Aqueous solubility of calcium L-lactate, calcium D-gluconate, and calcium D-lactobionate: importance of complex formation for solubility increase by hydroxycarboxylate mixtures

Among the calcium hydroxycarboxylates important for cheese quality, D-lactobionate [Ksp = (7.0 ± 0.3) × 10(-3) mol(3) L(-3)] and L-lactate [Ksp = (5.8 ± 0.2) × 10(-3) mol(3) L(-3)] were found more soluble than D-gluconate [Ksp = (7.1 ± 0.2) × 10(-4) mol(3) L(-3)], as indicated by the solubility products determined electrochemically for aqueous 1.0 M NaCl at 25.0 °C. Still, solubility of calcium L-lactate increases by 45% in the presence of 0.50 M sodium D-gluconate and by 37% in the presence of 0.50 M sodium D-lactobionate, while solubility of calcium D-gluconate increases by 66 and 85% in the presence of 0.50 M sodium L-lactate and 0.50 M sodium D-lactobionate, respectively, as determined by complexometric titration. Sodium L-lactate and sodium D-gluconate have only little influence on solubility of calcium D-lactobionate. The increased solubility is described quantitatively by calcium binding to D-gluconate (K1 = 14 ± 3 mol(-1) L) in 1.0 M NaCl at 25 °C, D-lactobionate (K1 = 11 ± 2 mol(-1) L), and L-lactate (K1 = 8 ± 2 mol(-1) L), as indicated by the association constants determined electrochemically. In mixed hydroxycarboxylate solutions, calcium binding is quantitatively described by the geometric mean of the individual association constants for both aqueous 1.0 and 0.20 M NaCl, indicating a 1:1 stoichiometry for complex formation.

Photooxidation of other B-vitamins as sensitized by riboflavin

Pyridoxal phosphate (PLP) was found to deactivate triplet-excited riboflavin (Rib) in aqueous solution with a deactivation constant of 3.0 ± 0.1 × 10(8) L mol(-1) s(-1) at 25 °C. Likewise, PLP was found to quench the fluorescence emission of (1)Rib* with (1)kq = 1.0 ± 0.1 × 10(11) L mol(-1) s(-1) as determined by steady state fluorescence. The rather high quenching constant suggests the formation of a ground state complex, which was further confirmed by time-resolved fluorescence measurements to yield a (1)Rib* deactivation constant of 3.4 ± 0.4 × 10(10) L mol(-1) s(-1). Triplet quenching is assigned as one-electron transfer rather than hydrogen-atom transfer from PLP to (3)Rib*, as the reaction quantum yield, Φ = 0.82, is hardly influenced by solvent change from water to D2O, Φ = 0.78. Neither biotin nor niacin deactivates the singlet- or triplet-excited riboflavin as it is expected from their higher oxidation potentials E > 2 V vs NHE.

Advanced glycation endproducts in food and their effects on health

Advanced glycation endproducts (AGEs) form by Maillard-reactions after initial binding of aldehydes with amines or amides in heated foods or in living organisms. The mechanisms of formation may include ionic as well as oxidative and radical pathways. The reactions may proceed within proteins to form high-molecular weight (HMW) AGEs or among small molecules to form low-molecular weight (LMW) AGEs. All free amino acids form AGEs, but lysine or arginine side chains dominate AGE formation within proteins. The analysis of AGEs in foods and body fluids is most often performed by ELISA or LC-MS; however, none of the methodologies cover all HMW and LMW AGEs. Most research is, therefore, carried out using 'representative' AGE compounds, most often N(ε)-carboxymethyl-lysine (CML). Only LMW AGEs, including peptide-bound forms, and carbonyls may be absorbed from the gut and contribute to the body burden of AGEs. Some AGEs interact with specific pro- or anti-inflammatory receptors. Most studies on the biological effects of AGEs have been carried out by administering heated foods. The pro-inflammatory and deteriorating biological effects of AGEs in these studies, therefore, need further confirmation. The current review points out several research needs in order to address important questions on AGEs in foods and health.

