacylmethyl radicals on interaction with triplet-excited riboflavin after spin trapping by 5,5-dimethyl-1-pyrroline N-oxide or 2-methyl-2-nitrosopropane, followed by electron paramagentic resonance spectroscopy combined with spectral simulation. Electron abstraction from the ionized beta-tricarbonyl chromophore, which is common to all five-membered ring hop derivatives, is the initial event on photoinduced degradation. Radicaloid decomposition of isohumulones leads to precursors for 3-methylbut-2-ene-1-thiol, the lightstruck constituent in beer. Interaction of reduced derivatives of isohumulones with triplet-excited riboflavin furnished radical precursors of volatile aldehydes, which may lead to the development of unpleasant stale or cardboard flavors.  相似文献   

3.
Tocopherols (alpha, beta, gamma, and delta) and Trolox were found to deactivate triplet-excited riboflavin in homogeneous aqueous solution (7:3 v/v tert-butanol/water) with second-order reaction rates close to diffusion control [k2 between 4.8 x 10(8) (delta-tocopherol) and 6.2 x 10(8) L mol(-1) s(-1) (Trolox) at 24.0 +/- 0.2 degrees C] as determined by laser flash photolysis transient absorption spectroscopy. In aqueous buffer (pH 6.4) the rate constant for Trolox was 2.6 x 10(9) L mol(-1) s1 and comparable to the rate constant found for ascorbate (2.0 x 10(9) L mol(-1) s(-1)). The deactivation rate constant was found to be inferior in heterogeneous systems as shown for alpha-tocopherol and Trolox in aqueous Tween-20 emulsion (approximately by a factor of 4 compared to 7:3 v/v tert-butanol/water). Neither beta-carotene (7:3 v/v tert-butanol/water and Tween-20 emulsion), lycopene (7:3 v/v tert-butanol/water), nor crocin (aqueous buffer at pH 6.4, 7:3 v/v tert-butanol/water, and Tween-20 emulsion) showed any quenching on the triplet excited state of riboflavin. Therefore, all carotenoids seem to reduce the formation of triplet-excited riboflavin through an inner-filter effect. Activation parameters were based on the temperature dependence of the triplet-excited deactivation between 15 and 35 degrees C, and the isokinetic behavior, which was found to include purine derivatives previously studied, confirms a common deactivation mechanism with a bimolecular diffusion-controlled encounter with electron (or hydrogen atom) transfer as rate-determining step. DeltaH for deactivation by ascorbic acid, Trolox, and homologue tocopherols (ranging from 18 kJ mol(-1) for Trolox in Tween-20 emulsion to 184 kJ mol(-1) for ascorbic acid in aqueous buffer at pH 6.4) showed a linear dependence on DeltaS (ranging from -19 J mol(-1) K(-1) for Trolox in aqueous buffer at pH 6.4 to +550 J mol(-1) K(-1) for ascorbic acid in aqueous buffer pH 6.4). Among photooxidation products from the chemical quenching, lumicrome, alpha-tocopherol quinones and epoxyquinones, and alpha-tocopherol dimers were identified by ESI-QqTOF-MS.  相似文献   

4.
The simultaneous determination of 19 phenolic compounds was performed directly in wort and beer by a combination of reverse-phase high-performance liquid chromatography coupled with coulometric array detection. Chromatographic separation was achieved with an appropriate gradient of flow and a binary solvent based on phosphate buffer, methanol, and acetonitrile in a 45-min run. Eight serial coulometric detectors were used for on-line generation of voltammetric data to resolve coeluting compounds. The method was reliable and sensitive, the regression coefficient of standard calibration curves is 0.972 < or = r < or = 1.000, and the standard deviation value ranges from 0.010 to 0.129 mg/L for wort and from 0.002 to 0.332 mg/L for beer. The mean concentrations of phenolic acids were 22.1 and 33.8 mg/L, respectively, in worts and beers produced in Italy. These amounts represent 5 and 10% of the non-tannic, non-flavonoid phenols in wort and beer, respectively.  相似文献   

