Double-Site Binding and Anti-/Pro-oxidation of Luteolin on Bovine Serum Albumin Mediated by Copper(II) Coordination.

The interactions of luteolin (Lut) with bovine serum albumin (BSA) mediated by Cu(II) were investigated by spectroscopic, calorimetric, and molecular dynamic (MD) methods. Fluorescence studies showed that the binding of Lut to BSA was significantly enhanced by Cu(II) coordination with the number of binding sites and binding constant increasing from n = 1 and K a = 3.2 × 105 L·mol-1 for Lut to n = 2 and K a = 7.1 × 105 L·mol-1 for a 1:1 Cu(II)-luteolin complex, in agreement with the results from isothermal titration calorimetry (ITC). Site-specific experiments with warfarin and ibuprofen and MD confirmed that two binding sites of BSA were sequentially occupied by two Cu(II)-luteolin complexes. Cu(II) coordination increased the antioxidant activity of luteolin by 60% in the inhibition of carbonyl formation from the oxidation of amino groups in the side chain of BSA induced by the peroxyl radical ROO•; however, it counteracted the antioxidant effects of luteolin and played pro-oxidative roles in BSA aggregation induced by •OH.

Peroxyl radical induced membrane instability of giant unilamellar vesicles and anti-lipooxidation protection.

The present work is intended to investigate the morphological instability of lipid membrane induced by peroxyl radical (ROO•) and the underlying mechanism. To this end, the giant unilamellar vesicle (GUV) made from phosphatidylcholine was employed as a membrane model, and the azo compounds 2,2'-azobis(2,4-dimethylvaleronitrile) (AMVN) and 2,2'-azobis(2-amidinopropane) dihydrochloride (AAPH) were used as the precursors of ROO•. Upon mild pyrolysis, the GUV immobilized in agarose gel was followed by conventional optical microscopy in real time, and the morphological variation was quantified by the image heterogeneity, perimeter and area all as a function of time for up to an hour. Lipid oxidation initiated from lipid phase with AMVN and from aqueous phase with AAPH led to different types of morphological changes, i.e. membrane coarsening and vesicle deformation/budding, respectively. Based on the compositional analysis of lipid oxidation products, we propose that ROO• as the primary radical initiator is responsible for the morphological changes of the GUV-AMVN while both ROO• and RO• are responsible for the morphological changes of the GUV-AAPH system. Lipophilic ß-carotene and amphipathic plant phenols as antioxidants are found to be able to stabilize the membrane integrity effectively, in corroboration with the proposed mechanisms for membrane destruction.

Promotion effects of flavonoids on browning induced by enzymatic oxidation of tyrosinase: structure-activity relationship.

Tyrosinase, widely distributed in nature, is a copper-containing polyphenol oxidase involved in the formation of melanin. Flavonoids are most often considered as tyrosinase inhibitors but have also been confirmed to be tyrosinase substrates. Four structure-related flavonoids including flavones (apigenin and luteolin) and flavonols (kaempferol and quercetin) are found to promote not inhibit browning induced by tyrosinase catalyzed oxidation both in model systems and in mushrooms under aerobic conditions. A comparison with enzymatic oxidation and autooxidation of flavonoids alone has helped to clarify why flavonoids function as a substrate rather than an inhibitor. Flavonoids almost do not affect the kinetics of melanin formation from enzymatic oxidation of l-dopa in excess. In addition, a new brown complex formed during the reaction of flavonoid quinone and dopaquinone is suggested to enhance the browning effects by competing with isomerization and autooxidation. Structure-activity relationships of the four flavonoids in melanin formation leading to browning induced by autooxidation and enzymatic oxidation confirm the enzymatic nature of the browning.

Kinetic Studies on Radical Scavenging Activity of Kaempferol Decreased by Sn(II) Binding.