Calcium binding to dipeptides of aspartate and glutamate in comparison with orthophosphoserine

Aspartate binds calcium(II) better than glutamate with Ka = 7.0 ± 0.9 L mol⁻¹ for Asp and Ka = 3.0 ± 0.8 L mol⁻¹ for Glu, respectively, as determined using calcium-selective electrodes for aqueous solutions of ionic strength 0.20 at 25 °C at pH of relevance for milk products. For the mixed peptides, the affinity seems additive with Ka = 27 ± 3 L mol⁻¹ for Asp-Glu and 22.7 ± 0.1 for Glu-Asp as compared to the expected 21 L mol⁻¹. In contrast, for Asp-Asp, the affinity is less than additive with Ka = 23 ± 5 L mol⁻¹ as compared to the expected 49 L mol⁻¹, whereas for Glu-Glu, the affinity is more than additive with Ka = 26 ± 4 L mol⁻¹ as compared to the expected 9.0 L mol⁻¹, indicating specific structural effects for Glu-Glu. Ionic strength effects, 1.0 versus 0.20 studied, are similar for Asp and Glu with decreasing affinity for higher ionic strength, whereas the dipeptides with Glu as C-terminus are more sensitive to increasing ionic strength than with Asp as C-terminus. Despite little affinity of calcium to serine with Ka = 0.9 ± 0.2 L mol⁻¹, Glu has increasing affinity for calcium in the serine dipeptide Ser-Glu with Ka = 10 ± 3 L mol⁻¹, which becomes comparable to phosphorylated serine with Ka = 22 ± 5 L mol⁻¹.

Reduction of ferrylmyoglobin by theanine and green tea catechins. Importance of specific Acid catalysis

Reduction of the hypervalent heme pigment ferrylmyoglobin by green tea catechins in aqueous solution of pH = 7.5 was investigated by stopped-flow spectroscopy. Reduction by the gallic acid esters epigallocatechin gallate (EGCG, k2 = 1460 L mol(-1) s(-1), 25.0 °C, 0.16 ionic strength) and epicatechin gallate (ECG, 1410 L mol(-1) s(-1)) was found faster than for epicatechin (EC, 300 L mol(-1) s(-1)) and epigallocatechin (EGC, 200 L mol(-1) s(-1)), even though the gallate ion (G, 330 L mol(-1) s(-1)) is similar in rate to EC. The rate for reduction by EC, EGC, ECG, EGCG, and G shows no correlation with their oxidation potentials or phenolic hydrogen-oxygen bond dissociation energy, but with the pKa of the most acidic phenol group. Theanine, with an acidity similar to that of EC, reduces ferrylmyoglobin with a similar rate (200 L mol(-1) s(-1)), in support of general acid catalysis with an initial proton transfer prior to electron transfer.

Reduction of ferrylmyoglobin by hydrogen sulfide. Kinetics in relation to meat greening

The hypervalent meat pigment ferrylmyoglobin, MbFe(IV)═O, characteristic for oxidatively stressed meat and known to initiate protein cross-linking, was found to be reduced by hydrogen sulfide to yield sulfmyoglobin. Horse heart myoglobin, void of cysteine, was used to avoid possible interference from protein thiols. For aqueous solution, the reactions were found to be second-order, and an apparent acid catalysis could be quantitatively accounted for in terms of a fast reaction between protonated ferrylmyoglobin, MbFe(IV)═O,H(+), and hydrogen sulfide, H2S (k2 = (2.5 ± 0.1) × 10(6) L mol(-1) s(-1) for 25.0 °C, ionic strengh 0.067, dominating for pH < 4), and a slow reaction between MbFe(IV)═O and HS(-) (k2 = (1.0 ± 0.7) × 10(4) L mol(-1) s(-1) for 25.0 °C, ionic strengh 0.067, dominating for pH > 7). For meat pH, a reaction via the transition state {MbFe(IV)═O···H···HS}([symbol: see text]) contributed significantly, and this reaction appeared almost independent of temperature with an apparent energy of activation of 2.1 ± 0.7 kJ mol(-1) at pH 7.4, as a result of compensation among activation energies and temperature influence on pKa values explaining low temperature greening of meat.

