Enol Form Research Articles

Triple Emission of 5'-(para-R-Phenylene)vinylene-2-(2'-hydroxyphenyl)benzoxazole (PVHBO). Part I: Dual Emission from the Neutral Species.

The effects of 5'-(para-R-phenylene)vinylene (PV) substituents on the emission properties of 2-(2'-hydroxyphenyl)benzoxazole (HBO) are analyzed using steady-state and time-resolved absorption and emission spectroscopies in addition to quantum chemical calculations. All members in the series of PVHBOs are capable of excited-state intramolecular proton transfer (ESIPT) with a solvent sensitivity that is typical of a HBO derivative to produce a normal (aka enol) emission and an excited-state tautomer (aka keto) emission. These two emission bands of the neutral dyes are discussed in the current paper. The intermolecular proton transfer, i.e., the deprotonation, of a PVHBO results in the third band of the triple emission, which is described in the succeeding paper. The placement of an electron-withdrawing substituent R on the PVHBO scaffold increases the intensity of the keto emission relative to the enol emission in hydrogen-bonding solvents. The R substituents do not significantly alter the wavelengths of the enol and keto emission bands, which are located in the blue and green regions, respectively, of the visible spectrum. The ultrafast time-resolved spectroscopies and quantum chemical calculations offer explanations on how the R group and the solvent affect the enol and keto emission properties (i.e., wavelength, lifetime, fluorescence quantum yield, and relative ratio of their emissions). The key findings include the following: (1) the emission energies of both enol and keto forms are not sensitively dependent on the R substituent and (2) the solvent-engaged enol excited state is quenched more efficiently as the R substituent becomes more electron-withdrawing. A PVHBO acts as a fusion of HBO and stilbenoid that intersect at the hydroxyphenyl moiety. Depending on the solvent and other environmental conditions, PVHBOs may exhibit the ESIPT property of HBO or the substituent-dependent emission of stilbenoid. This paper and the succeeding article provide a photophysical model of PVHBOs to explain the wavelengths and relative abundances of the three emission bands (enol, keto, and anion) that these compounds are able to produce. Judicial selection of the environmental factors may drive the emission of a PVHBO into the spectral regions of blue, green, and, in a couple of cases, orange or red.

Triple Emission of 5'-(para-R-Phenylene)vinylene-2-(2'-hydroxyphenyl)benzoxazole (PVHBO). Part II: Emission from Anions.

This paper is the second part of a study on the effects of a substituted 5'-phenylenevinylene (PV) functionality on the emission properties of 2-(2'-hydroxyphenyl)benzoxazole (HBO)─a dye that is known for excited-state intramolecular proton transfer. The topical compounds are referred to as PVHBOs, each of which is a structural fusion of HBO and a 4-hydroxy-4'-R-stilbene fluorophore that occurs at the hydroxyphenyl moiety. Therefore, the resulting fusion fluorophore manifests the properties of one component or the other, as governed by its interactions with the environment. In part I (the preceding paper), PVHBOs are divided into two groups depending on whether the R substituent is electron-donating/neutral (group I) or electron-withdrawing (group II). The difference in absorption and emission properties between groups I and II is explained based on observations from spectroscopic experiments (both steady-state and time-resolved) and quantum chemical calculations. In the current paper, the same set of tools is applied to characterize the photophysical properties of the conjugate bases─that is, the anions─of PVHBOs. The emission energy of the anion of any group I compound, where the R substituent is either electron-donating or neutral, is situated between those of the neutral enol and keto forms. The emission of the anion of any given group II compound, on the other hand, has a lower energy than both the enol and keto emissions. The frontier molecular orbitals (i.e., HOMO, LUMO, and LUMO + 1) of a PVHBO localized on either HBO or stilbenoid are impacted by the substituent R and the solvent/additive differently, which leads to the differences in the optical properties of group I and II PVHBOs in both neutral and anion forms.