Sn(II) binds to kaempferol (HKaem, 3,4',5,7-tetrahydroxy-2-(4-hydroxyphenyl)-4H-1-benzopyran-4-one) at the 3,4-site forming [Sn(II)(Kaem)2] complex in ethanol. DPPH• scavenging efficiency of HKaem is dramatically decreased by SnCl2 coordination due to formation of acid inhibiting deprotonation of HKaem as ligands and thus reduces the radical scavenging activity of the complex via a sequential proton-loss electron transfer (SPLET) mechanism. Moderate decreases in the radical scavenging of HKaem are observed by Sn(CH3COO)2 coordination and by contact between Sn and HKaem, in agreement with the increase in the oxidation potential of the complex compared to HKaem, leading to a decrease in antioxidant efficiency for fruits and vegetables with Sn as package materials.

Synergy between plant phenols and carotenoids in stabilizing lipid-bilayer membranes of giant unilamellar vesicles against oxidative destruction.

We have investigated the synergism between plant phenols and carotenoids in protecting the phosphatidylcholine (PC) membranes of giant unilamellar vesicles (GUVs) from oxidative destruction, for which chlorophyll-a (Chl-a) was used as a lipophilic photosensitizer. The effect was examined for seven different combinations of ß-carotene (ß-CAR) and plant phenols. The light-induced change in GUV morphology was monitored via conventional optical microscopy, and quantified by a dimensionless image-entropy parameter, ΔE. The ΔE-t time evolution profiles exhibiting successive lag phase, budding phase and ending phase could be accounted for by a Boltzmann model function. The length of the lag phase (LP in s) for the combination of syringic acid and ß-CAR was more than seven fold longer than for ß-CAR alone, and those for other different combinations followed the order: salicylic acid < vanillic acid < syringic acid > rutin > caffeic acid > quercetin > catechin, indicating that moderately reducing phenols appeared to be the most efficient membrane co-stabilizers. The same order held for the residual contents of ß-CAR in membranes after light-induced oxidative degradation as determined by resonance Raman spectroscopy. The dependence of LP on the reducing power of phenols coincided with the Marcus theory plot for the rate of electron transfer from phenols to the radical cation ß-CAR˙+ as a primary oxidative product, suggesting that the plant phenol regeneration of ß-CAR plays an important role in stabilizing the GUV membranes, as further supported by the involvement of CAR˙+ and the distinct shortening of its lifetime as shown by transient absorption spectroscopy.

Alkaline earth metal ion coordination increases the radical scavenging efficiency of kaempferol.

Flavonoids are used as natural additives and antioxidants in foods, and after coordination to metal ions, as drug candidates, depending on the flavonoid structure. The rate of radical scavenging of the ubiquitous plant flavonoid kaempferol (3,5,7,4'-tetrahydroxyflavone, Kaem) was found to be significantly enhanced by coordination of Mg(ii), Ca(ii), Sr(ii), and Ba(ii) ions, whereas the radical scavenging rate of apigenin (5,7,4'-trihydroxyflavone, Api) was almost unaffected by alkaline earth metal (AEM) ions, as studied for short-lived ß-carotene radical cations (ß-Car˙+) formed by laser flash photolysis in chloroform/ethanol (7 : 3) and for the semi-stable 2,2-diphenyl-1-picrylhydrazyl radical, DPPH˙, in ethanol at 25 °C. A 1 : 1 Mg(ii)-Kaem complex was found to be in equilibrium with a 1 : 2 Mg(ii)-Kaem2 complex, while for Ca(ii), Sr(ii) and Ba(ii), only 1 : 2 AEM(ii)-Kaem complexes were detected, where all complexes showed 3-hydroxyl and 4-carbonyl coordination and stability constants of higher than 109 L2 mol-2. The 1 : 2 Ca(ii)-Kaem2 complex had the highest second order rate constant for both ß-Car˙+ (5 × 108 L mol-1 s-1) and DPPH˙ radical (3 × 105 L mol-1 s-1) scavenging, which can be attributed to the optimal combination of the stronger electron withdrawing capability of the (n - 1)d orbital in the heavier AEM ions and their spatially asymmetrical structures in 1 : 2 AEM-Kaem complexes with metal ion coordination of the least steric hindrance of two perpendicular flavone backbones as ligands in the Ca(ii) complex, as shown by density functional theory calculations.

Copper(II) Coordination and Translocation in Luteolin and Effect on Radical Scavenging.