Quercetin and daidzein β-apo-14'-carotenoic acid esters as membrane antioxidants

Esterification by β-apo-14'-carotenoic acid was found to have opposite effects on antioxidant activity of quercetin (at B4', B3' hydroxyl) as of daidzein (at A7 hydroxyl) in phosphatidylcholine liposomes. The daidzein ester had increased activity, while quercetin had a significant decreased activity. Quantum mechanical calculations using density function theory (DFT) indicate a modest decrease in bond dissociation enthalpy, BDE, for (weakest) hydrogen-oxygen phenolic bond in daidzein from 368.4 kJ · mol(- 1) to 367.7 kJ · mol(- 1) compared to a significant increase in quercetin from 329.5 kJ · mol(- 1) to 356.6 kJ · mol(- 1) upon derivatization. These opposite changes in tendency for hydrogen atom transfer from phenolic groups to lipid radicals combined with an increase in A-to-B dihedral angle from 0.0° to 36.4° and in dipole moment from 0.40 D to 6.01 D for quercetin upon derivatization, while less significant for daidzein (36.4°-36.7° and 3.26 D-7.87 D, respectively), together provide a rationale for the opposite effect of esterification on antioxidation.

Riboflavin-photosensitized oxidation is enhanced by conjugation in unsaturated lipids

Methyl esters of polyunsaturated fatty acids were found to quench triplet-excited riboflavin ((3)Rib) in efficient bimolecular reactions with rate constants, as determined by laser flash photolysis, linearly depending upon the number of bis-allylic methylene (from 1 to 5). Deactivation of (3)Rib is predicted by combining the experimental second-order rate constants k2 determined for acetonitrile/water (8:2, v/v) at 25 °C with density functional theory (DFT) calculations of bond dissociation energy to have an upper limiting value of 1.22 × 10(7) L mol(-1) s(-1) for hydrogen abstraction from bis-allylic methylene groups in unsaturated lipid by (3)Rib. Still, ergosterol was found to deactivate (3)Rib with k2 = 6.2 × 10(8) L mol(-1) s(-1), which is more efficient than cholesterol, with 6.9 × 10(7) L mol(-1) s(-1). Likewise conjugated (9E,11E) methyl linoleate (CLA) reacts with 3.3 × 10(7) L mol(-1) s(-1), 30 times more efficient than previously found for methyl α-linolenate. Conjugation as in CLA and ergosterol is concluded to enhance (3)Rib deactivation, and dietary plant sterols and CLA may accordingly be important macronutrients for eye and skin health, protecting against light exposure through efficient deactivation of (3)Rib.

Formation of Advanced Glycation End Products (AGEs) are Influenced by Lipids in Milk Powders

Advanced glycation end products (AGEs) were determined by a polyclonal ELISA method in three milk powders of varying lipid content, during storage in sealed containers at 65°C for up to 20 days. AGEs content correlated with increased water activity (aw), decreased glass transition temperature (Tg), increased lactose crystallisation, and browning in the three milk powders. Formation of stable radicals as detected by electron spin resonance spectroscopy correlated with crystallisation of lactose and brown discoloration in the three powders indicating origin from Maillard reactions rather than lipid oxidation. AGEs content was greatest in whole milk powder with highest lipid content, while in butter milk powder formation of secondary lipid oxidation products increased faster as determined by thiobarbituric acid reactive substances.

