Keto Isomers Research Articles

Red‐Shifting ESIPT Fluorescence by Site‐Specific Functionalization in 2‐(2’‐hydroxyphenyl)benzazole Derivatives

AbstractWe describe the synthesis, full photophysical study, and ab initio calculations of 2‐(2’‐hydroxyphenyl)benzazole (HBX) fluorophores substituted, at the meta position of the phenol group, by pyridine derivatives. HBX are commonly used as model dyes to study the stimuli‐induced modulation of the Excited State Intramolecular Proton Transfer (ESIPT) process. The meta‐substituted fluorophores reported herein, display a photophysical profile different from the previously reported ortho‐ and para‐substituted HBO pyridine isomers. Indeed, while all dyes undergo spontaneous deprotonation in neutral conditions, leading to highly emissive anionic species; upon protonation, ortho‐ and para‐pyridine substitution leads to resonance‐stabilized keto isomers, formed after ESIPT. Protonated meta derivatives, unable to stabilize their excited structure by such electronic delocalization process, display sizable intramolecular charge transfer (ICT) processes, translating into significantly redshifted emission. In addition, all dyes present a strong emission intensity, not only in neutral and acidic solutions, but also in the solid‐state. The nature of the emissive transitions was confirmed in each case by theoretical calculations combining Time‐Dependent Density Functional Theory (TD‐DFT) and second‐order Coupled Cluster (CC2) methods.

On the Keto‐Enol‐Tautomerism of 2‐(1‐Hydroxyethylidene)malononitrile and Its Lewis Adduct Formation with B(C6F5)3

Abstract2‐(1‐Hydroxyethylidene)malononitrile (4) is commercially sold as the keto isomer and is described in the literature as either a keto (4 b) or enol (4 a) species. Using NMR techniques and single crystal X‐ray diffraction, we were able to show that 4 exists exclusively as the enol isomer (4 a) in solution as well as in the solid state, which agrees with quantum chemical calculations. By adding one or two equivalents of the Lewis acid B(C6F5)3, the mono‐ or diadducts are formed in exergonic reactions, in which the enol form is further thermodynamically stabilized. The structure and acidity of enol 4 a and its borane adducts are discussed based on experimental and theoretical results.

Water sensitive fluorescence tuning of V-shaped ESIPT fluorophores: Substituent effect and trace amount water sensing in DMSO

Highly sensitive nature of excited state intramolecular proton transfer (ESIPT) functionality in organic fluorophores made them potential candidates for developing environmental sensors and bioimaging applications. Herein, we report the synthesis of V-shaped Dapsone based Schiff base ESIPT derivatives (1–3) and water sensitive wide fluorescence tuning from blue to red in DMSO. Solid-state structural analysis confirmed the V-shaped molecular structure with intramolecular H-bonding and substituent dependent molecular packing in the crystal lattice. 1 showed strong solid-state fluorescence (λmax = 554 nm, Φf = 21.2 %) whereas methoxy substitution (2 and 3) produced tunable but significantly reduced fluorescence (λmax = 547 (2) and 615 nm (3), Φf = 2.1 (2) and 6.5 % (3)). Interestingly, aggregation induced emission (AIE) studies in DMSO-water mixture revealed water sensitive fluorescence tuning. The trace amount of water (less than 1 %) in DMSO converted the non-emissive 1–3 into highly emissive state due to keto tautomer formation. Further increasing water percentage produced deprotonated state of 1–3 in DMSO and enhanced the fluorescence intensity with red shifting of emission peak. At higher water fraction, 1–3 in DMSO produced aggregates and red shifted the emission with reduction of fluorescence intensity. The concentration dependent fluorescence study revealed the very low detection limit of water in DMSO. The limit of detection (LOD) of 1, 2 and 3 were 0.14, 1.04 and 0.65 % of water in DMSO. Hence, simple Schiff bases of 1–3 showed water concentration dependent keto isomer, deprotonated and aggregated state tunable fluorescence in DMSO. Further, scanning electron microscopic (SEM) studies of 1–3 showed water concentration controlled self-assembly and tunable fluorescence.

