Tautomeric Species Research Articles

Time-resolved photoluminescence of 6-thienyl-lumazine fluorophores in cellulose acetate nanofibers for detection of mercury ions.

Time-resolved photoluminescence measurements were used to characterize the photophysical properties of 6-thienyllumazine (TLm) fluorophores in cellulose acetate nanofibers (NFs) in the presence and absence of mercuric acetate salts. In solution, excited-state proton transfer (ESPT) from TLm to water molecules was investigated at pH from 2 to 12. The insertion of thienyl group into lumazine introduces cis and trans conformers while keeping the same tautomerization structures. Global and target analyses were employed to resolve the true emission spectra of all prototropic, tautomeric, and rotameric species for TLm in water. The results support the premise that only the cis conformers are related to the ESPT process. However, no ESPT from TLm to a nearby water molecule was observed in NFs. The addition of NFs increases the excited-state lifetime of TLm in the solid state because of combined polarity/confinement effects. The solid-state fluorescence of TLm (in NFs) was quenched by mercuric acetate through different mechanisms-dynamic and static-depending on the applied pressure-atmospheric and vacuum, respectively. The new solid-state sensor is simple, ecofriendly, and instantly fabricated. TLM-loaded NFs can detect mercuric ions at a concentration of 50 picomolar. The formation of non-fluorescent ground-state complex between TLm molecules and mercuric ions inside the pores of NFs was achieved under vacuum condition.

A comprehensive photophysical and NMR investigation on the interaction of a 4-methylumbelliferone derivative and cucurbit[7]uril

The host-guest interaction of a 7-hydroxycoumarin (3-[2-(diethylamino)ethyl]-7-hydroxy-4-methylcoumarin, 3ED4MU) with cucurbit[7]uril, CB7, was investigated in aqueous solution. From the steady-state fluorescence Job plot and 1H NMR spectroscopy the formation of a 1:1 inclusion complex, with a binding constant of 1.75 × 105 M−1 was obtained. Time-resolved fluorescence data in water, DMSO and dioxane for 3ED4MU and for umbelliferone (UM, here taken has the model compound) show that the decays are double or triple-exponentials mirroring the presence of excited neutral (N*), anionic (A*), tautomeric (T*) and zwitterionic (T*′) species (the latter only present in 3ED4MU). In the time-resolved experiments, the interaction of 3ED4MU with CB7 is mirrored by an increase of the pre-exponential factor associated to the zwitterionic species (thus giving support for the stabilization of this species when interacting with CB7) and a dramatic change in the pKa* value (from ~0.5 to ~8), associated to a displacement of the water environment probed by 3ED4MU.

Correlation among Hydrogen Bond, Excited-State Intramolecular Proton-Transfer Kinetics and Thermodynamics for −OH Type Proton-Donor Molecules

A series of new molecules bearing alkyl-substitutes on the parent molecule 1-hydroxy-11H-benzo[b]fluoren-11-one (HBF) has been designed and synthesized, which possesses an intramolecular hydrogen bond (H-bond) between −OH proton donor and carbonyl proton acceptor. All studied molecules present an equilibrium type of excited-state intramolecular proton transfer (ESIPT) at 298 K. The alkyl-substitutions at various positions subtly alter the intramolecular H-bond strength, which then fine-tune the excited-state equilibrium and hence thermodynamics between normal and tautomer species. These, in combination with finite rates of ESIPT resolved by femtosecond fluorescence upconversion techniques, lead to the establishment of an empirical relationship among H-bond strengths, ESIPT kinetics, and thermodynamics for HBF series of molecules, in which the stronger H-bond is, the faster and more exergonic ESIPT is. The intensity ratio for normal versus proton-transfer tautomer emission can be systematically tuned, demo...

Excited-state prototropism of 7-hydroxy-4-methylcoumarin in [Cnmim][BF4] series of ionic liquid-water mixtures: insights on reverse micelle-like water nanocluster formation.