5.
A method for the quantitative determination of riboflavin levels in beer was developed. The method is based on the quenching of riboflavin fluorescence, which occurs when riboflavin binds to the aporiboflavin-binding protein from egg white. The method does not require any pretreatment of the beer before analysis, other than dilution, and proved to be simple, reliable, and sensitive. The lowest concentration that could be detected was approximately 10 nM riboflavin. The possible interference of flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) with the determination of the riboflavin content of beer was excluded, because beer contains only a very small amount of FAD (0.03 microM) and no FMN. The riboflavin levels of the types and brands of beer investigated were in the range of 0.5-1.0 microM. The origin of the riboflavin in beer proved to be the malt. Hop and yeast hardly contributed to the riboflavin content of beer. Besides its use in the determination of riboflavin levels, the aporiboflavin-binding protein also provides a way to remove riboflavin from beer, which reduces the light sensitivity and the related lightstruck off-flavor formation in beer.  相似文献   

6.
The reaction between the triplet excited state of riboflavin and amino acids, peptides, and bovine whey proteins was investigated in aqueous solution in the pH range from 4 to 9 at 24 degrees C using nanosecond laser flash photolysis. Only tyrosine and tryptophan (and their peptides) were found to compete with oxygen in quenching the triplet state of riboflavin in aqueous solution, with second-order rate constants close to the diffusion limit, 1.75 x 10(9) and 1.40 x 10(9) L mol(-1) s(-1) for tyrosine and tryptophan, respectively, with beta-lactoglobulin and bovine serum albumin having comparable rate constants of 3.62 x 10(8) and 2.25 x 10(8) L mol(-1) s(-1), respectively. Tyrosine, tryptophan, and their peptides react with the photoexcited triplet state of riboflavin by electron transfer from the tyrosine and tryptophan moieties followed by a fast protonation of the resulting riboflavin anion rather than by direct H-atom abstraction, which could be monitored by time-resolved transient absorption spectroscopy as a decay of triplet riboflavin followed by a rise in riboflavin anion radical absorption. For cysteine- and thiol-containing peptides, second-order rate constants depend strongly on pH, for cysteine corresponding to pKaRSH = 8.35. H-atom abstraction seems to operate at low pH, which with rising pH gradually is replaced by electron transfer from the thiol anion. From the pH dependence of the second-order rate constant, the respective values for the H-atom abstraction (k = 1.64 x 10(6) L mol(-1) s(-1)) and for the electron transfer (k = 1.20 x 10(9) L mol(-1) s(-1)) were determined.  相似文献   

7.
Honeybee-collected pollen is promoted as a health food with a wide range of nutritional and therapeutic properties. A high-performance capillary electrophoresis with amperometric detection method has been developed for the simultaneous determination of bioactive ingredients in 10 samples of honeybee-collected pollen in this work. Under the optimum conditions, 13 phenolic components can be well-separated or nearly baseline-separated (apigenin and vanillic acid peaks) within 29 min at the separation voltage of 14 kV in a 50 mM borax running buffer (pH 9.0), and adequate extraction was obtained with ethanol for the determination of the above 13 compounds. Recovery (94.1-104.0%), repeatability of the peak current (<5.4%), and detection limits (6.9 x 10(-7)-6.4 x 10(-9) g mL(-1)) for the method were evaluated. This procedure was successfully used for the analysis and comparison of the phenolic content of honeybee-collected pollen samples originating from different floral origins based on their electropherograms or "phenolic profiles".  相似文献   