ESIPT‐Active Pyrene‐imidazole Fluorophores: GSIPT, Dual Solid‐ and Solution‐State Emission plus Counter‐Intuitive Crystal Packing**

AbstractWe report a new excited‐state intramolecular proton transfer (ESIPT)‐inspired hydroxyl (OH)‐substituted pyrene‐imidazole that promote unprecedented intramolecular proton transfer in the ground state (GSIPT). The enol and keto isomers are in equilibrium, with a high keto‐isomer component in the solid state. In conjunction with DFT calculations, the photophysical studies and crystal structure analysis shred substantial evidence for the ground state transformation. The ESIPT‐active compounds also show the rare feature of high dual solution (φf=70 %) and solid state (φf=41 %) emission characteristics and counter‐intuitive stronger intermolecular interactions in the solid state versus the ESIPT‐inactive counterparts.

Use of diformyl‐triazolo Schiff base for Zn2+ sensing and intracellular live cell imaging

A diformylphenol Schiff base of triazole‐amine, 4‐methyl‐2, 6‐bis‐[(1H‐[1,2,4] triazol‐3‐ylimino)‐methyl]‐phenol (PTR) has been synthesised and characterised by spectroscopic data (ESI‐MS, UV‐vis, FT‐IR, NMR spectra). The probe, PTR, emits at 610 nm upon excitation at 380 nm and the emission is strongly intensified on interacting with Zn2+ ion in DMSO‐water (99:1, v/v; HEPES buffer, pH, 7.2) solution even in the presence of 15 other cations (Cu2+, Mn2+, Co2+, Ni2+, Pd2+, Cd2+, Pb2+, Hg2+, Fe3+, Cr3+, Al3+, Na+, K+, Ba2+, Ca2+) and the emission band has been shifted to 530 nm. The limit of detection of Zn2+, 0.30 μM is much lower than WHO recommended value (76 μM). The binding constant (Kd) is 4.2585 × 104 M−1. Selective and sensitive chemosensing behaviour of PTR to Zn2+ has been explained by switching off ESIPT quenching of the keto‐enol tautomerisation of the probe along with the chelation enhancement of fluorescence (CHEF) by binding with Zn2+. The Job's plot and NMR titration have been extended to measure the 1:1 M composition [PTR + Zn2+]. The energy calculation by the DFT computation of keto and enol isomers supports easy tautomerisation and helps ESIPT quenching by proton transfer. Intracellular Zn2+ ions in living cells of HEK293 (PBS: phosphate buffer saline; pH, 7.2) have also been identified by the probe, PTR, using fluorescence microscopic imaging technique.

Identifying the role of excited‐state proton transfer and photoinduced electron transfer in detecting hypochlorous acid for a benzothiazole‐based colorimetric fluorescent probe

AbstractIn this work, the fluorescence mechanism of hypochlorous acid (HOCl) chemosensor 2‐(20‐hydroxyphenyl) benzothiazole‐4‐(1‐isocyanovinyl)pyridine (JBD) has been investigated by density functional theory (DFT) and time‐dependent DFT (TDDFT) methods. For JBD probe, the potential energy barrier for the forward excited‐state (S1) intramolecular proton transfer (ESIPT) is 0.71 kcal/mol, while for reversed ESIPT is 8.47 kcal/mol. Due to the keto isomers are stable, the two isomers can be existed in the S1 state. Dual fluorescence of JBD has been observed in the experiment, which is a typical feature for the ESIPT. By analyzing the change of the degree of intramolecular charge transfer (ICT), JBD could undergo a significant geometric rearrangement upon photoexcitation, leading to the large stokes shift. Adding HOCl, the double emission is turned off and the fluorescence disappears. In order to provide the possibility of ICT processes, the charge distribution has been analyzed by hole‐electrons.

Synthesis, in vitro, in silico and DFT studies of indole curcumin derivatives as potential anticancer agents

The curcumin derivatives were synthesized for the first time using simple starting materials through a three-component reaction, yielding high purity compounds. The synthesized indole curcumin derivatives were characterized by FT-IR, 1H NMR, 13C NMR and HRMS spectral techniques. The anticancer property of curcumin derivatives was evaluated against three cell lines such as, Hep-2, A549 and HeLa. Interestingly, it was found that methoxy substituted indole curcumin was showed the promising anticancer activity with IC50 values of 12 µM (Hep-2), 15 µM (A549) and 4 µM (HeLa) cancer cells lines and equated with standard drugs, doxorubicin and paclitaxel. Moreover, the indole curcumin derivatives were exhibited significant anti-oxidant activity with the reduced levels of 73.41%, 81.95%, and 90.50%, respectively at 50 µg concentration. The methoxy substituted indole curcumin molecule displayed the maximum antioxidant activity, confirming that the indole curcumin has dual action capabilities. The structural characteristics of these potential compounds were explored using DFT studies. The Keto and Enol isomers of indole curcumin derivatives were compared, and energy analysis suggested that the enol isomers were more stable than the keto isomer. Molecular docking results displayed that the keto form of indole curcumin molecules were efficiently binding with GSK-3β, EGFR, and Bcr-Abl proteins, implying that they possess significant anti-cancer activity.