Calcium hydroxy palmitate: possible precursor phase in calcium precipitation by palmitate

Calcium(II) precipitates with palmitate (Pal(-)), a process affecting calcium absorption in the gut, in a first-order reaction, as followed by a calcium electrode in neutral aqueous solution for excess of calcium(II), with a rate constant showing a minimum at physiological ionic strength of 0.32 ± 0.09 d(-1) at 25°C with a stoichiometry of 1.79 ± 0.03, lower than the expected 1:2. During ageing of precipitate, pH increases in the supernatant, while pH decreases for precipitation with excess of palmitate. Increasing pH, as in some aged food products, decreases the solubility, and Ksp(')=[Ca(2+)][Pal(-)][OH(-)] is found constant rather than Ksp=[Ca(2+)][Pal(-)](2) in agreement with an initial precipitation of Ca(OH)(Pal), which slowly may convert into Ca(Pal)2·Ca(OH)(Pal) has a solubility minimum at physiological ionic strength with Ksp(')=(1.4 ± 0.4)10(-16)M(3). X-ray diffraction showed a d-spacing of 44.15Å (different from 45.2Å reported for Ca(Pal)2), and infrared spectroscopy confirmed the presence of a hydroxy group.

Phenolic antioxidant scavenging of myosin radicals generated by hypervalent myoglobin

The scavenging activity of extracts of green tea (GTE), white grape (WGE), and rosemary (RE), all plant material with high phenolic content, and of the phenolic compounds 4-methylcatechol (4-MC), (+)-catechin, and carnosic acid toward long-lived myosin radicals generated by reaction with H2O2-activated myoglobin at room temperature (pH 7.5, I=1.0) was investigated by freeze-quench ESR spectroscopy. Myosin radicals were generated by incubating 16 μM myosin, 800 μM metmyoglobin, and 800 μM H2O2 for 10 min, and the phenolic extracts were subsequently added (1% (w/w) phenolic compounds relative to myosin). GTE was able to scavenge myosin radicals and reduce the radical intensity by 65%. Furthermore, a low concentration of 4-MC (33 μM) was found to increase the radical concentration when added to the myosin radicals, whereas a higher concentration of 4-MC and catechin (330 μM) was found to scavenge myosin radicals and reduce the overall radical concentration by ∼65%.

Antioxidants and physical integrity of lipid bilayers under oxidative stress

Giant unilamellar vesicles (GUVs of diameter 5-25 μm) of soy phosphatidylcholine (PC), resistant to intense light exposure (400-440 nm, ~15 mW·mm(-2)), underwent budding when containing chlorophyll a (Chla) in the lipid bilayer ([PC]:[Chla] = 1500:1). On the basis of image heterogeneity analysis using inverted microscopy, a dimensionless entropy parameter for the budding process was shown to increase linearly during an initial budding process. Lipophilic β-carotene (β-Car, [PC]:[β-Car] = 500:1) reduced the initial budding rate by a factor of 2.4, while the hydrophilic glycoside rutin ([PC]:[rutin] = 500:1) had no effect. Chla photosensitized oxidation of PC to form linoleoyl hydroperoxides, further leading to domains of higher polarity in the vesicles, is suggested to trigger budding. The average dipole moment (μ) of linoleic acid hydroperoxides was calculated using density functional theory (DFT) to have the value of 2.84 D, while unoxidized linoleic acid has μ = 1.86 D. β-Carotene as a lipophilic antioxidant and singlet-oxygen quencher seems to hamper oxidation in the lipid bilayers and delay budding in contrast to rutin located in the aqueous phase. The effect on budding of GUVs as a detrimental process for membranes is suggested for use in assays for evaluation of potential protectors of cellular integrity and functions under oxidative stress.

Oxidative stability of chilled pork chops following long term freeze storage

Colour stability and development of lipid oxidation were followed during chill storage for 6 days of chops from M. Longissimus dorsi produced from pigs with high (6.3) and low (5.5) ultimate pH (pH(u)). The chops from the same individual pigs were either chill stored at 2 days post-mortem or after frozen storage for 30 months (pre-frozen). Initial redness, measured as tristimulus parameter a(*), was lower for pre-frozen chops than for fresh chops. Chops with the high pH(u) had a stable a(*)-value during chill storage, while chops with the low pH(u) showed a rapidly decreasing a(*)-value both for fresh and pre-frozen chops. In contrast, initial lipid oxidation, measured as TBARS, was similar for pre-frozen and fresh chops prior to chill storage for both the high and the low pH(u) meat but developed most significantly in pre-frozen, low pH(u) meat. Individual differences in colour stability and development of lipid oxidation between pigs were notable for pre-frozen low pH(u) meat and need to be considered in quality control since meat from single pigs otherwise might give problems.