Photophysical properties and sensing mechanism of fluorescent coumarin–chalcone hybrid for biothiols: A theoretical study

AbstractThe photophysical properties and sensing mechanism of a novel coumarin–chalcone hybrid fluorescent probe (CCH) for biothiols were investigated by density functional theory (DFT) and time‐dependent density functional theory (TDDFT) methods. The theoretical calculations well reproduced the experimental spectra and clarified the experimental observed fluorescence “on–off” switching. Potential energy surface (PES) results revealed that the excited‐state intramolecular proton transfer (ESIPT) process is thermodynamically and kinetically viable for CCH, with the keto form more stable than the enol form. ESIPT process and the significant changes of the orbital energies should be responsible for the red fluorescence of CCH. The results revealed that the lowest lying transition of probe CCH corresponds to electronic transition between HOMO and LUMO with charge transfer (CT) character. The addition of cysteine to CCH breaks the conjugation and the intramolecular CT. A nonclassical photoinduced electron transfer (PET) process should contribute to the fluorescence quenching of CCH‐Cys. Thermodynamic and kinetic calculations on the recognition reaction predicted a moderate Gibbs free energy barrier, which proves the possibility of the Michael addition reaction and induces the rapid response of the probe CCH for cysteine.

A novel flavone-based ESIPT ratiometric fluorescent probe for selective sensing and imaging of hydrogen polysulfides

Hydrogen polysulfides (H2Sn) as an important member of reactive sulfur species is closely relevant to many physiological functions in redox homeostasis and metabolism. Dual-channel monitor the changes of H2Sn level in vivo is highly desired. Herein we design a simple ratiometric fluorescent probe based on flavone skeleton for highly selective detection of H2Sn. The probe HF-NA-MC bearing 2-fluoro-5-nitrobenzoic acid group inhibited the intramolecular ESIPT process, which show the blue fluorescence of adjacent naphthalene unit. In the presence of H2Sn, the enol form of probe is converted to conjugated keto form, resulted in a 90 nm red-shift of fluorescence emission from 450 nm to 540 nm. The ratiometric intensity (I540/I450) of the probe exhibits a good linear relationship toward H2Sn in the range of 0–120 μM, and the detection limit is estimated to be 0.63 μM. The ratiometric fluorescent probe shows high specificity and anti-interference ability for H2Sn over other related reactive sulfur species. The probe HF-NA-MC shows promising outlook and could be applied to the confocal imaging of H2Sn by dual emission channels in Hela cells.

PH-Responsive Semi-Interpenetrated Polymer Networks of pHEMA/PAA for the Capture of Copper Ions and Corrosion Removal.

Bronze artifacts constitute a fundamental portion of Cultural Heritage, but effective methodologies for the removal of corrosion layers, such as those produced by the “bronze disease”, are currently missing. We propose the formulation and application of novel poly(2-hydroxyethyl methacrylate) (pHEMA) networks semi-interpenetrated (SIPN) with poly(acrylic acid) (PAA) to achieve enhanced capture of copper ions and removal of corrosion products. The pHEMA/PAA SIPNs were designed to improve previous pHEMA/poly(vinylpyrrolidone) (PVP) networks, taking advantage of the chelating ability of pH-responsive carboxylic groups in PAA. Increasing the pH ionizes carboxyls, increases the porosity in pHEMA/PAA, and leads to the co-presence of enol and enolate forms of vinylpyrrolidone (VP), changing the macroporosity and decreasing the mesh size in pHEMA/PVP. The ion–matrix interaction is stronger in pHEMA/PAA, where the process occurs through an initial diffusion-limited step followed by diffusion in smaller pores or adsorption by less available sites. In pHEMA/PVP, the uptake is probably controlled by adsorption as expected, considering the porogen role of PVP in the network. Upon application of the SIPNs loaded with tetraethylenpentamine (TEPA) onto corroded bronze, copper oxychlorides dissolve and migrate inside the gels, where Cu(II) ions form ternary complexes with TEPA and carboxylates in PAA or carbonyls in PVP. The removal of oxychlorides is more effective and faster for pHEMA/PAA than its /PVP counterpart. The selective action of the gels preserved the cuprite layers that are needed to passivate bronze against corrosion, and the pH-responsive behavior of pHEMA/PAA allows full control of the uptake and release of the Cu(II)–TEPA complex, making these systems appealing in several fields even beyond Cultural Heritage conservation (e.g., drug delivery, wastewater treatment, agricultural industry, and food chemistry).