ESIPT and AIE characteristics of three Schiff base derivatives and the relevant photophysical mechanism analyses

Recently, three novel Schiff base derivatives (L1-3) were experimentally found to show water-induced aggregation-induced emission (AIE) and ultrafast excited state intramolecular proton transfer (ESIPT) characteristics, which have potential applications in fields of both optical material and fluoride ion detection. However, the corresponding optical properties have not been recognized completely and the relevant theoretical mechanism is lacking in analysis. In this work, based on the constructed relaxed potential energy curves, we investigated the stable configurations and clarified the proton transfer reaction mechanism for L1-3 with different substituents. The results show that the proton transfer associated with the interconversion from enol to keto isomer in S0 state is increasingly activated as substituted from electron-withdrawing group to electron-donating group, while the reverse trend is observed in S1 state. Above phenomenon can be ascribed to the change in strength of intramolecular hydrogen bond contributed to the reaction, which is demonstrated by hydrogen-bond parameters and interaction region indicator (IRI) analyses. Based on the optimized geometries, the single-fluorescence emission is found following the dual-absorption. The calculated fluorescence peaks (545 nm for L1, 536 nm for L2, 704 nm for L3) show little difference from those of experiment, which are reassigned to the ESIPT induced formation of keto isomer instead of enol form. In addition, the mechanism of water-induced AIE is clarified by increasing radiative rate and orbital overlap. To essentially comprehend the photophysical properties of organic luminogens with AIE and ESIPT characteristics is of fundamental interests in molecular electronics and photonics, and is also helpful to design new optical materials.

Identification of Elusive Keto and Enol Intermediates in the Photolysis of 1,3,5-Trinitro-1,3,5-Triazinane.

Enols have emerged as critical reactive intermediates in combustion processes and in fundamental molecular mass growth processes in the interstellar medium, but the elementary reaction pathways to enols in extreme environments, such as during the decomposition of molecular energetic materials, are still elusive. Here, we report on the original identification of the enol and keto isomers of oxy-s-triazine, as well as its deoxygenated derivative 1,3,5-triazine, formed in the photodecomposition processes of 1,3,5-trinitro-1,3,5-triazinane (RDX)-a molecular energetic material. The identification was facilitated by exploiting isomer-selective tunable photoionization reflectron time-of-flight mass spectrometry (PI-ReTOF-MS) in conjunction with quantum chemical calculations. The present study reports the first experimental evidence of an enol intermediate in the dissociation domain of a nitramine-based energetic material. Our investigations suggest that the enols like 1,3,5-triazine-2-ol could be the source of hydroxyl radicals, and their inclusion in the theoretical models is important to understand the unprecedented chemistry of explosive materials.

Comparison of synthetic, spectroscopic, computational and electrochemical aspects of ferrocenyl-containing β-diketones, β-ketoesters and β-ketoamides

Three different classes of ferrocenyl-containing β-keto compounds Fc-CO-CH2-CO-R were synthesised to obtain two β-diketones with R = CH3,3, and R = Fc (ferrocenyl = FeII(C5H5)(C5H4)), 4, a β-ketoester, 5, with R = OEt and a β-ketoamide, 6, with R = NH2. The β-diketones exhibited keto ⇌ enol tautomerization with the enol form Fc-CO-CH=C(OH)-R as the dominant isomer. Both the β-ketoester and β-ketoamide primarily existed in the keto form. FTIR and 1H NMR measurements could distinguish between keto and enol isomers. Dilute CHCl3 solutions of 3 and 4 exhibited FTIR ѵC=O keto vibrations between 1627 and 1712 cm−1 and ѵC=O enol vibrations between 1590 and 1660 cm−1. X-ray photoelectron spectroscopic (XPS) measurements showed keto isomer Fe 2p3/2 photoelectron lines at smaller binding energies than enol isomers. Significantly lower calculated molecular energies of the keto form of the β-ketoester 5 and β-ketoamide 6 utilising the Gaussian09 suite of programs at the B3LYP (uB3LYP for open-shell species), confirmed that 5 and 6 almost exclusively exists in the keto conformation. Cyclic voltammetry also distinguished between keto and enol isomers, and it was found that the electrochemical reversible ferrocenyl formal oxidation potential, E°ꞌ, of keto isomers of compounds can be estimated with the equation E°ꞌketo = 170.99χR–57.57 where χR is the group electronegativities of R in FcCOCH2COR. This result complements earlier research that enables calculation of the ferrocenyl formal oxidation potentials of enol isomers of β-diketones. Computational studies further showed that the HOMO orbitals of 3–6 is exclusively on the Fe centre and computed Mulliken spin densities on the dxy orbital of the oxidised iron species approached 1 e−.

