Enol Research Articles

Some Curcumin Isomers and Their Enol Tautomers - A DFT Treatment

Curcumin is a well known natural product having some health benefits. In the present study, within the constraints of density functional theory (at the level of B3LYP/6-31G(d,p)), some configurational isomers of curcumin and their keto-enol tautomers have been investigated. Some quantum chemical, QSAR and spectral data of them have been obtained and discussed.

Regioselective Synthesis of 6-Chlorofulvene and 6-Aminofulvene via Keto–Enol Tautomerism

Keto–enol tautomerism is a special example of structural isomerism. In this experiment, 6-chlorofulvene is predominantly yielded during the chlorination reaction of 6′-hydroxyfulvene with POCl3 in ...

21-Epi-taichunamide D and (±)-versicaline A, three unusual alkaloids from the endophytic Aspergillus versicolor F210

Three new alkaloids, 21-epi-taichunamide D (1) and (±)-versicalin A ((±)-6), along with four known analogues (2–5) were isolated from the endophytic Aspergillus versicolor F210, which harbored in the bulbs of Lycoris radiata. Structurally, compound 1 is the third natural indole alkaloid with a rare methylsulfonyl unit, and (±)-6 existing as two pairs of keto-enol tautomers ((+)-6a/(+)-6b and (−)-6a/(−)-6b, respectively), represent the first naturally occurring alkaloids with a rare 2,3-dihydro-1H-isoindol-1-one unit fused to a tautomeric diisopentenyl cyclohexenone unit. The absolute structures of 1 and (±)-6 were characterized by extensive analyses of spectroscopic data, comparison of the electronic circular dichroic (ECD) data, and single-crystal X-ray diffraction analysis, respectively. Compound 6 exhibited moderate cytotoxicity against HL-60 cells with an IC50 value of 5.6 μM.

Curcumin in silver nanoparticles aqueous solution: Kinetics of keto-enol tautomerism and effects on AgNPs

The tautomerism of curcumin (CUR) in aqueous solution was investigated in the presence of silver nanoparticles (AgNPs) in the temperature range 294−314 K. The kinetic data indicate that the conversion of the keto-enol into the diketo form is associated to higher rate constants in comparison to those obtained in pure water. Moreover, nonlinear Arrhenius behavior was observed in the investigated temperature range. The effects of CUR on the AgNPs were determined by zeta potential and dynamic light scattering measurements and supported by spectroscopic analysis suggesting that interactions with metal surface occur preserving nanoparticles from self-aggregation in the investigated conditions. The results reported herein contribute to enhance the kinetics of CUR tautomerism in aqueous media.

Role of complexation in the photochemical reduction of chromate by acetylacetone

Organic ligands can alter the redox behavior of metal species through the generation of metal-ligand complexes. Photo-induced complexation between ligands and metals is an important, but under-appreciated, aspect of process. Acetylacetone (AA) is a good chelating agent due to keto-enol tautomerization. In the presence of AA, photoreduction of Cr(VI) is accelerated; however, it is unclear exactly how complexation is involved in UV/AA mediated Cr(VI) reduction. On the basis of spectral and kinetic analyses, this study shows that the formation of {Cr(VI)-AA}* complexes is the main mechanism of Cr(VI) reduction by UV/AA. Evidence for this includes (1) the formation rate constant of Cr(III)-AA complexes in the UV system was 2–3 orders of magnitude greater than that in the thermal system; (2) there was a linear relationship between the photons absorbed by AA and the reduction rate constants of Cr(VI); and (3) the reaction appeared initially zero-order in Cr(VI) and turned to first-order as the pool of available Cr(VI) ran out. The results presented here are not only important for the better understanding of the complexation effects in the reduction of Cr(VI), but also crucial for the possible application of the UV/AA process in many other scenarios.