Absolute configuration of azaphilones from Monascus kaoliang KB9 and solvent effects on their keto and enol forms

Monascus fermented rice, also known as red yeast rice, exhibits a broad spectrum of biological activities due to its chemical constituents, such as monacolins and azaphilone pigments. Here, we cultured Monascus kaoliang KB9 in a liquid malt medium instead of on rice as a carbon source. Eleven known compounds (1–11) containing azaphilones and their early intermediate were isolated and identified. However, this was the first time that angular tricyclic azaphilones, monasfluols A (4) and B (7), acetyl-monasfluol A (5) and monasfluore A (6), were isolated from this species. Interestingly, all isolated tricyclic azaphilones existed exclusively in enol form in CD3OD, as evidenced by NMR spectroscopy. The absolute configuration of compounds 4–7 was also first experimentally identified based on ECD spectroscopy combined with conformational analyses using computational techniques. The assigned stereochemistry of Monascus azaphilones in this work provides essential structural information that will benefit future biological and pharmaceutical investigations.

Reductive Ring Opening of Arylcyclopropanecarboxamides Accompanied by Borylation and Enolate Formation.

Treatment of arylcyclopropanecarboxamides with a sodium dispersion in the presence of methoxypinacolborane as a reduction-resistant electrophile leads to reductive cleavage of the cyclopropane ring followed by instant trapping with the boron electrophile to yield the enolates of γ-aryl-γ-borylalkanamides. The enolates react further with a different electrophile to yield the corresponding α-substituted amides with anti selectivity.

Engineering an ESIPT-based fluorescent probe for dual-channel (Vis/ NIR) ratiometric monitoring of intracellular sulfur dioxide by single wavelength excitation

ESIPT-based probes have large stokes shift and dual emission, and can quantify the analytes through dual-channel LSCM imaging under one excitation wavelength without of the need of two lasers or corresponding parameter corrections. To exploit these advantages of ESIPT mechanism and achieve a big breakthrough in their excitation-emission wavelength and solubility which of the old probes dissatisfied the requirements of biology experiments, we challenged to develop a favorable and practical SO2 probe. In this work, the benzopyrylium moiety which had both a built-in SO2 site and good water solubility, was conjugated with 10-hydroxybenzo[h]quinoline to construct ESIPT-based probe NIR-PT with dual emission (enol form:544 nm; keto form: 733 nm). NIR-PT could ratiometricly visualize SO2 levels in aqueous solution with 460 nm as a excitation. Moreover, we have successfully applied NIR-PT to visualize SO2 levels in cells and mice under oxidative stress.

Exploring curcumin interactions with BN nanostructures: A DFT approach

Density functional theory (DFT) calculations were performed to investigate the curcumin adsorption at the surfaces of two boron nitride (BN) nanostructures including nanosheet (BNNS) and nanotube (BNNT). The singular models were optimized to reach the stabilized structures and to evaluate electronic features. Next, performing optimization processes on interacting systems yielded formations of bimolecular complexes through occurrence of physical interactions. For curcumin, keto and enol tautomeric forms were investigated for participating in interactions with the BN nanostructures, in which the enol form was seen for participating in stronger interactions with both of BNNS and BNNT surfaces in comparison with the keto form. Based on such interactions, electronic molecular orbital features detected the effects of molecular communications to show benefit of employing BN nanostructures for drug delivery purposes. Moreover, BNNS was seen to work better than BNNT for such purpose of adsorption and detection of curcumin substance.

Tandem Pd-Catalyzed Cyclization/Coupling of Non-Terminal Acetylenic Activated Methylenes with (Hetero)Aryl Bromides.

We report a new method for a tandem Pd-catalyzed intramolecular addition of active methylene compounds to internal alkynes followed by coupling with aryl and heteroaryl bromides. Highly substituted vinylidenecyclopentanes were obtained with good yields, complete selectivity, and excellent functional group tolerance. A plausible mechanism, supported by DFT calculations, involves the oxidative addition of bromoarene to Pd(0), followed by cyclization and reductive elimination. The excellent regio- and stereoselectivity arises from the 5-exo-dig intramolecular addition of the enol form of the substrate to alkyne activated by the π-acidic Pd(II) center, postulated as the rate-determining step.