8.
A comparison between the results obtained by using HPLC-UV, HPLC-MS, and CE-UV for characterizing the deterioration of extra-virgin olive oil during heating (180 degrees C) was investigated, taking into account phenolic compounds. The concentration of several compounds belonging to four families of phenols (simple phenols, lignans, complex phenols, and phenolic acids) was determined in the samples after the thermal treatment by all three techniques. Hydroxytyrosol, elenolic acid, decarboxymethyl oleuropein aglycon, and oleuropein aglycon reduced their concentration with the thermal treatment more quickly than other phenolic compounds present in olive oil. HYTY-Ac and Lig Agl were demonstrated to be quite resistant to this kind of treatment, and the behavior of lignans could be outstanding, as they belong to the family most resistant to thermal treatment. Several "unknown" compounds were determined in the phenolic profiles of the oils after the thermal treatment, and their presence was confirmed in refined olive oils. The oxidative stability index (OSI time) was reduced from 25 to 5 h after 3 h of heating, whereas the peroxide value showed a minimum after 1 h of heating.  相似文献   

9.
Lumichrome and lumiflavin were formed from riboflavin under light. pH had a significant influence on the formation of lumichrome and lumiflavin from riboflavin. Lumichrome was the only major product from riboflavin under neutral or acidic pH values. Lumiflavin was also formed from riboflavin in basic pH. The maximum concentration of lumiflavin from 100 microM riboflavin at pH 8.5 was 30.9 microM, and it was reached after 2 h of exposure at 1500 lux. The maximum concentration of lumichrome formed from 100 microM riboflavin at pH 4.5, 6.5, or 8.5 was 79.9, 58.7, and 73.1 microM, respectively, after 8, 6, or 2 h of light exposure. The formation of lumichrome and lumiflavin from riboflavin was due to the type I mechanism of the riboflavin photosensitized reaction. Singlet oxygen was also involved in the photosensitized degradation of lumiflavin and lumichrome. The reaction rates of riboflavin, lumiflavin, and lumichrome with singlet oxygen were 9.66 x 10(8), 8.58 x 10(8), and 8.21 x 10(8) M(-1) s(-1), respectively. The headspace oxygen depletion and headspace volatile formation were significant in soy milk containing lumichrome or lumiflavin under light (p < 0.05) and were insignificant (p > 0.05) in the dark. Ascorbic acid could inhibit the total volatile changes of soy milk under light. Soy milk should be protected from light to prevent the photodegradation of riboflavin and the oxidation of soy milk.  相似文献   

10.
The effects of 0, 0.3, 0.6, and 0.9 mM Trolox and ascorbic acid on the singlet oxygen oxidation of tryptophan and tyrosine containing 25 ppm of riboflavin were determined by measuring tryptophan and tyrosine concentration by high-performance liquid chromatography analysis. The samples were stored in the a 1000 lx light storage box for 4 h at 30 degrees C. As the concentration of Trolox and ascorbic acid increased, the degradation of tryptophan and tyrosine decreased significantly at p < 0.05. Trolox reduced tryptophan and tyrosine degradation by quenching both singlet oxygen and excited triplet riboflavin, whereas ascorbic acid quenched singlet oxygen only. The total singlet oxygen quenchings of Trolox in the presence of tryptophan and tyrosine were 1.55 x 10(7) and 1.32 x 10(7) M(-1) s(-1), respectively. The total singlet oxygen quenchings of ascorbic acid in the presence of tryptophan and tyrosine were 1.16 x 10(7) and 1.10 x 10(7) M(-1) s(-1), respectively. Trolox was more effective than ascorbic acid in preventing the degradation of tryptophan and tyrosine.  相似文献   