This study explores the excited state prototropic behavior of the fluorophore, 7-hydroxy-4-methylcoumarin (7H4MC), in the [Cnmim][BF4] (n = 2, 4, 6, 8, 10) series of ionic liquid (IL)-water mixtures at low water contents. In pure IL media, 7H4MC exists in the neutral form in both ground and excited states. However, on addition of water to the ILs, the excited neutral form of the dye is gradually converted to the anionic and the tautomeric species, leading to characteristic changes in the emission spectra. The similarity in the spectral features of 7H4MC in the IL-water system with that in a conventional reverse micelle system rather than with organic solvent-water mixtures, suggests that in the presence of water, the ILs are organized into reverse micelle-like structures with the consequent formation of confined water pockets. The results further suggest that formation of water nanoclusters and the ensuing changes in excited state prototropic behavior of the dye, is facilitated by increase in the alkyl chain length of the IL cation. These propositions are supported by time-resolved fluorescence studies. To the best of our knowledge this is the first report on proton transfer reaction in IL-water mixtures at low water contents. Considering that ILs are useful as solvents and surfactants, and IL-water mixtures in particular have applications in chemical extractions and biocatalysis, an understanding of the structural organization and water pool formation in these systems is quite important. The insights obtained from the prototropic transformations of 7H4MC are significant not only for fundamental self-assembly studies, but also for the development of ILs as chemical reaction media.

A new perfluoromethyl aminoenone derivative and the role of the hydrogen bonding in the intra- and intermolecular interactions

The structure of a new perfluoromethyl derivative, (Z)-4,4,4-trifluoro-1-(2-hydroxyphenyl)-3-(methylamino)-2-buten-1-one (1), was investigated in both crystalline state and solution since the prototropy enable the existence of three tautomeric species in equilibrium. The preference in the configurational E/Z arrangement is strongly dependent on the intramolecular hydrogen bond interactions. The vibrational and electronic spectra of 1 were discussed and assigned by using DFT calculations. The crystal, solved by single-crystal X-ray diffraction, shows the prevalence of the tautomer aminoenone over the imino-enol and imino-ketone forms. Therefore, an almost planar molecular conformation is observed due to the formation of two intramolecular amino and phenolic hydrogen bonds to the same carbonyl oxygen acceptor atom that stabilize the Z configuration. The molecular packing is held by the OH⋯O(keto) and NH⋯O hydrogen bonds, F⋯F, CH⋯π and π-stacking interactions that contribute to the crystal stabilization. The intra and intermolecular contacts were also characterized by Natural Bond Orbital (NBO) analysis and Atoms in Molecules (AIM) approach. Hirshfeld surfaces and their associated fingerprint plots were generated for 1 and for a related structure, to visualize different intermolecular interactions and their relative contributions to the total surface.

Dynamic basis for dG•dT misincorporation via tautomerization and ionization.

Tautomeric and anionic Watson-Crick-like mismatches play important roles in replication and translation errors through mechanisms that are not fully understood. Using NMR relaxation dispersion, we resolved a sequence-dependent kinetic network connecting G•T/U wobbles with three distinct Watson-Crick mismatches consisting of two rapidly exchanging tautomeric species (Genol•T/U⇌G•Tenol/Uenol; population <0.4%) and one anionic species (G•T−/U−; population ≈0.001% at neutral pH). Inserting the sequence-dependent tautomerization/ionization step into a minimal kinetic mechanism for correct incorporation during replication following initial nucleotide binding leads to accurate predictions of dG•dT misincorporation probability across different polymerases, pH conditions, and for a chemically modified nucleotide, and provides mechanisms for sequence-dependent misincorporation. Our results indicate that the energetic penalty for tautomerization/ionization accounts for ≈10−2−10−3–fold discrimination against misincorporation, which proceeds primarily via tautomeric dGenol•dT and dG•dTenol with contributions from anionic dG•dT− dominating at pH ≥8.4 or for some mutagenic nucleotides.

Construction of 6-thioguanine and 6-mercaptopurine carriers based on βcyclodextrins and gold nanoparticles

As a novel strategy to overcome some of the therapeutic disadvantages of 6-thioguanine (TG) and 6-mercaptopurine (MP), we propose the inclusion of these drugs in βcyclodextrin (βCD) to form the complexes βCD-TG and βCD-MP, followed by subsequent interaction with gold nanoparticles (AuNPs), generating the ternary systems: βCD-TG-AuNPs and βCD-MP-AuNPs. This modification increased their solubility and improved their stability, betting by a site-specific transport due to their nanometric dimensions, among other advantages.The formation of the complexes was confirmed using powder X-ray diffraction, thermogravimetric analysis and one and two-dimensional NMR. A theoretical study using DFT and molecular modelling was conducted to obtain the more stable tautomeric species of TG and MP in solution and confirm the proposed inclusion geometries. The deposition of AuNPs onto βCD-TG and βCD-MP via sputtering was confirmed by UV–vis spectroscopy. Subsequently, the ternary systems were characterized by TEM, FE-SEM and EDX to directly observe the deposited AuNPs and evaluate their sizes, size dispersion, and composition. Finally, the in vitro permeability of the ternary systems was studied using parallel artificial membrane permeability assay (PAMPA).

