Curcumin-derived transients: a pulsed laser and pulse radiolysis study

Successful Application of TD-DFT in Transient Absorption Spectra Assignment

[Reaction: see text] Despite the wide application of time-resolved spectra technology in investigating various chemical and biological processes, it is still a great challenge to give an unambiguous spectra assignment for observed transient species, e.g., the curcumin-derived radicals. Here we report that time-dependent density functional theory with the B3LYP functional and an appropriate basis set can give reliable absorption spectra for carbon- and oxygen-centered radicals derived from curcumin, which indicates the potential of theoretical methods in helping assign transient spectra.

Reactions of superoxide radicals with curcumin: probable mechanisms by optical spectroscopy and EPR

Reactions of superoxide-crown ether complex with curcumin have been studied in acetonitrile. Optical absorption spectra showed that curcumin on reaction with superoxide forms a blue color intermediate absorbing at 560 nm, which subsequently decayed in a few hours with the development of the absorption band corresponding to the parent curcumin. The regeneration was 100% at low superoxide concentrations (1:1, or 1:2 or 1:3 of curcumin:superoxide) but reduced to 60% at high superoxide concentration (>1:5). The regeneration of curcumin is confirmed by HPLC analysis. Stopped-flow studies in acetonitrile following either the decay of parent curcumin at 420 nm or formation of 560 nm absorption have been used to determine the rate constant for the reaction of superoxide with curcumin. EPR studies confirmed the disappearance of characteristic superoxide signal in presence of curcumin with the formation of new featureless signal with g = 2.0067. Based on these studies it is concluded that at low superoxide concentrations curcumin effectively causes superoxide dismutation without itself undergoing any chemical change. At higher concentrations of superoxide, curcumin inhibits superoxide activity by reacting with it.

Abnormal Solvent Effects on Hydrogen Atom Abstraction. 2. Resolution of the Curcumin Antioxidant Controversy. The Role of Sequential Proton Loss Electron Transfer

The rates of reaction of 1,1-diphenyl-2-picrylhydrazyl (dpph*) radicals with curcumin (CU, 1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione), dehydrozingerone (DHZ, "half-curcumin"), and isoeugenol (IE) have been measured in methanol and ethanol and in two non-hydroxylic solvents, dioxane and ethyl acetate, which have about the same hydrogen-bond-accepting abilities as the alcohols. The reactions of all three substrates are orders of magnitude faster in the alcohols, but these high rates can be suppressed to values essentially equal to those in the two non-hydroxylic solvents by the addition of acetic acid. The fast reactions in alcohols are attributed to the reaction of dpph* with the CU, DHZ, and IE anions (see J. Org. Chem. 2003, 68, 3433), a process which we herein name sequential proton loss electron transfer (SPLET). The most acidic group in CU is the central keto-enol moiety. Following CU's ionization to a monoanion, ET from the [-(O)CCHC(O)-](-) moiety to dpph* yields the neutral [-(O)CCHC(O)-]* radical moiety which will be strongly electron withdrawing. Consequently, a phenolic proton is quickly lost into the alcohol solvent. The phenoxide anion so formed undergoes charge migration to produce a neutral phenoxyl radical and the keto-enol anion, i.e., the same product as would be formed by a hydrogen atom transfer (HAT) from the phenolic group of the CU monoanion. The SPLET process cannot occur in a nonionizing solvent. The controversy as to whether the central keto-enol moiety or the peripheral phenolic hydroxyl groups of CU are involved in its radical trapping (antioxidant) activity is therefore resolved. In ionizing solvents, electron-deficient radicals will react with CU by a rapid SPLET process but in nonionizing solvents, or in the presence of acid, they will react by a slower HAT process involving one of the phenolic hydroxyl groups.

Role of phenolic O-H and methylene hydrogen on the free radical reactions and antioxidant activity of curcumin