Quantum-Chemical Search for Keto Tautomers of Azulenols in Vacuo and Aqueous Solution

Keto-enol prototropic conversions for carbonyl compounds and phenols have been extensively studied, and many interesting review articles and even books appeared in the last 50 years. Quite a different situation takes place for derivatives of biologically active azulene, for which only scanty information on this phenomenon can be found in the literature. In this work, quantum-chemical studies have been undertaken for symmetrically and unsymmetrically substituted azulenols (constitutional isomers of naphthols). Stabilities of two enol (OH) rotamers and all possible keto (CH) tautomers have been analyzed in the gas phase {DFT(B3LYP)/6-311+G(d,p)} and also in aqueous solution {PCM(water)//DFT(B3LYP)/6-311+G(d,p)}. Contrary to naphthols, for which the keto forms can be neglected, at least one keto isomer (C1H, C2H, and/or C3H) contributes significantly to the tautomeric mixture of each azulenol to a higher degree in vacuo (non-polar environment) than in water (polar amphoteric solvent). The highest amounts of the CH forms have been found for 2- and 5-hydroxyazulenes, and the smallest ones for 1- and 6-hydroxy derivatives. The keto tautomer(s), together with the enol rotamers, can also participate in deprotonation reaction leading to a common anion and influence its acid-base properties. The strongest acidity in vacuo exhibits 6-hydroxyazulene, and the weakest one displays 1-hydroxyazulene, but all azulenols are stronger acids than phenol and naphthols. Bond length alternation in all DFT-optimized structures has been measured using the harmonic oscillator model of electron delocalization (HOMED) index. Generally, the HOMED values decrease for the keto tautomers, particularly for the ring containing the labile proton. Even for the keto tautomers possessing energetic parameters close to those of the enol isomers, the HOMED indices are low. However, some kind of parallelism exists for the keto forms between their relative energies and HOMEDs estimated for the entire molecules.

Revised excited-state intramolecular proton transfer of the 3-Aminophthalimide molecule: A TDDFT study

The spectroscopic properties of 3-Aminophthalimide (3AP) molecule were investigated [Chem. Phys. 2002, 283, 249, New J. Chem. 2018, 42, 1181]. The result was that the 3AP molecule was exhibiting excited-state intramolecular proton transfer (ESIPT). In the research, we revised previous result using time-dependent density functional theory (TDDFT) method. The fluorescence spectrum shows that the only fluorescence peak is from initial enol form, which is different from the traditional case of ESIPT. The red shift of characteristic peaks in infrared vibration spectra is not induced by ESIPT process. The change in the vibration mode of the amino group causes the red shift of characteristic peak in the infrared spectrum. Energy curves indicate that the barrier (19.71kcal/mol) is anomalously high in the first excited state. In addition, there are not stable points to lead the ESIPT to form a keto isomer. Together, these results demonstrate that there is not an ESIPT process happening of 3AP molecule.

Photoinitiated Intramolecular Proton Transfer in Deprotonated para-Coumaric Acid.

Deprotonated para-coumaric acid is commonly considered as a model for the chromophore in photoactive yellow protein, which undergoes E → Z isomerization following absorption of blue light. Here, tandem ion mobility mass spectrometry is coupled with laser excitation to study the photochemistry of deprotonated para-coumaric acid, to show that the E isomers of the phenoxide and carboxylate forms have distinct photochemical responses with maxima in their action spectra at 430 and 360 nm, respectively. The E isomer of the phenoxide anion undergoes efficient autodetachment upon excitation of its lowest ππ* transition. For the E isomer of the carboxylate deprotomer, a one-way photoinitiated proton transfer generates the phenoxide deprotomer through a mechanism postulated to involve an excited-state enol-keto tautomerism followed by a series of ground-state rearrangements including a second proton transfer. This mechanism is supported by experiments in which the relevant intermediate keto isomer is prepared and spectroscopically probed and through master equation modeling of possible ground-state isomerization processes. The Z isomer of the carboxylate deprotomer shows a weak Z → E photoisomerization response that occurs in competition with photodestruction (presumably electron detachment), demonstrating that the E and Z isomers undergo different processes in their excited states. The study highlights the utility of isomer-selective spectroscopy for characterizing the photochemistry of isolated anions possessing multiple deprotonation sites.