Luteolin differs as a radical scavenger dramatically from apigenin in response to Cu(II) coordination despite a minor structural difference. Coordination of Cu(II) increases the radical scavenging efficiency of luteolin, especially at low pH, while decreases the efficiency of apigenin at both low and higher pH as studied by ABTS•+ radical scavenging. Luteolin forms a 1:1 complex with Cu(II) binding to 4-carbonyl and 5-phenol for pH <6 and to 3',4'-catechol for pH >6. Apigenin forms a 1:2 complex independent of pH coordinated to 4-carbonyl and 5-hydroxylyl. Cu(II) coordinated to luteolin, as studied by pH jump stopped-flow, translocates with rate constants of 11.1 ± 0.3 s-1 from 4,5 to 3',4' sites and 1.0 ± 0.1 s-1 from 3',4' to 4,5 sites independent of Cu(II) concentration, pointing toward the dissociation of Cu(II) from an intermediate with two Cu(II) coordination as rate determining. 3',4'-Catechol is suggested to be a switch for Cu(II) translocation with deprotonation initiating 4,5 to 3',4' translocation and protonation initiating 3',4' to 4,5 translocation. For dicoordinated apigenin, the coordination symmetry balances an electron withdrawal effect of Cu(II) resulting in a decrease of phenol acidity and less radical scavenging efficiency compared to parent apigenin. Compared to that of parent luteolin, the radical scavenging rate of both 4,5 and 3',4' Cu(II)-coordinated luteolin is enhanced through increased phenol acidity by electron withdrawal by Cu(II), as confirmed by density functional theory (DFT) calculations. Coordination and translocation of Cu(II) accordingly increases the antioxidant activity of luteolin at pH approaching the physiological level and is discovered as a novel class of natural molecular machinery derived from plant polyphenols, which seems to be of importance for protection against oxidative stress.

Kaempferol Binding to Zinc(II), Efficient Radical Scavenging through Increased Phenol Acidity.

Zinc(II) enhances radical scavenging of the flavonoid kaempferol (Kaem) most significantly for the 1:1 Zn(II)-Kaem complex in equilibrium with the 1:2 Zn(II)-Kaem complex both with high affinity at 3-hydroxyl and 4-carboxyl coordination. In methanol/chloroform (7/3, v/v), 1:1 Zn(II)-Kaem complex reduces ß-carotene radical cation, ß-Car•+, with a second-order rate constant, 1.88 × 108 L·mol-1·s-1, while both Kaem and 1:2 Zn(II)-Kaem complex are nonreactive, as determined by laser flash photolysis. In ethanol, 1:1 Zn(II)-Kaem complex reduces the 2,2-diphenyl-1-picrylhydrazyl radical, DPPH•, with a second-order rate constant, 2.48 × 104 L·mol-1·s-1, 16 times and 2 times as efficient as Kaem and 1:2 Zn(II)-Kaem complex, respectively, as determined by stopped-flow spectroscopy. Density functional theory calculation results indicate significantly increased acidity of Kaem as ligand in 1:1 Zn(II)-Kaem complex other than in 1:2 Zn(II)-Kaem complex. Kaem in 1:1 Zn(II)-Kaem complex loses two protons (one from 3-hydroxyl and one from phenolic hydroxyl) forming 1:1 Zn(II)-(Kaem-2H) during binding with Zn(II), while Kaem in 1:2 Zn(II)-Kaem complex loses one proton in each ligand forming Zn(II)-(Kaem-H)2, as confirmed by UV-vis absorption spectroscopy. Zn(II)-(Kaem-2H) is a far stronger reductant than Kaem and Zn(II)-(Kaem-H)2 as determined by cyclic voltammetry. Significant rate increases for the 1:1 complex in both ß-Car•+ scavenging by electron transfer and DPPH• scavenging by hydrogen atom transfer were ascribed to decreases of ionization potential and of bond dissociation energy of 4'-OH for deprotonated Zn(II)-(Kaem-2H), respectively. Increased phenol acidity of plant polyphenols by 1:1 coordination with Zn(II) may explain the unique function of Zn(II) as a biological antioxidant and may help to design nontoxic metal-based drugs derived from natural bioactive molecules.

Integrity of Membrane Structures in Giant Unilamellar Vesicles as Assay for Antioxidants and Prooxidants.