To understand the relative importance of phenolic O-H and the CH-H hydrogen on the antioxidant activity and the free radical reactions of Curcumin, (1,7-bis[4-hydroxy-3-methoxyphenyl]-1,6-heptadiene-3,5-dione), biochemical, physicochemical, and density functional theory (DFT) studies were carried out with curcumin and dimethoxy curcumin (1,7-bis[3, 4-dimethoxy phenyl]-1,6-heptadiene-3,5-dione). The antioxidant activity of these compounds was tested by following radiation-induced lipid peroxidation in rat liver microsomes, and the results suggested that at equal concentration, the efficiency to inhibit lipid peroxidation is changed from 82% with curcumin to 24% with dimethoxy curcumin. Kinetics of reaction of (2,2'-diphenyl-1-picrylhydrazyl) DPPH, a stable hydrogen abstracting free radical was tested with these two compounds using stopped-flow spectrometer and steady state spectrophotometer. The bimolecular rate constant for curcumin was found to be approximately 1800 times greater than that for the dimethoxy derivative. Cyclic voltammetry studies of these two systems indicated two closely lying oxidation peaks at 0.84 and 1.0 V vs. SCE for curcumin, while only one peak at 1.0 V vs. SCE was observed for dimethoxy curcumin. Pulse radiolysis induced one-electron oxidation of curcumin and dimethoxy curcumin was studied at neutral pH using (*)N(3) radicals. This reaction with curcumin produced phenoxyl radicals absorbing at 500 nm, while in the case of dimethoxy curcumin a very weak signal in the UV region was observed. These results suggest that, although the energetics to remove hydrogen from both phenolic OH and the CH(2) group of the beta-diketo structure are very close, the phenolic OH is essential for both antioxidant activity and free radical kinetics. This is further confirmed by DFT calculations where it is shown that the -OH hydrogen is more labile for abstraction compared to the -CH(2) hydrogen in curcumin. Based on various experimental and theoretical results it is definitely concluded that the phenolic OH plays a major role in the activity of curcumin.

Photophysical studies on binding of curcumin to bovine serum albumins

The excited-state photophysical properties of curcumin in the presence of bovine serum albumin (BSA) have been studied. The absorption and fluorescence changes in curcumin on binding to BSA have been followed at varying concentrations of either curcumin or BSA to determine the binding constant, which has been found to be approximately 10(4) to 10(5) M(-1). Stopped-flow kinetics studies suggested at least two distinct kinetic steps for the binding of curcumin to BSA. The photophysical properties of the singlet-excited state of the curcumin-BSA complex have also been studied. Whereas the absorption spectrum of curcumin is redshifted, the fluorescence spectrum of curcumin was blueshifted in the presence of BSA. The fluorescence quantum yield of curcumin on complexing with BSA was approximately 0.05. Steady-state fluorescence anisotropy studies showed a significant increase in the anisotropy value of 0.37 in BSA-bound curcumin. The fluorescence decay of the curcumin-BSA complex followed a biexponential decay with fluorescence lifetimes of 413 ps (33%) and 120 ps (67%). On the basis of these complementary results, it has been concluded that curcumin shows very high binding to BSA, probably at the hydrophobic cavities inside the protein.

Photophysical studies on binding of curcumin to bovine serum albumins

The excited-state photophysical properties of curcumin in the presence of bovine serum albumin (BSA) have been studied. The absorption and fluorescence changes in curcumin on binding to BSA have been followed at varying concentrations of either curcumin or BSA to determine the binding constant, which has been found to be approximately 10(4) to 10(5) M(-1). Stopped-flow kinetics studies suggested at least two distinct kinetic steps for the binding of curcumin to BSA. The photophysical properties of the singlet-excited state of the curcumin-BSA complex have also been studied. Whereas the absorption spectrum of curcumin is redshifted, the fluorescence spectrum of curcumin was blueshifted in the presence of BSA. The fluorescence quantum yield of curcumin on complexing with BSA was approximately 0.05. Steady-state fluorescence anisotropy studies showed a significant increase in the anisotropy value of 0.37 in BSA-bound curcumin. The fluorescence decay of the curcumin-BSA complex followed a biexponential decay with fluorescence lifetimes of 413 ps (33%) and 120 ps (67%). On the basis of these complementary results, it has been concluded that curcumin shows very high binding to BSA, probably at the hydrophobic cavities inside the protein.

Curcumin (diferuloylmethane), a singlet oxygen ((1)O(2)) quencher

Curcumin (diferuloylmethane) is a major component of food flavoring turmeric (Curcuma longa), and has been reported to be anticarcinogenic and anti-inflammatory. Although curcumin was shown to have antioxidant properties, its exact antioxidant nature has not been fully investigated. In this report we have investigated the possible antioxidant properties of curcumin using EPR spectroscopic techniques. Curcumin was found to inhibit the (1)O(2)-dependent 2,2,6,6-tetramethylpiperidine N-oxyl (TEMPO) formation in a dose-dependent manner. (1)O(2) was produced in a photosensitizing system using rose bengal as sensitizer, and was detected as TEMP-(1)O(2) adducts by electron paramagnetic resonance (EPR) spectroscopic techniques using TEMP as a spin-trap. Curcumin at 2.75 microM caused 50% inhibition of TEMP-(1)O(2) adduct formation. However, curcumin only marginally inhibited (24% maximum at 80 microM) reduction of ferricytochrome c in a xanthine-xanthine oxidase system demonstrating that it is not an effective superoxide radical scavenger. Additionally, there was minor inhibition of DMPO-OH adduct formation by curcumin (solubilized in ethanol) when an ethanol control was included in the EPR spin-trapping study, suggesting that curcumin may not be an effective hydroxyl radical scavenger. Together these data demonstrate that curcumin is able only to effectively quench singlet oxygen at very low concentration in aqueous systems.