Photophysics of the Blue Light Using Flavin Domain

Light activated proteins are at the heart of photobiology and optogenetics, so there is wide interest in understanding the mechanisms coupling optical excitation to protein function. In addition, such light activated proteins provide unique insights into the real-time dynamics of protein function. Using pump-probe spectroscopy, the function of a photoactive protein can be initiated by a sub-100 fs pulse of light, allowing subsequent protein dynamics to be probed from femtoseconds to milliseconds and beyond. Among the most interesting photoactive proteins are the blue light using flavin (BLUF) domain proteins, which regulate the response to light of a wide range of bacterial and some euglenoid processes. The photosensing mechanism of BLUF domains has long been a subject of debate. In contrast to other photoactive proteins, the electronic and nuclear structure of the chromophore (flavin) is the same in dark- and light-adapted states. Thus, the driving force for photoactivity is unclear.To address this question requires real-time observation of both chromophore excited state processes and their effect on the structure and dynamics of the surrounding protein matrix. In this Account we describe how time-resolved infrared (IR) experiments, coupled with chemical biology, provide important new insights into the signaling mechanism of BLUF domains. IR measurements are sensitive to changes in both chromophore electronic structure and protein hydrogen bonding interactions. These contributions are resolved by isotope labeling of the chromophore and protein separately. Further, a degree of control over BLUF photochemistry is achieved through mutagenesis, while unnatural amino acid substitution allows us to both fine-tune the photochemistry and time resolve protein dynamics with spatial resolution.Ultrafast studies of BLUF domains reveal non-single-exponential relaxation of the flavin excited state. That relaxation leads within one nanosecond to the original flavin ground state bound in a modified hydrogen-bonding network, as seen in transient and steady-state IR spectroscopy. The change in H-bond configuration arises from formation of an unusual enol (imine) form of a critical glutamine residue. The dynamics observed, complemented by quantum mechanical calculations, suggest a unique sequential electron then double proton transfer reaction as the driving force, followed by rapid reorganization in the binding site and charge recombination. Importantly, studies of several BLUF domains reveal an unexpected diversity in their dynamics, although the underlying structure appears highly conserved. It is suggested that this diversity reflects structural dynamics in the ground state at standard temperature, leading to a distribution of structures and photochemical outcomes. Time resolved IR measurements were extended to the millisecond regime for one BLUF domain, revealing signaling state formation on the microsecond time scale. The mechanism involves reorganization of a β-sheet connected to the chromophore binding pocket via a tryptophan residue. The potential of site-specific labeling amino acids with IR labels as a tool for probing protein structural dynamics was demonstrated.In summary, time-resolved IR studies of BLUF domains (along with related studies at visible wavelengths and quantum and molecular dynamics calculations) have resolved the photoactivation mechanism and real-time dynamics of signaling state formation. These measurements provide new insights into protein structural dynamics and will be important in optimizing the potential of BLUF domains in optobiology.

Silver nanoparticles from Tabernaemontana divaricate leaf extract: mechanism of action and bio-application for photo degradation of 4-aminopyridine.

Silver nanoparticles (Ag NPs) were synthesised by the reduction of Ag+ to Ag0 in the presence of enol form of flavonoids present in plant extract of Tabernaemontana divaricate (T. divaricate). Prepared Ag NPs were characterised using UV-Vis, XRD, HR-TEM with EDX and XPS techniques. XPS spectra exhibited peaks at 366eV and 373eV, which specified spin orbits for Ag 3d3/2, and Ag 3d5/2 that confirmed the formation of Ag NPs. Ag NPs were spherical in shape with an average size of 30nm as revealed by HR-TEM and FE-SEM techniques. EDX studies verified the high purity of Ag NPs with silver 46.96%, carbon 16.35%, oxygen 16.22%, nitrogen 20.25% and sulphur 0.21%. LC-MS analysis of plant extract confirmed the qualitative presence of alkaloids, tannins, flavonoids, phenols, and carbohydrates. Prepared Ag NPs showed good photocatalytic activity towards degradation of 4-Amniopyridine with 61% degradation efficiency at optimum conditions in 2h of reaction time under visible light. The ten intermediates were found within the mass number of 0-450. Ag NPs synthesised using bio-extract have also shown good inactivation against Escherichia coli (E. coli) and Bacillus subtilis (B. subtilis) bacteria due to the availability of free radicals.