Protein and lipid oxidation in Parma ham during production

Lipid oxidation adds to the characteristic flavor of dry-cured products, but the role of protein oxidation in the production of such meats is unknown. Lipid and protein oxidations in biceps femoris (BF) and semimembranosus (SM) muscles were evaluated throughout the production period of Parma ham (0-13 months). Lipid oxidation, determined as primary and secondary lipid oxidation products (POV and TBARS), and protein thiol loss commenced immediately without any lag phase and preceded the initiation of protein radicals and protein carbonyls, which increased after a lag phase of 3 months. TBARS reached a maximum value of 2.5 mg MDA/kg dry matter in SM after 1 month and leveled off at 1 mg MDA/kg dry matter in both SM and BF between 6 and 12 months. Loss in protein thiols proceeded similarly for SM and BF from initial ∼50 to ∼27 nmol/mg protein after 6 months and stayed constant. Gel electrophoresis showed that myosin was lost primarily by proteolytic degradation and not by polymerization through protein disulfides. Overall, oxidation accelerated during the first stages of production but stabilized toward the final stages of maturation.

Flavonoid deactivation of excited state flavins: reaction monitoring by mass spectrometry

Flavin mononucleotide (FMN, as a B(2) vitamin model) was shown to induce dimerization of flavonoids (flavanone, apigenin, naringenin, eriodictyol, taxifolin, catechin, kaempferol, luteolin, quercetin, rutin, and seven smaller model phenols studied) as the major photoreaction, when aqueous solutions were exposed to visible light using a new, real-time electrospray ionization mass-spectrometric (ESI-MS) technique supported by LC-MS and MS(2) analysis. Electrophilic intermediates such as transient radical cations, o-quinones, and p-quinone methide were proposed to be involved in the coupling process. The C(3)-OH in flavon-3-ols gave rise to atypical compounds such as a depside or a dioxane-linked dimer. Flavonoid dimers, formed in vegetal extracts added to food during storage in light and for which structures are proprosed based on MS and MS(2), may affect colloidal stability, color, astringency, and antioxidative capacity.

Green tea extract as food antioxidant. Synergism and antagonism with α-tocopherol in vegetable oils and their colloidal systems

The antioxidant effects of α-tocopherol (TOH) in combination with green tea extract (GTE), the green tea polyphenol (-)-epicatechin (EC) or the isomeric (+)-catechin (C), were investigated using different lipid systems based on high linoleic sunflower oil: bulk oil, o/w-emulsion and a phosphatidylcholine-based liposome system. Both polyphenols as well as TOH were efficient antioxidants in all systems when used alone, as detected by the formation of free radicals and conjugated dienes and by oxygen consumption. Strong synergistic effect was found for the combination of TOH and GTE in a methyl linoleate o/w-emulsion and in the pure bulk oil, while only an additive effect was observed in a liposome system. The synergism was already evident for the tendency for radical formation in the bulk oil as detected by electron spin resonance (ESR) spectroscopy. On the contrary, combinations of TOH with either EC or C showed clear synergistic effects in both heterogeneous systems, but antagonistic or additive effects in bulk oil. GTE may accordingly be used to protect both vegetable oils and their emulsions against oxidation through enhancement of the activity of their endogenous antioxidants, while GTE is less efficient in the protection of phospholipids as in liposomes.