Experimental and computational studies on hydroxamic acids as environmental friendly chelating corrosion inhibitors for mild steel in aqueous acidic medium

In the present study, three hydroxamic acids (HAs) namely acetohydroxamic acid (AHA), benzohydroxamic acid (BHA) and oxalohydroxamic acid (OHA) were synthesized, characterized and used as inhibitors for mild steel corrosion in1M HCl using chemical, electrochemical, surface and computational methods. Results of the studies show that the HAs act as effective corrosion inhibitors and their inhibition efficiencies follow the order: OHA (96.37%) > BHA (95.69%) > AHA (93.29%). EIS study showed that studied HAs act as interface type inhibitors. Polarization study revealed that HAs demonstrate mixed-type corrosion inhibitors characteristics and adsorb on the active sites of metallic surface. . Adsorption of HAs on metal-1 M HCl interfaces followed the Langmuir adsorption isotherm model. Surface morphological analyses of inhibited and uninhibited metallic surface were carried out using SEM-EDX and XRD methods. DFT analyses showed that studied compounds act as chelating type of ligands. Effect of Keto-enol tautomerism and different possible conformational isomers on metallic corrosion inhibition was demonstrated. The conformational isomers in which >C=O and –OH (hydroxyl) groups present in same side behave as chelating ligands and form relatively more stable complex than that of conformational isomers in which >C=O and –OH groups present in opposite side. Experimental and DFT studies complimented each other well.

Under Glass Weathering of Hemp Fibers Reinforced Polypropylene Biocomposites: Degradation Mechanisms Based on Emitted Volatile Organic Compounds

The durability of hemp fibers reinforced polypropylene biocomposites was investigated after one year under glass exposure. Volatile organic compounds emissions were assessed using a new passive sampling method. Degradation pathways were examined in order to understand the weathering mechanisms. The polymer matrix was decomposed into oxygenated products due to UV rays and high temperatures. As regards hemp fibers, different degradation steps of the carbohydrates were highlighted according to the nature of the detected furans. At a non-weathered state, dehydrations preceded the ring-opening mechanism, often catalyzed by Maillard reactions. The further cyclization induced the formation of 2-or 5-substituted furans emitted by non-weathered materials. Reactions between identified products after weathering which were not yet found in literature were proposed in this paper. They often implied a keto-enol tautomerism but also dehydrations that induced the formation of 3-and 4-substituted furanones. These differences can be explained by a primary decomposition of carbohydrates favored at a non-weathered state and a secondary one occurring at a weathered state.

Using principal component analysis for neural network high-dimensional potential energy surface.

Potential energy surfaces (PESs) play a central role in our understanding of chemical reactions. Despite the impressive development of efficient electronic structure methods and codes, such computations still remain a difficult task for the majority of relevant systems. In this context, artificial neural networks (NNs) are promising candidates to construct the PES for a wide range of systems. However, the choice of suitable molecular descriptors remains a bottleneck for these algorithms. In this work, we show that a principal component analysis (PCA) is a powerful tool to prepare an optimal set of descriptors and to build an efficient NN: this protocol leads to a substantial improvement of the NNs in learning and predicting a PES. Furthermore, the PCA provides a means to reduce the size of the input space (i.e., number of descriptors) without losing accuracy. As an example, we applied this novel approach to the computation of the high-dimensional PES describing the keto-enol tautomerism reaction occurring in the acetone molecule.

Tautomers of homophthalic anhydride in the ground and excited electronic states: analysis through energy, hardness and vibrational signatures.

The keto-enol tautomerisation in homophthalic anhydride (HA) is investigated in the ground (S0) and excited (S1) electronic states. The keto form with a dicarbonyl structure is found to be the most stable form in S0 and enol form with a monocarbonyl structure in S1 indicating an excited state intramolecular proton transfer (ESIPT) process. The computed results show consistency with the change in basis sets and methods of calculations. Apart from the two tautomers, transition states are also identified. The barrier to interconversion is found to reduce substantially in S1. Internal reaction coordinate (IRC) calculations confirm the pathway of interconversion between the two forms in S0 and S1. The observed FT-IR spectra corroborate well with our computed spectra. The appearance of two strong lines around 1800cm-1 confirms the lowest energy structure to be the keto tautomer with a dicarbonyl form in S0. Our computations corroborate well with the crystal structure data for an analogous molecule. Electron distribution in HOMO and LUMO indicate the excitation process as π → π* in nature. The qualitative chemical concepts like hardness and electrophilicity are calculated to estimate the stability of the tautomers. The energy and hardness profiles with the variation of IRC are opposite to each other, verifying the principle of maximum hardness.

Temperature-Dependent Formation of Acetophenone Oligomers Accompanied by Keto–Enol Tautomerism: Real Space Distribution

We present a mechanistic study addressing formation, adsorption configuration, and stability of acetophenone oligomers on a catalytically active Pt(111) surface by a combination of scanning tunneli...