Direct NMR Evidence that Transient Tautomeric and Anionic States in dG·dT Form Watson-Crick-like Base Pairs.

The replicative and translational machinery utilizes the unique geometry of canonical G·C and A·T/U Watson-Crick base pairs to discriminate against DNA and RNA mismatches in order to ensure high fidelity replication, transcription, and translation. There is growing evidence that spontaneous errors occur when mismatches adopt a Watson-Crick-like geometry through tautomerization and/or ionization of the bases. Studies employing NMR relaxation dispersion recently showed that wobble dG·dT and rG·rU mismatches in DNA and RNA duplexes transiently form tautomeric and anionic species with probabilities (≈0.01-0.40%) that are in concordance with replicative and translational errors. Although computational studies indicate that these exceptionally short-lived and low-abundance species form Watson-Crick-like base pairs, their conformation could not be directly deduced from the experimental data, and alternative pairing geometries could not be ruled out. Here, we report direct NMR evidence that the transient tautomeric and anionic species form hydrogen-bonded Watson-Crick-like base pairs. A guanine-to-inosine substitution, which selectively knocks out a Watson-Crick-type (G)N2H2···O2(T) hydrogen bond, significantly destabilized the transient tautomeric and anionic species, as assessed by lack of any detectable chemical exchange by imino nitrogen rotating frame spin relaxation (R1ρ) experiments. An 15N R1ρ NMR experiment targeting the amino nitrogen of guanine (dG-N2) provides direct evidence for Watson-Crick (G)N2H2···O2(T) hydrogen bonding in the transient tautomeric state. The strategy presented in this work can be generally applied to examine hydrogen-bonding patterns in nucleic acid transient states including in other tautomeric and anionic species that are postulated to play roles in replication and translational errors.

BITHIENOLs: Promising C2‐Symmetric Biheteroaromatic Diols for Organic Transformation

2,2′,5,5′‐Tetraphenyl‐3,3′‐bithiophene‐4,4′‐diol, the first member of a new class of chiral C2‐symmetric atropisomeric diols based on a biheteroaromatic scaffold, has been synthesized. Dynamic enantioselective HPLC experiments revealed a racemization barrier of 22.9 kcal/mol at 25 °C, which is high enough to permit the chromatographic collection of both enantiomers on a semi‐preparative scale, but at the borders of application as a promoter in asymmetric catalysis. The ground and transition states involved in the enantiomerization process have been identified through DFT calculations, which suggest the intermediacy of a tautomeric species in which a thiophen‐3‐ol unit and a 2,3‐dihydrothiophen‐3‐one ring are present. The proposed racemization mechanism is supported by the results of dynamic HPLC experiments. Calculations also suggested which substituent groups at the 2,2′‐positions of the thiophene rings could lead to an increase in enantiomerization barriers sufficient to confer full configurational stability on the 3,3′‐bithiophene framework.

Spectral Features and Excited-State Transformations of Hydroxy Derivatives of 4′-N,N-Dimethylaminoflavone in PVA Films and on Plasmonic Platforms

Compounds which undergo excited-state intramolecular proton-transfer (ESIPT) have been widely applied in biophysical investigations as fluorescence sensors enabling several analytical signals. Low fluorescence quantum yields in protic media seem to be, however, their main drawback. In this study, we investigated spectral features of 4′-N,N-dimethylaminoflavone derivatives containing hydroxyl groups at positions 3 and/or 7 in protic solid media—poly(vinyl alcohol) (PVA) films at various concentrations and under conditions of fluorescence intensity enhancement by plasmonic resonance. Our experimental findings indicate that, due to ESIPT and susceptibility of the investigated compounds to the polarity and hydrogen-bonding ability of the medium, various tautomeric species localized in different domains of PVA can be distinguished in the ground and electronically excited states, which determines multiple spectral parameters of such materials. According to the evaluated rate constants, relaxation and aggregatio...