Liquid-phase hydrodeoxygenation of lignin-derived phenolics on Pd/Fe: A mechanistic study

Abstract Although Pd/Fe bimetallic catalysts have been extensively studied in vapor-phase hydrodeoxygenation of phenolics (i.e., guaiacol, cresol), little is yet known about their performance in liquid-phase reactions. In this work, we present a mechanistic study on the Pd/Fe bimetallic catalysts in liquid-phase hydrodeoxygenation of phenolics. The role of tautomerization in hydrodeoxygenation of the lignin-derived phenolics, particularly for ring saturation, is systematically studied by employing two representative modeling compounds: phenol (a molecule that is keto-enol tautomeric) and diphenyl ether (a molecule that does not allow ketol-enol tautomerization). It was found that although the addition of oxyphilic Fe inhibits the direct aromatic ring saturation reaction typically occurring on Pd, tautomerization opens another reaction pathway toward ring saturation products (i.e. cyclohexanone, cyclohexanol, cyclohexane et al.), where both tautomerization and the hydrogenation of keto isomers are significantly enhanced to produce cyclohexanol followed by direct hydrogenolysis of the cyclohexanol to cyclohexane.

Syntheses, characterization, and biological activity of novel mono- and binuclear transition metal complexes with a hydrazone Schiff base derived from a coumarin derivative and oxalyldihydrazine

A hydrazone Schiff base ligand was synthesized by the condensation of 3-formyl-4-hydroxycoumarin and oxalyldihydrazide in the molar ratio 2:1. The Schiff base ligand acts as a mono-, bi-, tri- or even tetradentate ligand with metal cations in the molar ratios 1:1 or 2:1 (M:L) to yield either mono- or binuclear complexes as keto or enol isomers, where M = Co(II), Ni(II), Cu(II), VO(IV), and Fe(III). The ligand and its metal complexes were characterized by elemental analyses, IR, 1H NMR, mass, and UV–Vis spectroscopy. Furthermore, the magnetic moments were calculated from the measured electric conductivities of the complexes. According to the received data, the dihydrazone ligand contains one or two units of ONO domains and can bind to the metal ions via the azomethine nitrogen, the carbonyl oxygen atoms, and/or the phenolic oxygen atoms. Electronic spectra and the magnetic moments of all complexes show that the complexes’ geometries are either octahedral, tetrahedral, square planar, or square pyramidal. Cyclic voltammograms of the mononuclear Co(II) and Ni(II) complexes show quasi-reversible peaks. Tests against two pathogenic bacteria as Gram-positive and Gram-negative bacteria for both, the Schiff base ligand and its metal complexes were carried out. In addition, also one kind of fungi was tested. The synthesized complexes demonstrate mild antibacterial and antifungal activities against these organisms.Graphical abstract

A STUDY ON EFFECT OF INDOLE AS A SUBSTITUENT ON A KETO-ENOL TAUTOMER: A SYNTHETIC APPROACH ON β-DIKETONE

Objective: The existence of keto-enol tautomerism in β-diketones can typically study by a choice of analytical technique. The position of the keto-enol equilibrium depends on a number of factors like solvent, temperature, and substituents. Here an attempt was made to examine the effect of indole, a heterocyclic moiety with the moderately high polar surface area to examine its effect on ketonisation of β-diketone.Methods: The β-diketone studied and synthesized is a structural analog of magical drug curcumin. The structural influence of indole on ketonisation of β-diketone is studied to give a hypothesis on factors contributing towards ketonisation. This work involves the synthesis of 6-(1H-Indol-3-yl)-hex-5ene-2, 4-dione and the study on the single crystal structure of indole-3-carboxaldehyde, major functional component to result in the reaction. The tautomer was studied for its ability to bind with tetrahydrofolate reductase enzyme using Discovery Studio 3.5 version to differentiate the pharmacological significance of conformations.Results: The single crystal XRD structure of this compound was deposited in Cambridge crystallographic data center bearing CCDC No.1536311. The structural characterization of synthesized ligand was carried out by using IR, Mass, 1H NMR spectroscopic techniques. The docking study reveals that keto isomer found to exhibit more inhibition of the enzyme tetrahydrofolate reductase hence more pharmacologically active.Conclusion: The experimental evidence proves that indole substitution shifted the keto-enol equilibrium towards keto form of 6-(1H-Indol-3-yl)-hex-5ene-2, 4-dione.