Not only AIE: Light-sensitivity of 4-dimethylamino-2′-hydroxychalcones beneficial to highly efficient photochemical synthesis of 4′-dimethylaminoflavanones

4-dimethylamino-2′-hydroxychalcone in crystals is well known for its aggregation induced emission (AIE) in the red region of spectrum. We however observe that in liquid solutions this dye and its analogues undergo reversible wavelength-dependent light-induced cyclization to the flavanone derivatives. Special care thus should be taken when this compound is used for optoelectronic applications requiring high purity, especially via solution-processed methods. The discussed intramolecular cyclization proceeds faster in nonpolar medium and is quenched completely in the presence of protic solvents or via formation of aggregates in crystal phase. In spite of that quantum yield of a single photoinduced transformation does not exceed 1%, continuous irradiation affords 92% of product which makes it the most efficient preparation route for 4′-dimethylminoflavanone and its derivatives among the ones reported before. According to the DFT and TDDFT calculations supported by the experimentally investigated spectral features, the mechanism of photoinduced formation of a 4′-dimethylaminoflavanone involves several transformations: excited state intramolecular proton transfer (ESIPT), excited and ground state rotational isomerization from s-trans to s-cis isomer, cyclization and enol-keto tautomerization via proton transfer in the ground state.

Heat capacity and standard thermodynamic functions of the fullerenol C60(OH)24

In the present investigation, the heat capacity of fullerenol C60(OH)24 was measured by precision adiabatic and differential scanning calorimetry over the temperature range from T = (6 to 670) K. The keto-enol tautomerism was revealed in the temperature range T = (355–575) K and enthalpy of transition was determined. The standard thermodynamic functions: molar heat capacity Cp°, enthalpy [H°(T) − H°(0)], entropy S°(T), and Gibbs energy [G°(T) − H°(0)] of the fullerenol were determined over the temperature range from T = (6 to 350) K from the obtained experimental results. Also, DFT calculations of the heat capacities were performed. The standard thermodynamic properties of the investigated fullerenol were compared to other fullerene derivatives.

In Situ FTIR Spectroscopic Monitoring of the Formation of the Arene Diazonium Salts and its Applications to the Heck-Matsuda Reaction.

This study depicts the use of a fiber-optic coupled Fourier transform infrared spectroscopy-attenuated total reflection (FTIR-ATR) probe for the in-depth study of arene diazonium salt formation and their utilization in the Heck–Matsuda reaction. The combination of these chemical reactions and in situ IR spectroscopy enabled us to recognize the optimum parameters for arene diazonium salt formation and to track the concentrations of reactants, products and intermediates under actual reaction conditions without time consuming HPLC analysis and the necessity of collecting the sample amid the reaction. Overall advantages of the proposed methodology include precise reaction times as well as identification of keto enol tautomerization in allylic alcohols supporting the ‘path a’ elimination mechanism in the Heck–Matsuda reaction.

A near-infrared fluorescent probe for observing thionitrous acid-mediated hydrogen polysulfides formation and fluctuation in cells and in vivo under hypoxia stress

Hydrogen polysulfides (H2Sn, n>1) as important intracellular reactive sulfur species (RSS) are believe to be responsible for cellular redox regulation. Lots of researches about H2Sn focusing on their formation, detection and bio-function in signalling regulation are spring up but with poor understanding, especially for biosynthesis and bio-function remain complicated and confusing. Recent studies reveal that thionitrous acid (HSNO) as potential intermediate linked signalling molecules of nitrogenous and sulphureous during biotic redox regulation. However, there are limited evidences for supporting the interrelation and bioeffect between HSNO and H2Sn. Herein, we have successfully designed a near-infrared (NIR) fluorescent probe ((2-fluoro-5-nitrobenzoyl)oxy)-Benzo[e]cyanine (BCy-FN) for detection H2Sn and for the first time observing HSNO-mediated H2Sn generation in cells and in vivo. The probe is harvested from fluorophore BCy-Keto and 2-fluoro-5-nitrobenzoic acid in one step, featuring mitochondria localization. The unique Enol-Keto tautomerization of fluorophore enables the probe becomes more sensitive and has powerful application. Hypoxia model has been constructed and powerfully interpreted the pretreatment of HSNO for zebrafish hypoxia process effectively improves H2Sn levels and defends the hypoxia induced brain damage. We believe the present studies will help environmentalist and biologist for better understanding of biosynthesis and bio-function in HSNO-mediated H2Sn formation process under hypoxia stress.