We have attempted to evaluate, on the basis of optical microscopy for a single giant unilamellar vesicle (GUV), the potency of antioxidants in protecting GUV membranes from oxidative destruction. Photosensitized membrane budding of GUVs prepared from soybean phosphatidylcholine with chlorophyll a (Chl a) and ß-carotene (ß-Car) as photosensitizer and protector, respectively, were followed by microscopic imaging. A dimensionless entropy parameter, ΔE, as derived from the time-resolved microscopic images, was employed to describe the evolution of morphological variation of GUVs. As an indication of membrane instability, the budding process showed three successive temporal regimes as a common feature: a lag phase prior to the initiation of budding characterized by LP (in s), a budding phase when ΔE increased with a rate of kΔE (in s-1), and an ending phase with morphology stabilized at a constant ΔEend (dimensionless). We show that the phase-associated parameters can be objectively obtained by fitting the ΔE-t kinetics curves to a Boltzmann function and that all of the parameters are rather sensitive to ß-Car concentration. As for the efficacy of these parameters in quantifying the protection potency of ß-Car, kΔE is shown to be most sensitive for ß-Car in a concentration regime of biological significance of <1 × 10-7 M, whereas LP and ΔEend are more sensitive for ß-Car concentrations exceeding 1 × 10-7 M. Furthermore, based on the results of GUV imaging and fluorescence and Raman spectroscopies, we have revealed for different phases the mechanistic interplay among 1O2* diffusion, PC-OOH accumulation, Chl a and/or ß-Car consumption, and the morphological variation. The developed assay should be valuable for characterizing the potency of antioxidants or prooxidants in the protection or destruction of the membrane integrity of GUVs.

Genistein Binding to Copper(II)-Solvent Dependence and Effects on Radical Scavenging.

Genistein, but not daidzein, binds to copper(II) with a 1:2 stoichiometry in ethanol and with a 1:1 stoichiometry in methanol, indicating chelation by the 5-phenol and the 4-keto group of the isoflavonoid as demonstrated by the Jobs method and UV-visible absorption spectroscopy. In ethanol, the stability constants had the value 1.12 × 1011 L²âˆ™mol-2 for the 1:2 complex and in methanol 6.0 × 105 L∙mol-1 for the 1:1 complex at 25 °C. Binding was not detected in water, as confirmed by an upper limit for the 1:1 stability constant of K = 5 mol-1 L as calculated from the difference in solvation free energy of copper(II) between methanol and the more polar water. Solvent molecules compete with genistein as demonstrated in methanol where binding stoichiometry changes from 1:2 to 1:1 compared to ethanol and methanol/chloroform (7/3, v/v). Genistein binding to copper(II) increases the scavenging rate of the stable, neutral 2,2-diphenyl-1-picrylhydrazyl radical by more than a factor of four, while only small effects were seen for the short-lived but more oxidizing ß-carotene radical cation using laser flash photolysis. The increased efficiency of coordinated genistein is concluded to depend on kinetic rather than on thermodynamic factors, as confirmed by the small change in reduction potential of -0.016 V detected by cyclic voltammetry upon binding of genistein to copper(II) in methanol/chloroform solutions.

Singlet Fission Reaction of Light-Exposed ß-Carotene Bound to Bovine Serum Albumin. A Novel Mechanism in Protection of Light-Exposed Tissue by Dietary Carotenoids.

We have attempted to investigate the role of carotenoids (Car) in protecting pigment-protein complexes against light-induced degradation. Upon direct photoexcitation of ß-carotene (ß-Car), nanosecond flash photolysis and femtosecond time-resolved spectroscopy detected a substantial population of triplet states for ß-Car aggregates associated with bovine serum albumin (BSA) or dispersed in aqueous phase with 10% tetrahydrofuran (THF), but none were observed for monomeric ß-Car in neat THF. The direct photogeneration of triplet states was on the time scale of <1 ps, indicating that the underlying reaction mechanism was singlet fission (SF). Efficient triplet-triplet annihilation in the time regime from picoseconds to microseconds resulted in a <1 µs triplet lifetime for ß-Car aggregates, in contrast to a 20 µs lifetime for monomeric ß-Car as determined by anthracene-sensitized flash photolysis. The short-lived triplet excitations of ß-Car aggregates associated with BSA or dispersed in aqueous phase were found to be insensitive to the presence of oxygen, which are considered to be important for the protection of both protein and carotenoid against light-induced degradation via reaction with oxidative species.