Mechanism of Excited-State Intramolecular Proton Transfer for 1,2-Dihydroxyanthraquinone: Effect of Water on the ESIPT.

Mechanisms of excited-state intramolecular proton transfer (ESIPT) of 1,2-dihydroxyanthraquinone (ALR) in ethanol solvent and binary solvent of water and ethanol are investigated using the density functional theory and time-dependent density functional theory. The intramolecular hydrogen bond is found to be reinforced in the excited state based on the bond lengths, bond angles, and infrared vibrational spectra of relevant group. The reinforcement of intramolecular hydrogen bond is attributed to the charge transfer in the excited state, which leads the ESIPT to form a keto isomer. The absorption and fluorescence spectra of ALR in binary solvent with different water percentage are obtained and demonstrate the inhibition effect of water on the ESIPT process, which are consistent with the experimentally observation. Furthermore, more water molecules are considered near the carbonyl group and hydroxyl group related to the intramolecular proton transfer to form intermolecular hydrated hydrogen bond with ALR for clarifying the block mechanism of water on ESIPT. The potential energy curves, frontier molecular orbitals, and NBO analysis are calculated for the several complexes in the ground and excited states. The results show that the interrupt role of water on the ESIPT originated from the forming of hydrated hydrogen bond between the carbonyl oxygen atom and the water molecule, which weakens the intramolecular hydrogen bond associated with proton transfer, increases the energy barrier of ESIPT, and thus precludes the transition of ALR-E to ALR-K in the excited state. In addition, the weakening of intramolecular hydrogen bonds is increased as the water molecule number increases. So the inhibitory effect is enhanced by the water quantity, which reasonably explains the experimental attenuating of keto emission spectra as the water percentage in binary solvent increases.

Cyclization mechanisms of the cyclic dimer of aziridine aldehyde with vinyl aldehyde

Aziridine aldehydes are the notable representative of a limited number of reported amphoteric molecules containing aziridine and aldehyde groups, which can react both as an acid and as a base. In the present work, the intriguing dimerization mechanism of aziridine aldehyde free from any substituents and solvents, as well as the addition and cyclization mechanisms of its dimer with vinyl aldehyde in acetonitrile were investigated systematically. The NH bond of aziridine aldehyde couples with the CO bond in another aziridine aldehyde molecule, and then the OH bond couples to the adjacent CO bond to form a stable polycyclic dimer. In acetonitrile, aziridine aldehyde and CH2CHCHO are inclined to cyclize to polycyclic compounds. The addition reaction of the dimer and CH2CHCHO will form various enol and keto isomers, and the enol isomers cyclizing to 8-exo-trig aldol is energetically favorable.

Computational Study of the Malonic Acid Tautomerization Products in Highly Concentrated Particles.

Knowing the tautomeric form of malonic acid (MA) in concentrated particles is critical to understanding its effect on the atmosphere. Energies and vibrational modes of hydrated MA particles were calculated using density functional theory (DFT) at the B3LYP/6-31G(d,p) level and the effective fragment potential (EFP) method. Visualization of the keto and enol isomer vibrational modes enabled the assignment of keto isomer peaks in the 1710-1750 cm-1 range, and previously unidentified experimental IR peaks in the 1690-1710 cm-1 can now be attributed to the enol isomer. Comparison of calculated spectra of pure hydrated enol or keto isomers confirm recent experimental evidence, presented by Ghorai et al. ( J. Phys. Chem. A 2011 , 115 , 4373 - 4380 ) of a shift in the keto-enol tautomer equilibrium when MA exists as concentrated particles.

Reverse Stability of Oxyluciferin Isomers in Aqueous Solutions.

We investigated the stability of oxyluciferin anions (keto, enol, and enolate isomers) in aqueous solution at room temperature by performing a nanosecond time scale first-principles molecular dynamics simulation. In contrast to all previous quantum chemistry calculations, which suggested the keto-type to be the most stable, we show that the enol-type is slightly more stable than the keto-type, in agreement with some recent experimental studies. The simulation highlights the remarkable hydrophobicity of the keto-type by the cavity formed at the oxyluciferin-water interface as well as a reduction in hydrophobicity with the number of hydrating water molecules. It is therefore predicted that the isomeric form in a hydrated cluster is size-dependent.