Low-Temperature Luminescent Studies of Emissive Guanine Substitute for the Detection of Biopolymers

The optical absorption at 300 K and the fluorescence and phosphorescence at 78 K of the emissive guanine substitute, deoxythienoguanosine, (dthG) were investigated in aqueous and TRIS-HCl-buffer solutions. Two optical absorption and fluorescence centers at room temperature were attributed to two keto-enol tautomers of dthG, which confirms previously obtained results. In contrast to room temperature, only one emission band was observed at 78 K in fluorescence spectra that was close to the long-wave fluorescence band at room temperature and could be associated with the tautomer with long-wave absorption. This phenomenon can be explained by the energy transfer by excitations in a frozen solution between two types of the optical centers mentioned above. The similar conclusion is drawn for the phosphorescence: only one tautomer phosphorescence band is observed. The spectral positions of this band maximum are essentially different for aqueous and buffer solutions (∼50 nm).

INTERACTION OF α-CARBANIONS OF LITHIUM ACYLATES WITH 1,2-DIBROMOALKANES

We have studied the interaction of α-carbanions of lithium acylates with 1,2-dibromoalkanes with different length of the carbon chain in order to establish the direction of the reaction. Interaction of α-carbanions of lithium acylates obtained by metallation of acetic, butyric and isobutyric acids with lithium diisopropylamide and 1,2-dibromoalkanes (1,2-dibromohexane, 1,2-dibromoheptane or 1,2-dibromononane) with a molar ratio of reactants equal to 1: 2: 1 in argon atmosphere in tetrahydrofuran under normal conditions (20-25 °C) for 2 h leads to the products of the oxidative coupling of enolate anions: succinic acid, 2,3-diethylsuccinic acid (mixture of diastereomers of meso- and (±) -form in a ratio of ~ 2: 1) and 2,2,3,3-tetramethylsuccinic acid with 10-70% yields. We didn’t observe any products of sequential nucleophilic substitution in reaction. The yields of dicarboxylic acids depend on the structure of the initial α-carbanion of lithium acylate and on the carbon chain length of 1,2-dibromoalkane involved in the reaction. We established that the oxidative coupling proceeds most effectively in the interaction of 1,2-dibromoalkanes with an α-carbanion containing an anionoid center at the secondary α-carbon atom - α-carbanion of lithium butyrate (2,3-diethylsuccinic acid yields are 31-70%). We suggested the most probable anion-radical scheme of the formation of the products of the oxidative coupling of α-carbanions of lithium acylates through electron transfer from the enolate anion to 1,2-dibromoalkane with the generation of its radical anion. The pathways for the formation of primary bromoalkanes (1-bromohexane, 1-bromoheptane, 1-bromononane) and terminal olefins (hexene-1, heptene-1, nonene-1) that are generated from the corresponding 1,2-dibromoalkanes during the elimination of their radical anions with the loss of the bromine anion and the subsequent stabilization of 1-bromoalkyl radicals by abstraction of the hydrogen atom from the solvent, or by bromine elimination, are shown.

Advanced oxidation process of coumarins by hydroxyl radical: Towards the new mechanism leading to less toxic products

The excessive use of coumarins and their derivatives is becoming an ecological concern due to their toxicity towards different organisms. One of the possible ways of their removal is the advanced oxidation processes. The oxidative breakdown of 4-hydroxycoumarin and its two derivatives, induced by a very powerful oxidizer hydroxyl radical (HO•), was investigated experimentally and theoretically. The new mechanism, namely radical adduct formation followed by hydrogen atom abstraction (RAF-HAA), was proposed. The thermodynamic parameters and rate constants, calculated by the Transition State Theory for several active positions indicated a possible reaction in which less toxic products were obtained, as confirmed by the ecotoxicity assessment. The mechanism included the reaction with two HO• and the introduction of an additional OH group to the structure. These results were compared to the more common mechanism that includes HAA between the OH group of 4-hydroxycoumarin and HO•. Based on the Quantum Theory of Atoms in Molecules and Natural Bond Orbital theory the exo-coupled electron transfer (PCET) mechanism was pointed out as a dominant pathway. Radical addiction with another HO•, followed by keto-enol tautomerism, led to the formation of the stable final product, identical to one obtained in the RAF-HAA mechanism. The reaction rates for RAF-HAA were higher then those for pure HAA and reactions were more spontaneous, therefore leading to the conclusion that the newly proposed mechanism could be a dominant pathway for the aromatic molecules’ breakdown in the advanced oxidation processes.