Tautomeric equilibria in solutions of 1-methyl-2-phenacylbenzimidazoles

Until now the susceptibility of 1-methyl-2-phenacylbenzimidazoles to the proton transfer has not been carefully examined. There only have been selective trials to recognize tautomeric equilibrium of substituted compounds. Unfortunately, conclusions of these studies are often conflicting. Therefore, the aim of this work was to analyze the influence of the factors affecting the tautomeric processes of substituted 1-methyl-2-phenacylbenzimidazoles in solutions of chloroform by spectroscopic technique of 1H and 13C NMR. Complex equilibria may only take place when molecules of tautomeric species contain multiple basic and/or acidic centres. Analysis of NMR spectra show unequivocally that 1-methyl-2-phenacylbenzimidazoles (ketimine tautomeric form) are in equilibrium with (Z)-2-(1-methyl-1H-benzo[d]imidazol-2yl)-1-phenylethenols (enolimine).

β-Oxo-δ-diimine Nickel Complexes: A Comparison of Tautomeric Active Species in Ethylene Polymerization Catalysis

A series of mono- and bimetallic Ni alkyl complexes of a β-oxo-δ-diimine (BODDI) ligand are reported. The monometallic complexes have a second binding pocket, of which the free “arm” can exist as either an enamine (e.g., 8, BODEI, β-oxo-δ-enamineiminato) or imine (e.g., 3, BODII, β-oxo-δ-imineiminato) tautomer. The identity of the tautomer in the secondary Ni coordination sphere has a significant effect on ethylene polymerization behavior: the enamine tautomer, which hydrogen bonds to the central O atom and is in conjugation with the N,O backbone chelate, is significantly more electron rich and yields a much lower molecular weight polymer than the imine tautomer, which rotates away from Ni to a distal position and has little effect on polymerization. Deprotonation of the second binding pocket with M(HMDS) (M = Li, Na, K) yields the Ni-alkali metal heterobimetallic complexes 3Li, 3Na, and 3K. The deprotonated alkali metal enamides display ethylene polymerization behavior similar to the neutral imine comple...

Studying Gas-Phase Interconversion of Tautomers Using Differential Mobility Spectrometry.

In this study, we report on the use of differential mobility spectrometry (DMS) as a tool for studying tautomeric species, allowing a more in-depth interrogation of these elusive isomers using ion/molecule reactions and tandem mass spectrometry. As an example, we revisit a case study in which gas-phase hydrogen-deuterium exchange (HDX)-a probe of ion structure in mass spectrometry-actually altered analyte ion structure by tautomerization. For the N- and O-protonated tautomers of 4-aminobenzoic acid, when separated using DMS and subjected to subsequent HDX with trace levels of D2O, the anticipated difference between the exchange rates of the two tautomers is observed. However, when using higher levels of D2O or a more basic reagent, equivalent and almost complete exchange of all labile protons is observed. This second observation is a result of the interconversion of the N-protonated tautomer to the O-protonated form during HDX. We can monitor this transformation experimentally, with support from detailed molecular dynamics and electronic structure calculations. In fact, calculations suggest the onset of bulk solution phase properties for 4-aminobenzoic acid upon solvation with eight CH3OH molecules. These findings also underscore the need for choosing HDX reagents and conditions judiciously when separating interconvertible isomers using DMS. Graphical Abstract ᅟ.

Excited-State Proton Transfer and Intramolecular Charge Transfer in 1,3-Diketone Molecules.

The photophysical signature of the tautomeric species of the asymmetric (N,N-dimethylanilino)-1,3-diketone molecule are investigated using approaches rooted in density functional theory (DFT) and time-dependent DFT (TD-DFT). In particular, since this molecule, in the excited state, can undergo proton transfer reactions coupled to intramolecular charge transfer events, the different radiative and nonradiative channels are investigated by making use of different density-based indexes. The use of these tools, together with the analysis of both singlet and triplet potential energy surfaces, provide new insights into excited-state reactivity allowing one to rationalize the experimental findings including different behavior of the molecule as a function of solvent polarity.

Detecting local heterogeneity and ionization ability in the head group region of different lipidic phases using modified fluorescent probes.