Structural identification of new curcumin dimers and their contribution to the antioxidant mechanism of curcumin

As a part of the research project on the elucidation of the chain-breaking antioxidant mechanism of natural phenolics against the oxidation of food components, curcumin, a main turmeric pigment, was investigated. A relatively high concentration of curcumin gave three dimers as radical termination products in addition to the coupling products with curcumin and the lipid hydroperoxide. The structural analysis of these dimers and quantitative analysis of their production rates revealed that radical-radical termination mainly occurred at the 2-position of curcumin. The contribution of the pathway for production of these dimers to the antioxidant mechanism of curcumin was estimated from the concentration-dependent data of the antioxidant activity and formation rates of these termination products. The A-A termination (dimer formation) was estimated to contribute at least about 40% of the entire antioxidant process against ethyl linoleate oxidation.

A laser flash photolysis study of curcumin in dioxane-water mixtures

Curcumin [bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione] was studied by means of UV-VIS absorption spectroscopy and nanosecond laser flash photolysis in 1,4-dioxane-water mixtures in a series of dioxane-water volume ratios. The transient characteristics were found to be dependent on the amount of water. In pure dioxane the triplet state of the molecule in its enolic form was detected (lambda(max) = 720 nm, tau = 3.2 micros), whereas upon water addition, the diketo form was found to prevail, because of the perturbation of intramolecular H-bonded structure. This led to hydrogen abstraction from dioxane by curcumin triplet state and the formation of the corresponding ketyl radical (lambda(max) = 490 nm, tau approximately 10 micros). Laser flash photolysis measurements, carried out in solvents of different polarity and proticity (benzene, cyclohexane and various alcohols), allowed the transient assignments to be confirmed, supporting our interpretation.

Protection of radiation-induced protein damage by curcumin

Free radical reactions of lysozyme (Lz), tryptophan and disulfides were studied with curcumin, a lipid-soluble antioxidant from turmeric, in aqueous solution using a pulse radiolysis technique. The binding of curcumin with lysozyme was confirmed using absorption, fluorescence and stopped-flow techniques. The free radicals of curcumin generated after repairing radicals of disulfides, lysozyme and tryptophan absorb at 500-510 nm. Implication of this in evaluating the antioxidant behavior of curcumin in protecting proteins is discussed.

How curcumin works preferentially with water soluble antioxidants

In this study we investigated physicochemical characteristics of the curcumin radical by pulse radiolysis and laser flash photolysis. Two methylated curcumin derivatives, methylcurcumin and trimethylcurcumin, were synthesized to explore the role of phenol hydroxy and beta-diketone moieties in the free radical chemistry of curcumin. Our results show that the initially generated beta-oxo-alkyl transforms rapidly, probably via an intramolecular H-atom shift, into the phenoxyl-type curcumin radical. This phenoxyl does not react with oxygen, k < 10(5) M(-1) s(-1), and can be repaired by any water-soluble antioxidant with appropriate redox potential, E(6) < 0.83 V, for example, with vitamin C, k = (6 +/- 1) x 10(6) M(-1) s(-1). A molecular mechanism of cancer chemoprevention by curcumin is proposed, with special emphasis on the synergism with water-soluble antioxidants.

Effect of solvent on the excited-state photophysical properties of curcumin

Photophysical properties of curcumin, 1,7-bis-(4-hydroxy-3-methoxy phenyl)-1,6-heptadiene-2,5-dione, a pigment found in the rhizomes of Curcuma longa (turmeric) have been studied in different kinds of organic solvent and also in Triton X-100 aqueous micellar media using time-resolved fluorescence and transient absorption techniques having pico and nanosecond time resolution, in addition to steady-state absorption and fluorescence spectroscopic techniques. Steady-state absorption and fluorescence characteristics of curcumin have been found to be sensitive to the solvent characteristics. Large change (delta mu = 6.1 Debye) in dipole moments due to photoexcitation to the excited singlet state (S1) indicates strong intramolecular charge transfer character of the latter. Curcumin is a weakly fluorescent molecule and the fluorescence decay properties in most of the solvents could be fitted well to a double-exponential decay function. The shorter component having lifetime in the range 50-350 ps and percent contribution of amplitude more than 90% in different solvents may be assigned to the enol form, whereas the longer component, having lifetime in the range 500-1180 ps with less than 10% contribution may be assigned to the di-keto form of curcumin. Our nuclear magnetic resonance study in CDCl3 and dimethyl sulfoxide-D6 also supports the fact that the enol form is present in the solution by more than about 95% in these solvents. Excited singlet (S1) and triplet (T1) absorption spectrum and decay kinetics have been characterized by pico and nanosecond laser flash photolysis. Quantum yield of the triplet is low (phi T < or = 0.12). Both the fluorescence and triplet quantum yields being low (phi f + phi T < 0.18), the photophysics of curcumin is dominated by the energy relaxation mechanism via the internal conversion process.