Regeneration of ß-Carotene from Radical Cation by Eugenol, Isoeugenol, and Clove Oil in the Marcus Theory Inverted Region for Electron Transfer.

The rate of regeneration of ß-carotene by eugenol from the ß-carotene radical cation, an initial bleaching product of ß-carotene, was found by laser flash photolysis and transient absorption spectroscopy to be close to the diffusion limit in chloroform/methanol (9:1, v/v), with a second-order rate constant (k2) of 4.3 × 109 L mol-1 s-1 at 23 °C. Isoeugenol, more reducing with a standard reduction potential of 100 mV lower than eugenol, was slower, with k2 = 7.2 × 108 L mol-1 s-1. Regeneration of ß-carotene following photobleaching was found 50% more efficient by eugenol, indicating that, for the more reducing isoeugenol, the driving force exceeds the reorganization energy for electron transfer significantly in the Marcus theory inverted region. For eugenol/isoeugenol mixtures and clove oil, kinetic control by the faster eugenol determines the regeneration, with a thermodynamic backup of reduction equivalent through eugenol regeneration by the more reducing isoeugenol for the mixture. Clove oil, accordingly, is a potential protector of provitamin A for use in red palm oils.

Binding to Bovine Serum Albumin Protects ß-Carotene against Oxidative Degradation.

Binding to bovine serum albumin (BSA) was found to protect ß-carotene (ß-Car) dissolved in air-saturated phosphate buffer solution/tetrahydrofuran (9:1, v/v) efficiently against photobleaching resulting from laser flash excitation at 532 nm. From dependence of the relative photobleaching yield upon the BSA concentration, an association constant of Ka = 4.67 × 10(5) L mol(-1) for ß-Car binding to BSA was determined at 25 °C. Transient absorption spectroscopy confirmed less bleaching of ß-Car on the microsecond time scale in the presence of BSA, while kinetics of triplet-state ß-Car was unaffected by the presence of oxygen. The protection of ß-Car against this type of reaction seems accordingly to depend upon dissipation of excitation energy from an excited state into the protein matrix. Static quenching of BSA fluorescence by ß-Car had a Stern-Volmer constant of Ksv = 2.67 × 10(4) L mol(-1), with ΔH = 17 kJ mol(-1) and ΔS = 142 J mol(-1) K(-1) at 25 °C. Quenching of tryptophan (Trp) fluorescence by ß-Car suggests involvement of Trp in binding of ß-Car to BSA through hydrophobic interaction, while the lower value for the Stern-Volmer constant Ksv compared to the binding constant, Ka, may indicate involvement of ß-Car aggregates. Bound ß-Car increased the random coil fraction of BSA at the expense of α-helix, as shown by circular dichroism, affecting the ß-Car configuration, as shown by Raman spectroscopy.

Astaxanthin Protecting Membrane Integrity against Photosensitized Oxidation through Synergism with Other Carotenoids.

Incorporation of astaxanthin or zeaxanthin in giant unilamellar vesicles (GUVs) of phosphatidylcholine resulted in a longer lag phase than incorporation of ß-carotene or lycopene for the onset of budding induced by chlorophyll a photosensitization and quantified by a dimensionless entropy parameter using optical microscopy and digital image heterogeneity analysis. The lowest initial rate of GUV budding after the lag phase was seen for GUVs with astaxanthin as the least reducing carotenoid, while the lowest final level of entropy appeared for those with lycopene or ß-carotene as a more reducing carotenoid. The combination of astaxanthin and lycopene gave optimal protection against budding with respect to both a longer lag phase and lower final level of entropy by combining good electron acceptance and good electron donation. Quenching of singlet oxygen by carotenoids close to chlorophyll a in the membrane interior in parallel with scavenging of superoxide radicals by astaxanthin anchored in the surface may explain the synergism between carotenoids involving both type I and type II photosensitization by chlorophyll a.