11.
The free phenols have been measured in 15 lagers, 6 porters and ales, and 11 light and nonalcoholic beers. Phenols were measured colorimetrically using an oxidation-reduction reaction with Folin-Ciocalteu reagent and catechin as the standard. The order of phenol concentration was ales > lagers > low calorie > nonalcoholic. The quality of antioxidants of the major phenols in beers and the quality of beer antioxidants were measured by (1) dose-response inhibition of lower density lipoprotein oxidation and (2) concentration of phenols in the beers at which 50% of the peroxide was destroyed in a luminescent assay for antioxidant activity. The beers' lipoprotein antioxidant quality was clearly superior to that of vitamin antioxidants and to that of the phenol ingredients, suggesting synergism among the antioxidants in the mixture. The average per capita consumption of beer in the United States in 2000 was 225 mL/day, equivalent to 42 mg/day of catechin equivalents. Beer provides more antioxidants per day than wine in the U.S. diet. A dark beer and a lager beer were given at two concentrations to cholesterol-fed hamsters, an animal model of atherosclerosis. At the high dose ((1)/(2)-diluted beer) both lager and dark beer significantly inhibited atherosclerosis compared to a control of 2% alcohol. At the high dose, lager significantly decreased cholesterol and triglycerides, and both beers acted as in vivo antioxidants by decreasing the oxidizability of lower density lipoproteins. At the low dose ((1)/(10)-diluted beer) only the lager beer significantly decreased atherosclerosis compared to the 0.4% alcohol control. The polyphenols in the beers appear to be responsible for the benefits of beer in this model. Lager beer inhibited atherosclerosis at a human equivalent dose in this hamster model of atherosclerosis.  相似文献   

12.
To protect the nutrient and flavor stability of milk under light, the effects of 0, 0.01, 0.03, and 0.05 M 1,4-diazabicyclo[2,2,2]octane (DABCO) and 2,5-dimethylfuran (DMF) on the riboflavin photosensitized oxidation of milk were studied. The oxidation of milk was studied by measuring the headspace oxygen in sample bottles after 3 h of light exposure at 3000 lux. As the concentration of DABCO and DMF, which are water soluble compounds, increased in the sample from 0, 0.01, and 0.03 to 0.05 M, the depleted headspace oxygen content significantly decreased (P < 0.05). Steady state kinetic studies of singlet oxygen oxidation showed that the antioxidant activity of DABCO and DMF was due to singlet oxygen quenching. The reaction rate constant of singlet oxygen with milk fat was 8.1 x 10(5) M(-1) s(-1). Total singlet oxygen quenching rates of DABCO and DMF were 1.5 x 10(7) and 2.6 x 10(7) M(-1) s(-1), respectively. DABCO and DMF could be used to slow the reaction between singlet oxygen and milk components to protect nutrients, especially riboflavin, and to improve the oxidative stability of milk fat during storage or processing under light.  相似文献   

13.
Light-induced formation of lipid peroxides in a water-in-oil emulsion based on purified rape-seed oil (80%) was found to increase with decreasing wavelength and to have the (apparent) quantum yields (1.1 +/- 0.1) x 10(-)(3) for 436 nm, (2.6 +/- 0.1) x 10(-)(3) for 405 nm, and (4.5 +/- 0.4) x 10(-)(3) for 366 nm irradiation, as determined after 12 h of exposure to monochromatic light of an approximate intensity of 10(18) quanta.min(-)(1).mL(-)(1) and related to total light absorption. Riboflavin (0.8 ppm) had no effect on lipid peroxidation, but photodegraded with a quantum yield ((1.5 +/- 0.3) x 10(-)(5) for 436 nm, (1.7 +/- 0.2) x 10(-)(5) for 405 nm and (1.39 +/- 0.09) x 10(-)(5) for 366 nm irradiation) independent of irradiation wavelength. beta-Carotene was only photodegraded to a minor extent, but protected riboflavin against photodegradation and the lipids against peroxidation for 436 and 405 nm irradiation (reduction in quantum yield three times for 4.5 ppm beta-carotene for lipid oxidation and more for riboflavin degradation), but not for 366 nm irradiation, where beta-carotene has an absorption minimum.  相似文献   