The mechanism of proton transfer and tautomerism in barbituric acid: A theoretical study

Intermolecular proton transfer in barbituric acid (BA) both in the gas phase and in the presence of water molecule was investigated using the B3LYP method with aug-cc-pVDZ basis set. The overall process involves keto-enol and lactam-lactim tautomerism that occurs. The most stable form of barbituric acid turns into the intermediates with three-step proton transfer at three different positions of the structure of this compound. Then, these intermediates become to the final products passing through two competing processes. In order to determine the priority of each competitive process, energy changes of all possible tautomeric forms were investigated along with the role of solvent molecules in multiple routes. Also, transposition in the proton transfer process makes changes in the electron transfer energy which was investigated both in the gas phase and in the presence of water molecules. Frequency calculations performed to characterize ground and transition states and calculation of zero point energies. Natural bond orbital analysis and topological property of electron density were investigated using natural bond orbital (NBO) and atoms in molecules (AIM) analysis. From the results, all proton transfer process in the gas phase and in the presence of the solvent molecule (water) is exothermic and non-spontaneous. In the gas phase and in the water assisted condition, the route 1b is kinetically more preferred. Despite the increase of the increase of ring resonance of stronger hydrogen bonds in the product compared to the reactant, the products are more unstable than the reactant. Therefore, the only factor can make the product unstable is the severe decrease of LP (N) →σ* (C-O) electron transfer energy.

The Study on Curcuminoids in Chromatography, Spectroscopy and Regioisomerism

The turmeric or Curcuma species was macerated with organic solvents. By executing both analytical and preparative thin-layer chromatographic analysis of the crude material, the chromatographic profile of the curcuminoids could be improved. The Nuclear Magnetic Resonance (NMR) spectra of the pure curcumin, demethoxycurcumin and bisdemethoxy-curcumin was acquired. Additional focus for discussion includes the keto–enol tautomerisation of the products. It is concluded that the understanding of this medicinal herb and its curcuminoidal content, is heightened with advanced technological aid.

Curcumin in Health and Diseases: Alzheimer's Disease and Curcumin Analogues, Derivatives, and Hybrids.

Worldwide, Alzheimer’s disease (AD) is the most common neurodegenerative multifactorial disease influencing the elderly population. Nowadays, several medications, among them curcumin, are used in the treatment of AD. Curcumin, which is the principal component of Curcuma longa, has shown favorable effects forsignificantly preventing or treating AD. During the last decade, the scientific community has focused their research on the optimization of therapeutic properties and on the improvement of pharmacokinetic properties of curcumin. This review summarizes bibliographical data from 2009 to 2019 on curcumin analogues, derivatives, and hybrids, as well as their therapeutic, preventic, and diagnostic applications in AD. Recent advances in the field have revealed that the phenolic hydroxyl group could contribute to the anti-amyloidogenic activity. Phenyl methoxy groups seem to contribute to the suppression of amyloid-β peptide (Aβ42) and to the suppression of amyloid precursor protein (APP) andhydrophobic interactions have also revealed a growing role. Furthermore, flexible moieties, at the linker, are crucial for the inhibition of Aβ aggregation. The inhibitory activity of derivatives is increased with the expansion of the aromatic rings. The promising role of curcumin-based compounds in diagnostic imaging is highlighted. The keto-enol tautomerism seems to be a novel modification for the design of amyloid-binding agents. Molecular docking results, (Q)SAR, as well as in vitro and in vivo tests highlight the structures and chemical moieties that are correlated with specific activity. As a result, the knowledge gained from the existing research should lead to the design and synthesis ofinnovative and multitargetedcurcumin analogues, derivatives, or curcumin hybrids, which would be very useful drug and tools in medicine for both diagnosis and treatment of AD.