Regeneration of ß-Carotene from the Radical Cation by Tyrosine and Tryptophan.

The phenolic amino acid tyrosine (Tyr) was found more efficient in regenerating ß-carotene (ß-Car) from the radical cation (ß-Car(•+)) than tryptophan (Trp) in the presence of base for conditions where the reduction potentials for Trp and Tyr are comparable. Electron transfer from Tyr in 4:1 chloroform/methanol to ß-Car(•+) in the presence of excess base, (CH3)4N(+)OH(-), had a rate close to diffusion control and a second-order rate constant in agreement with the Marcus theory for electron transfer when compared to plant phenols. A maximum of 40% ß-Car was regenerated for ten times excess of Tyr as studied by 532 nm laser flash photolysis followed by transient absorption spectroscopy in the visible and near-infrared regions. The nonregenerated fraction of ß-Car is assigned to secondary degradation processes. For Trp, the rate constant for regeneration of ß-Car(•+) was 1 order of magnitude smaller compared to Tyr and slower than expected from Marcus theory by comparison with plant phenols.

Dinitrosyl iron complexes with cysteine. Kinetics studies of the formation and reactions of DNICs in aqueous solution.

Kinetics studies provide mechanistic insight regarding the formation of dinitrosyl iron complexes (DNICs) now viewed as playing important roles in the mammalian chemical biology of the ubiquitous bioregulator nitric oxide (NO). Reactions in deaerated aqueous solutions containing FeSO4, cysteine (CysSH), and NO demonstrate that both the rates and the outcomes are markedly pH dependent. The dinuclear DNIC Fe2(µ-CysS)2(NO)4, a Roussin's red salt ester (Cys-RSE), is formed at pH 5.0 as well as at lower concentrations of cysteine in neutral pH solutions. The mononuclear DNIC Fe(NO)2(CysS)2(-) (Cys-DNIC) is produced from the same three components at pH 10.0 and at higher cysteine concentrations at neutral pH. The kinetics studies suggest that both Cys-RSE and Cys-DNIC are formed via a common intermediate Fe(NO)(CysS)2(-). Cys-DNIC and Cys-RSE interconvert, and the rates of this process depend on the cysteine concentration and on the pH. Flash photolysis of the Cys-RSE formed from Fe(II)/NO/cysteine mixtures in anaerobic pH 5.0 solution led to reversible NO dissociation and a rapid, second-order back reaction with a rate constant kNO = 6.9 × 10(7) M(-1) s(-1). In contrast, photolysis of the mononuclear-DNIC species Cys-DNIC formed from Fe(II)/NO/cysteine mixtures in anaerobic pH 10.0 solution did not labilize NO but instead apparently led to release of the CysS(•) radical. These studies illustrate the complicated reaction dynamics interconnecting the DNIC species and offer a mechanistic model for the key steps leading to these non-heme iron nitrosyl complexes.

ß-Carotene As a Lipophilic Scavenger of Nitric Oxide.

The efficient bleaching following continuous bubbling of gaseous nitric oxide (NO(•)) to ß-carotene (ß-Car) dissolved in n-hexane under anaerobic conditions results from an initial addition of two NO(•) followed by fragmentation coupled with further NO(•) addition as shown by mass spectrometry (MS). Density functional theory (DFT) calculations demonstrated that hydrogen atom transfer (HAT) and electron transfer (ET) from ß-Car to NO(•) are strongly energetically unfavorable in contrast to radical adduct formation (RAF) followed by degradation. The results indicated the lowest energy for addition of the first NO(•) at C7 with an activation free energy of ΔG(≠) = 74.40 kJ mol(-1) and a rate constant of 0.56 s(-1), followed by trans-addition of a second NO(•) at C8 with ΔG(≠) = 55.51 kJ mol(-1). MS confirmed the formation of a dinitrosyl-ß-Car (596.6 m/z), and of a ß-Car fragment (400.4 m/z) formed by C7/C8 bond cleavage and suggested to be of importance for progression of bleaching. Up to eight reaction products with increasing mass of 28 m/z are assigned to continuous addition of NO(•) to the initially formed fragment forming nitroxides. Continuous wave photolysis of sodium nitroprusside (SNP) as a NO(•) source dissolved together with ß-Car in 4:1 (v/v) methanol:tetrahydrofuran gradually bleached ß-Car. Nanosecond laser flash photolysis at 355 nm followed by transient absorption spectroscopy showed a ß-Car derived intermediate with an absorption maximum around 420 nm in agreement with a prediction (425 nm) from time-dependent DFT (TDDFT) for the trans-C7,8 dinitrosyl adduct of ß-Car. The NO(•) adduct of ß-Car decays with a rate constant of ∼10(7) s(-1) at 25 °C.