14.
Metolachlor is one of the most widely used herbicides in the world for controlling weeds. It has been detected in both ground and surface waters in the United States, and there are rising concerns in regard to its health risks and in developing effective treatment processes for its removal from water. Degradation of metolachlor via ultraviolet (UV) photolysis and an UV/hydrogen peroxide advanced oxidation process (AOP) was studied. The quantum yield of metolachlor at 254 nm was found to be 0.302 +/- 0.001 mol E-1 through direct UV photolysis in the range of pH 6-8. The second-order rate constant of the reaction between metolachlor and hydroxyl radical was determined to be 9.07 (+/-0.21) x 10(9) M-1 s-1 by using a competition kinetics model that utilized nitrobenzene as a reference compound. In addition, these parameters were successfully applied in modeling the kinetics of elimination of metolachlor using an UV/H2O2 process in both laboratory and natural waters. The formation of several photolysis byproducts was identified using gas chromatography/mass spectrometry, and a scheme for the metolachlor photodegradation pathway is proposed.  相似文献   

15.
The possibility that beer and other alcoholic beverages could be antimutagenic against the heterocyclic amines (HAs), a group of carcinogens produced on cooking proteinaceous foods, has been explored. In the Salmonella mutation assays, beer showed inhibitory effects against several HAs [preactivated Trp-P-1, Trp-P-2(NHOH), and Glu-P-1(NHOH)] that are directly mutagenic in bacteria. Japanese sake, red and white wines, and brandy were also effective. However, ethyl alcohol alone did not show these effects. The formation of O(6)-methylguanine by N-methyl-N'-nitro-N-nitrosoguanidine in the DNA of Salmonella YG7108 was also inhibited by beer. Nonvolatile beer components were administered orally to CDF(1) mice together with Trp-P-2. Adducts in the liver DNA were significantly decreased by the beer, as compared to those in controls fed Trp-P-2 only. Although several phenolic compounds known to be present in beer were antimutagenic toward these mutagens, their effects were very small. It was concluded that some yet to be identified component(s) of beer is (are) responsible for this antimutagenicity.  相似文献   

16.
Phenolic compounds are found in both free and bound forms in cereals. The majority is in the insoluble bound form, that is, bound to cell wall material, such as ferulic acid and its derivatives. The antioxidant properties of the phenolic compounds in grains are associated with the health benefits of grains and grain products. The extraction capacity of several solvent mixtures, for extracting free phenols from barley flours, and the possibility of employing a rapid automated solvent extraction method were studied. The extraction yield of each method was evaluated by correlating several spectrophotometric indices (absorption at 280, 320, and 370 nm and total phenolic compounds by the Folin-Ciocalteu method) with the antioxidant activities of the barley extracts (scavenging activity by the 2,2-diphenyl-1-picrylhydrazyl method). Interesting results were obtained when ethanol and acetone-based extraction mixtures were employed to extract free phenols. A comparison was made between alkaline and acid hydrolysis. The extraction yield of bound phenolic compounds increased when the digestion time for alkaline hydrolysis was prolonged.  相似文献   

17.
Capillary electrophoresis (CE) can be effectively used as a fast screening tool to obtain qualitative and semiquantitative information about simple and complex phenolic compounds of extra virgin olive oil. Three simple phenols (tyrosol, hydroxytyrosol, and vanillic acid), a secoiridoid derivative (deacetoxy oleuropein aglycon), and two lignans (pinoresinol and acetoxypinoresinol) were detected as the main compounds in extra virgin olive oils by high-performance liquid chromatography (HPLC) and capillary zone electrophoresis (CZE). Spectrophotometric indices, radical scavenging activity, and oxidative stability of extra virgin olive oil samples obtained from olives hand-picked at different ripening degrees were statistically correlated with the CZE and HPLC quantification. The concentration of phenols in extra virgin olive oil decreased with ripeness of olive fruits. The high correlations found between CZE and the other analytical results indicate that CE can be applied as a rapid and reliable tool to routinely determine phenolic compounds in extra virgin olive oils.  相似文献   