Local heterogeneity in lipid self-assembly is important for executing the cellular membrane functions. In this work, we chemically modified 2-(2′-hydroxyphenyl)benzoxazole (HBO) and attached a C8 alkyl chain in two different locations to probe the microscopic environment of four lipidic phases of dodecyl β-maltoside. The fluorescence change in HBO and the new probes (HBO-1 and HBO-2) shows that in all phases (micellar, hexagonal, cubic and lamellar) three HBO tautomeric species (solvated syn-enol, anionic, and closed syn-keto) are stable. The formation of multi tautomers reflects the heterogeneity of the lipidic phases. The results indicate that HBO and HBO-1 reside in a similar location within the head group region, whereas HBO-2 is slightly pushed away from the sugar-dominated area. The stability of the solvated syn-enol tautomer is due to the formation of a hydrogen bond between the OH group of the HBO moiety and an adjacent oxygen atom of a sugar unit. The detected HBO anions was proposed to be a consequence of this solvation effect where a hydrogen ion abstraction by the sugar units is enhanced. Our results point to a degree of local heterogeneity and ionization ability in the head group region as a consequence of the sugar amphoterism.

Solid state photochromism and thermochromism of two related N-salicylidene anilines

The crystalline structure and optical properties of N-salicylidene-p-cyanoaniline and N-salicylidene-p-carboxyaniline were investigated in solid state (microcrystalline powder), with the purpose to connect the effects of substitution and crystal packing with their optical properties. Diffuse reflectance and fluorescence spectroscopy were used to study the absorption and emission properties upon photoirradiation and cooling down to the liquid nitrogen temperature. The Stokes-shifted fluorescence, with a quantum yield of about 10−2–10−1, is given by the tautomeric cis-keto species formed as a result of the excited state intramolecular proton transfer from the initial enol structure. Despite their similar geometrical parameters, only SA-COOH is photochromic, which is in contradiction with literature. On the other side, the thermochromism is especially strong at SA-CN. The study shows the behavior of SA-CN similar to that of a classical anil, while SA-COOH presents interesting features that contradict the presumed behavior based only on its supramolecular structure.

Water-catalyzed excited-state proton-transfer reactions in 7-azaindole and its analogues.

The mechanism of the water-catalyzed excited-state proton-transfer (ESPT) reaction for 7-azaindole (7AI) has long been investigated, but there are some controversial viewpoints. Recently, owing to the superiority of sensing biowaters in proteins by a 7AI analogue, 2,7-diazatryptophan, it is timely to reinvestigate water-catalyzed ESPT in 7AI and its analogues in an attempt to unify the mechanism. Herein, a series of 7AI analogues and their methylated derivatives were synthesized to carry out a systematic study on pKa, pKa*, and the associated fluorescence spectroscopy and dynamics. The results conclude that all 7AI derivatives undergo water-catalyzed ESPT in neutral water. However, for those derivatives with -H (7AI) and a electron-donating substituent at C(3), they follow water-catalyzed ESPT to form an excited N(7)-H proton-transfer tautomer, T*. T* is rapidly protonated to generate an excited cationic (TC*) species. TC* then undergoes a fast deactivation to the N(1)-H normal species in the ground state. Conversely, protonation in T* is prohibited for those derivatives with an electron-withdrawing group at the C(2) or C(3) or with the C(2) atom replaced by an electron-withdrawing nitrogen atom (N(2) in, e.g., 2,7-diazatryptophan), giving a prominent green T* emission. Additional support is given by the synthesis of the corresponding N(7)-CH3 tautomer species, for which pKa* of the cationic form, that is, the N(7)-CH3N(1)-H(+) species, is measured to be much greater than 7.0 for those with electron-donating C(3) substituents, whereas it is lower than 7.0 upon anchoring electron-withdrawing groups. For 7AI, the previously missing T* emission is clearly resolved with a peak wavelength at 530 nm in the pH interval of 13.0-14.3 (H- 14.2).