Nutritional aspects of ß-carotene and resveratrol antioxidant synergism in giant unilamellar vesicles.

Giant unilamellar vesicles of soy phosphatidylcholine are found to undergo budding when sensitized with chlorophyll a ([phosphatidylcholine] : [chlorophyll a] = 1500 : 1) under light irradiation (400-440 nm, 16 mW mm(-2)). 'Entropy' as a dimensionless image heterogeneity measurement is found to increase linearly with time during an initial budding process. For ß-carotene addition ([phosphatidylcholine] : [ß-carotene] = 500 : 1), a lag phase of 23 s is observed, followed by a budding process at an initial rate lowered by a factor of 3.8, whereas resveratrol ([phosphatidylcholine] : [resveratrol] = 500 : 1) has little if any protective effect against budding. However, resveratrol, when combined with ß-carotene, is found to further reduce the initial budding rate by a total factor of 4.7, exhibiting synergistic antioxidation effects. It is also interesting that ß-carotene alone determines the lag phase for the initiation of budding, while resveratrol supports ß-carotene in reducing the rate of the budding process following the lag phase; however, it alone has no observable effect on the lag phase. Resveratrol is suggested to regenerate ß-carotene following its sacrificial protection of unsaturated lipids from oxidative stress, modeling the synergistic effects in cell membranes by combinations of dietary antioxidants.

Electron transfer from plant phenolates to carotenoid radical cations. Antioxidant interaction entering the Marcus theory inverted region.

ß-Carotene, lycopene, and zeaxanthin are maximally regenerated by plant phenolates from their radical cations formed during laser flash photolysis in 9:1 (v/v) chloroform/methanol for a driving force corresponding to the reorganization energy according to the Marcus theory. For ß-carotene, the reorganization energy has values of 0.41 ± 0.04 and 0.40 ± 0.04 eV for the plant phenols in the presence of 1 and 2 equiv of base, respectively, at 23 °C. For a driving force lower than the reorganization energy, regeneration of the carotenoids is less efficient as is seen for m-hydroxybenzoic acid, vanillic acid, and p-coumaric acid. For a driving force above the maximum rate as determined to have kET = 6.3 × 10(9) L·mol(-1)·s(-1) for syringic acid and ß-carotene, the reaction becomes gradually slower and regeneration less efficient as is seen for the more reducing caffeic acid, rutin, and quercetin corresponding to an inverted region for the rate of electron transfer. Lycopene and zeaxanthin show a similar behavior for the same series of plant phenols with slightly lower reorganization energy, in agreement with the lower reduction potential of their radical cations, while, for the ketocarotenoids astaxanthin and canthaxanthin, fast reactions with a solvent of radical cations inhibit regeneration from being detected. Intermediate reducing plant phenols accordingly yield maximal protection of carotenoids against photobleaching in foods and beverages.

Efficient scavenging of ß-carotene radical cations by antiinflammatory salicylates.

The radical cation generated during photobleaching of ß-carotene is scavenged efficiently by the anion of methyl salicylate from wintergreen oil in a second-order reaction approaching the diffusion limit with k2 = 3.2 × 10(9) L mol(-1) s(-1) in 9 : 1 v/v chloroform-methanol at 23 °C, less efficiently by the anion of salicylic acid with 2.2 × 10(8) L mol(-1) s(-1), but still of possible importance for light-exposed tissue. Surprisingly, acetylsalicylate, the aspirin anion, reacts with an intermediate rate in a reaction assigned to the anion of the mixed acetic-salicylic acid anhydride formed through base induced rearrangements. The relative scavenging rate of the ß-carotene radical cation by the three salicylates is supported by DFT-calculations.