Hydroxyl radical reaction with trans-resveratrol: initial carbon radical adduct formation followed by rearrangement to phenoxyl radical

In the reaction between trans-resveratrol (resveratrol) and the hydroxyl radical, kinetic product control leads to a short-lived hydroxyl radical adduct with an absorption maximum at 420 nm and a lifetime of 0.21 ± 0.01 μs (anaerobic acetonitrile at 25 °C) as shown by laser flash photolysis using N-hydroxypyridine-2(1H)-thione (N-HPT) as a "photo-Fenton" reagent. The transient spectra of the radical adduct are in agreement with density functional theory (DFT) calculations showing an absorption maximum at 442 or 422 nm for C2 and C6 hydroxyl adducts, respectively, and showing the lowest energy for the transition state leading to the C2 adduct compared to other radical products. From this initial product, the relative long-lived 4'-phenoxyl radical of resveratrol (τ = 9.9 ± 0.9 μs) with an absorption maximum at 390 nm is formed in a process with a time constant (τ = 0.21 ± 0.01 μs) similar to the decay constant for the C2 hydroxyl adduct (or a C2/C6 hydroxyl adduct mixture) and in agreement with thermodynamics identifying this product as the most stable resveratrol radical. The hydroxyl radical adduct to phenoxyl radical conversion with concomitant water dissociation has a rate constant of 5 × 10(6) s(-1) and may occur by intramolecular hydrogen atom transfer or by stepwise proton-assisted electron transfer. Photolysis of N-HPT also leads to a thiyl radical which adds to resveratrol in a parallel reaction forming a sulfur radical adduct with a lifetime of 0.28 ± 0.04 μs and an absorption maximum at 483 nm.

Carotenoids in antioxidant networks. Colorants or radical scavengers

Optical and electronic properties of carotenoids as also reflected in their colors have been fine-tuned through evolution, resulting in a structural diversity important for carotenoid properties as radical scavengers and as quenchers of electronically excited states. Carotenoids form antioxidant networks based on one-electron transfer with other carotenoids depending on the balance between ionization energy and electron affinity of the individual carotenoids as has been demonstrated by real-time kinetic studies and later confirmed by quantum mechanical calculations. The more hydrophilic xanthophylls serve as molecular wiring across membranes in these networks through anchoring in water/lipid interfaces resulting in synergism with more lipophilic carotenoids. Radical scavenging of such networks seems to be thermodynamically controlled according to a two-dimensional classification of potential antioxidants. Carotenoids in birds' plumage, as reflected by their color and color intensity, seem to be indicators of good antioxidant status and health of the bird, and such antioxidant networks appear to be in "equilibrium". Carotenoids are under other conditions involved in networks with other types of antioxidants as in egg yolk and in some fish. For the more hydrophilic (iso)flavonoids and their glycosides, antioxidant synergism through regeneration of the lipophilic carotenoids active as radical scavengers becomes kinetically controlled at interfaces. Carotenoids appear accordingly, and also in food, as antioxidants under two types of conditions: (i) in "equilibrium" with other antioxidants in thermodynamically controlled networks serving as color indicators of good antioxidant status and (ii) as antioxidants active through radical scavenging in networks with kinetically controlled regeneration.

Reaction dynamics of flavonoids and carotenoids as antioxidants

Flavonoids and carotenoids with rich structural diversity are ubiquitously present in the plant kingdom. Flavonoids, and especially their glycosides, are more hydrophilic than most carotenoids. The interaction of flavonoids with carotenoids occurs accordingly at water/lipid interfaces and has been found important for the functions of flavonoids as antioxidants in the water phase and especially for the function of carotenoids as antioxidants in the lipid phase. Based on real-time kinetic methods for the fast reactions between (iso)flavonoids and radicals of carotenoids, antioxidant synergism during protection of unsaturated lipids has been found to depend on: (i) the appropriate distribution of (iso)flavonoids at water/lipid interface, (ii) the difference between the oxidation potentials of (iso)flavonoid and carotenoid and, (iii) the presence of electron-withdrawing groups in the carotenoid for facile electron transfer. For some (unfavorable) combinations of (iso)flavonoids and carotenoids, antioxidant synergism is replaced by antagonism, despite large potential differences. For contact with the lipid phase, the lipid/water partition coefficient is of importance as a macroscopic property for the flavonoids, while intramolecular rotation towards coplanarity upon oxidation by the carotenoid radical cation has been identified by quantum mechanical calculations to be an important microscopic property. For carotenoids, anchoring in water/lipid interface by hydrophilic groups allow the carotenoids to serve as molecular wiring across membranes for electron transport.