Ground state spectroscopy of hydroxyquinolines: evidence for the formation of protonated species in water-rich dioxane–water mixtures

We have recently used 6-, 7-, and 8-hydroxyquinolines (HQs) as fluorescent probes to study the binding mechanism in one of the drug binding sites of human serum albumin. In the present work we study the absorption spectra of the HQ molecules in neat and binary mixtures of dioxane and water in order to identify the different tautomeric species in the ground state. This study should help in identifying the environment in nanocavities of macromolecules when HQs are used as local reporters. The enol form is shown to be the only tautomer for the three HQs in dioxane and water, with the exception of 7HQ in which both the enol and the zwitterion tautomers exist in equilibrium in water. The results are confirmed by the density functional theory (DFT) calculations using the B3LYP method with a 6-311++G(2d,p) basis set. In water-rich dioxane mixtures, all HQs are protonated. The results were confirmed by comparing the absorption spectra in binary solvents with those in acidic and basic aqueous solutions, and by DFT calculations of the Franck-Condon S1 ← S0 transitions. The number of water molecules solvating the polar sites in each HQ molecule was estimated from the spectral change in the binary solvent mixtures, and structures were calculated by DFT. Mapping the water density around the polar sites in each HQ using molecular dynamics (MD) simulations shows well-defined hydrogen bonds around the N-heteroatom in each HQ molecule. Water density is only well-defined around the hydroxyl group in 8HQ. The MD simulations indicate free rotation of the OH group in 6HQ and 7HQ, and the stability of the cis-isomer in 8HQ. The results point to the unique spectral signatures of 7HQ in water which make this molecule a potential probe to detect the presence of water in nanocavities of macromolecules, and to the ability of the three HQs to detect acidic media in binding sites.

Brazilwood Reds: The (Photo)Chemistry of Brazilin and Brazilein

The ground and excited state (in the singlet state, S1) acid–base equilibria, together with the photophysical properties of the two main constituents of brazilwood, brazilin and brazilein, have been investigated in aqueous solutions in the pH range: −1 < pH < 10. Brazilin is the colorless reduced form of brazilein where three ground and three excited state species (B(red)H(n), with n = 2–4 representing the protonated hydroxyl groups) are observed with two corresponding acidity constants: pKa1 = 6.6 and pKa2 = 9.4 (pKa1* = 4.7 and pKa2* = 9.9, obtained from the Förster cycle). In the case of brazilein, three ground species (pKa1 = 6.5 and pKa2 = 9.5) and four excited state species were identified (again from the Förster cycle: pKa1* = 3.9 and pKa2* = 9.8). The colorless species (brazilin) presents a high fluorescence quantum yield (F = 0.33) and competitive radiative channel (kF = 1.3 × 10(9) s(–1)) over radiationless processes (kNR = 2.6 × 10(9) s(–1)). In contrast to this behavior, brazilein displays a F value 2 orders of magnitude lower and a dominance of the radiationless decay pathways, which is suggested to be linked to an excited state proton transfer leading to a quinoidal-like structure. This is further supported by time-resolved data (obtained in a ps time domain). The overall data indicates that brazilin is more prone to degradation than brazilein, mainly due to the high efficiency of the radiationless decay channel (likely through internal conversion), which confers a stabilizing inherent characteristic to the latter. In the case of brazilein, the efficiency of the radiationless channel is linked to an excited state intramolecular proton transfer resulting from an excited state equilibrium involving neutral and zwitterionic tautomeric species of this compound. Furthermore, a theoretical study has been performed with the determination of the optimized ground-state and excited molecular geometries for the two compounds together with the prediction of the lowest vertical one-electron excitation energy and the relevant molecular orbital contours and charge densities changes using density functional theory calculations. These were found to corroborate differences in acidity in the ground and excited states.

Surface enhancement Raman scattering of tautomeric thiobarbituric acid. Natural bond orbitals and B3LYP/6-311+G (d, p) assignments of the Fourier Infrared and Fourier Raman Spectra

Surface enhancement Raman scattering (SERS) of two tautomer of thiobarbituric acid was obtained using silver and gold nanoparticles. Large band enhancement in the region of the ν(CS), ν(CC), δ(CH2), and δ(CNH) vibrational modes was found. Natural bond analysis of the tautomer species revealed expressive values of charge transfer, principally from lone pair electron orbitals of the S, N, and O atoms. Complete vibrational assignment was done for the two tautomers using the B3LYP/6-311+G (d, p) procedure, band deconvolution analysis, and from a rigorous interpretation of the normal modes matrix. The calculated spectra agree well with the experimental ones.