Effect of solvent on the excited-state photophysical properties of curcumin

Photophysical properties of curcumin, 1,7-bis-(4-hydroxy-3-methoxy phenyl)-1,6-heptadiene-2,5-dione, a pigment found in the rhizomes of Curcuma longa (turmeric) have been studied in different kinds of organic solvent and also in Triton X-100 aqueous micellar media using time-resolved fluorescence and transient absorption techniques having pico and nanosecond time resolution, in addition to steady-state absorption and fluorescence spectroscopic techniques. Steady-state absorption and fluorescence characteristics of curcumin have been found to be sensitive to the solvent characteristics. Large change (delta mu = 6.1 Debye) in dipole moments due to photoexcitation to the excited singlet state (S1) indicates strong intramolecular charge transfer character of the latter. Curcumin is a weakly fluorescent molecule and the fluorescence decay properties in most of the solvents could be fitted well to a double-exponential decay function. The shorter component having lifetime in the range 50-350 ps and percent contribution of amplitude more than 90% in different solvents may be assigned to the enol form, whereas the longer component, having lifetime in the range 500-1180 ps with less than 10% contribution may be assigned to the di-keto form of curcumin. Our nuclear magnetic resonance study in CDCl3 and dimethyl sulfoxide-D6 also supports the fact that the enol form is present in the solution by more than about 95% in these solvents. Excited singlet (S1) and triplet (T1) absorption spectrum and decay kinetics have been characterized by pico and nanosecond laser flash photolysis. Quantum yield of the triplet is low (phi T < or = 0.12). Both the fluorescence and triplet quantum yields being low (phi f + phi T < 0.18), the photophysics of curcumin is dominated by the energy relaxation mechanism via the internal conversion process.

Free radical scavenging ability and antioxidant efficiency of curcumin and its substituted analogue

Free radical reactions of curcumin and its ethoxy substituted derivative (C1) 1,7-bis-(4-hydroxy-3-ethoxy phenyl)-1,6-heptadiene-3,5-dione have been studied using a pulse radiolysis technique in homogeneous aqueous-organic solutions like acetonitrile-water and isopropanol-water mixtures, as well as in neutral TX-100 and cationic CTAB micellar solutions. The phenoxyl radicals of curcumin or C1 were generated by one-electron transfer to several oxidants like N(3)(.), Br(2)(-.), CCl(3)O(2)(.), glutathione radicals which exhibit absorption from a 300-600-nm wavelength region with the maximum at 490-500 nm. Other important properties of the phenoxyl radicals such as extinction coefficient, radical lifetime and their formation and decay rate constants were also determined in these systems. The antioxidant property of curcumin and C1 were estimated in terms of their ability to inhibit the lipid peroxidation in liposomes and also in terms of trolox equivalent antioxidant capacity (TEAC). The results were compared with alpha-tocopherol.

Free radical reactions of curcumin in membrane models

Free radical reactions of curumin, a lipid soluble antioxidant from turmeric (Curcuma longa), have been studied with a variety of oxidants using TX 100 micelle as a model membrane. The phenoxyl radicals of curcumin generated by one electron oxidizing azide radicals in acetonitrile-water mixture and TX 100 micelles show very similar spectral behavior. However, in membrane models the radical lifetimes and the molar extinction coefficients are significantly different from the homogeneous solutions. Micellized curcumin reacts with haloperoxyl radicals, superoxide, and lipid peroxyl radicals with rate constants of 5 X 10(3), 4.6 X 10(4), and 5.3 X 10(5) M-1s-1, respectively. Curcumin derived phenoxyl radicals decay by radical-radical reactions in homogeneous solutions, while in the micelles, radical decay is mostly first order when the average occupancy of the micelle is less than 1. Implications of these results in evaluating curcumin as an antioxidant is discussed.