18.
In view of the essential role of phenolic compounds in the development of pathogen resistance in plants, and given the influence that fungicides exert over phenolic metabolism, the aim of the present study was to determine the effect of the application of different rates of fungicide on the metabolism of phenolic compounds in tobacco plants (Nicotiana tabacum L. cv. Tennessee 86). The fungicide applied was carbendazim, with a purity of 100%, at three different rates: 1.3 mM (carb(1)), 2.6 mM (this being the recommended concentration, carb(2)), and 5.2 mM (carb(3)). The control treatment was without carbendazim. The results in relation to control plants indicate that the application of carb(1) in tobacco plants not afflicted by damaging biotic and abiotic agents boosts phenolic accumulation. Therefore, in the case of carbendazim, the application of 50% less (carb(1), 1.3 mM) than the recommended dosage (carb(2), 2.6 mM) of this fungicide could be more effective, because the foliar accumulation of phenolics presented at carb(1) may imply an increased resistance of plants to pathogen infection. On the other hand, we found an inhibition of the phenolic oxidation by the application of carbendazim, principally at carb(3). These results suggest that the excessive application of carbendazim (5.2 mM) could be harmful for healthy plants, because, on inhibiting phenolic metabolism (biosynthesis and oxidation), such treatment would also sharply reduce the capacity of these plants to respond against pathogen attack.  相似文献   

19.
Antioxidants are released during the NaOH extraction of soils, and this also releases phenolic compounds from plant and soil material. We hypothesized that phenolics would be important contributors to the antioxidant capacity (AOC) of soils. The concentrations of 12 phenolic compounds and the AOCs in 4 m ‐NaOH extracts of a range of UK soil types were measured. Samples of both surface and subsurface horizons were taken from 24 sites representing the major soil types (brown soils, gleys, podzols, peats and lithomorphic soils). The land use/vegetation varied from deciduous woodland through heather moorland to agricultural crops. The internal standards method was used to quantify the phenolics, which were detected by gas chromatography. The AOCs of the extracts and of standards of the individual phenolic compounds were measured using the TEAC (Trolox Equivalent Antioxidant Capacity) method. There were differences in the phenolic distributions extracted from soils with different land uses/plant inputs, as well as differences between surface and subsurface samples. A statistically significant linear relationship was found between the AOCs of the extracts and the sum of the phenolic compounds, as both these variables were positively correlated with the organic matter content of the soils. The AOCs of the individual phenols were used to calculate their contribution to the overall AOC of the extracts, and this was found to be small (approximately 1% of the total AOC). We concluded that the measured phenolic compounds were not important contributors to the AOCs of the soil extracts, and therefore that other unidentified antioxidant compounds were probably present.  相似文献   

20.
The negative effect of fatty acids on the foam stability of beer has been assessed. Long-chain fatty acids are far more damaging than short-chain fatty acids on the foam stability of beer at the concentrations employed. Polypeptides have been isolated from an all malt beer by hydrophobic interaction chromatography. Using this technique five groups of polypeptides were isolated, group 1 being the least hydrophobic and group 5 the most hydrophobic, all of which exhibited similar polypeptide compositions by SDS-PAGE. All five hydrophobic polypeptide groups bound [(14)C]linoleic acid; however, group 5, the most hydrophobic group, bound the most linoleic acid. Groups 1 and 5 were titrated with cis-parinaric acid (CPA) to produce binding curves, which were compared with a binding curve obtained for bovine serum albumin (BSA). Groups 1 and 5 both produced binding curves that saturated at approximately 5.5 microM and 4 microM CPA and had association constants (K(a)) of 6.27 x 10(7) and 1.62 x 10(7) M(-1), respectively. In comparison, BSA produced a binding curve that saturated at 6 microM CPA and had a K(a) of 3.95 x 10(7) M(-1). Further investigation has shown that group 1 is pH sensitive and group 5 pH insensitive with respect to lipid binding. The lipid-binding activity of group 5 was also shown to be unaffected by ethanol concentration. Linoleic acid (5 microM) when added to beer resulted in unstable foam. Group 5 was added to the lipid-damaged beer and was shown to restore the foam stability to values that were obtained for the control beer. It has therefore been demonstrated that proteins isolated from beer have a lipid-binding capacity and that they can convey a degree of protection against lipid-induced foam destabilization.  相似文献   

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