Altering the binding determinant on the interdigitating loop of mandelate racemase shifts specificity towards that of d-tartrate dehydratase

The enolase superfamily (ENS) has served as a paradigm for understanding how enzymes that share a conserved structure, as well as a common partial reaction (i.e., metal-assisted, Brønsted base-catalyzed enol(ate) formation), evolved from a common progenitor to catalyze mechanistically diverse reactions. Enzymes of the mandelate racemase (MR)-subgroup of the ENS share interdigitating loops between adjacent, 2-fold symmetry-related protomers of the tightly associated homodimers that comprise their quaternary structures. For the MR-subgroup members MR and d-tartrate dehydratase (TarD), the tip of the loop contributes a binding determinant to the adjacent active site (i.e., Leu 93 and Lys 102, respectively). To assess the role of Leu 93 of MR in substrate specificity and catalysis, we constructed L93 variants bearing hydrophobic (L93A, L93F, and L93W), polar neutral (L93N), acidic (L93D), or basic (L93K and L93R) residues at position 93. Gel filtration-HPLC revealed that wild-type MR and all L93 MR variants, apart from L93R MR (dimeric), were tetrameric in solution. The catalytic efficiency (kcat/Km) was reduced in the R→S and S→R reaction directions for all variants, primarily due to reduced turnover (kcat). Substitution of Leu 93 by Lys or Arg to mimic Lys 102 of TarD enhanced the binding of malate and tartrate, with meso- and d-tartrate exhibiting linear mixed-type inhibition of L93K MR. Despite the striking 500-fold increase in the binding affinity of d-tartrate, relative to (S)-mandelate, L93K MR exhibited no TarD activity. MD simulations suggested that the failure of L93K MR to catalyze α-deprotonation (i.e., H-D exchange) arises from inappropriate positioning of the Brønsted base (Lys 166). Thus, a change in binding determinant on the interdigitating loop can play a significant role in governing substrate specificity within the ENS, but does not necessarily confer ‘new’ catalytic activity despite similarities in catalytic machinery.

Structural Insight into the Reaction Mechanism of Ketosynthase-Like Decarboxylase in a Loading Module of Modular Polyketide Synthases.

Ketosynthase-like decarboxylase (KSQ) domains are widely distributed in the loading modules of modular polyketide synthases (PKSs) and are proposed to catalyze the decarboxylation of a malonyl or methylmalonyl unit for the construction of the PKS starter unit. KSQ domains have high sequence similarity to ketosynthase (KS) domains, which catalyze transacylation and decarboxylative condensation in polyketide and fatty acid biosynthesis, except that the catalytic Cys residue of KS domains is replaced by Gln in KSQ domains. Here, we present biochemical analyses of GfsA KSQ and CmiP4 KSQ, which are involved in the biosynthesis of FD-891 and cremimycin, respectively. In vitro analysis showed that these KSQ domains catalyze the decarboxylation of malonyl and methylmalonyl units. Furthermore, we determined the crystal structure of GfsA KSQ in complex with a malonyl thioester substrate analogue, which enabled identification of key amino acid residues involved in the decarboxylation reaction. The importance of these residues was confirmed by mutational analysis. On the basis of these findings, we propose a mechanism of the decarboxylation reaction catalyzed by GfsA KSQ. GfsA KSQ initiates decarboxylation by fixing the substrate in a suitable conformation for decarboxylation. The formation of enolate upon decarboxylation is assisted by two conserved threonine residues. Comparison of the structure of GfsA KSQ with those of KS domains suggests that the Gln residue in the active site of the KSQ domain mimics the acylated Cys residue in the active site of KS domains.

Hydrogen bonding in the crystal of 1,1'-((1E,1'E)-(pyridine-3,4-diylbis(azanylylidene))bis(methanylylidene))-bis(naphthalen-2-ol) acetonitrile solvate: combined experimental and theoretical study

The title compound, 1,1'-((1E,1'E)-(pyridine-3,4-diylbis(azanylylidene))bis(methanylylidene))­bis(naphthalen-2-ol) (1), was synthesized and structurally characterized. The compound cocrystallized with one MeCN molecule. Interestingly, one of two salicylaldehyde Schiff base fragments exists in enol form, while the other one — in a ketone form. Moreover, cocrystallized acetonitrile molecule forms hydrogen bonding with three hydrogen atoms of the dye molecule. The nature and energies of intermolecular and intramolecular hydrogen bonds were studied theoretically using DFT calculations and topological analysis of the electron density distribution within the formalism of Bader's theory (QTAIM method).

Synthesis, Characterization, Electrochemical and Anitmicrobial Studies of Manganese(IV) Complexes Derived from Polyfunctional Glutaryldihydrazone

The synthesis and structure elucidation of a new series of manganese(IV) complexes from disalicylaldehyde glutaryldihydrazone (slghH4) of the composition [MnIV(slgh)](A)2]·H2O and [MnIV(slgh)(NN)] (where A= H2O, (1); pyridine, (2); 2-picoline, (3); 3-picoline, (4); 4-picoline, (5) and NN = 2,2′-bipyridine, (6); 1,10-phenanthroline, (7)) were carried out. The composition and structures of all the complexes of Mn(IV) have been evaluated by elemental analysis, thermal studies, molar conductance, mass spectral data, magnetic moment, electronic, electron paramagnetic resonance and infrared spectral studies. Molar conductances of these Mn(IV) complexes suggest their non-electrolytic nature. Magnetic moment and EPR studies suggested that the Mn(IV) ions are six-coordinated octahedral geometry around the metal ions. The IR spectral studies confirmed that the ligand coordinates to the Mn(IV) ion in enolic form and behave as a tetradentate ligand in anti-cis configuration chelating Mn(IV) ion with NNOO coordination sites. The electrochemical and antimicrobial studies of the Mn(IV) complexes have also been carried out.

Autocatalytic methylthiomethylation of carboxylic acid/phenol involving the formation of DMSO enolate: convenient synthesis of methylthiomethyl ester/ether.

This work reported a simple and practical protocol for the preparation of methylthiomethyl (MTM) esters/ethers directly from carboxylic acid/phenol and dimethylsulfoxide (DMSO) as solvent and methylthiomethyl source. With different types of carboxylic acids/phenols the reactions underwent smooth transformation to afford the corresponding MTM esters/ethers in moderate to excellent yields. This method features catalyst-free, easy to operate, broad substrate scope, good functional group tolerance and involvement of the formation of DMSO enolate.

Studies of adsorption of α,β-unsaturated carbonyl compounds on heterogeneous Au/CeO2, Au/TiO2 and Au/SiO2 catalysts during reduction by hydrogen.

Our research focuses on phenomena accompanying adsorption of mesityl oxide (4-methylpent-3-en-2-one) on the surface of heterogeneous supported gold catalysts: Au/CeO2, Au/TiO2 and Au/SiO2. We have studied reduction in the gas phase of (volatile) α,β-unsaturated carbonyl compounds (R-(V)ABUCC) which mesityl oxide is a basic model of. In situ infrared (IR) spectroscopy was employed to establish that the most active catalysts allow adsorption of conjugated ketones or aldehydes in the enolate (i.e. bridge-like adsorption through the oxygen from the carbonyl group and the β-carbon) and carboxylic form or with the αC Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> βC double bond on a Lewis acidic site. Reductive properties of the catalysts and pure supports were studied by temperature-programmed reduction (TPR). We show that cerium(iv) oxide (CeO2, ceria) and titanium(iv) oxide (TiO2, titania) when decorated with gold nanoparticles (AuNP) can interact with hydrogen at temperatures approx. 150 °C lower than typical for pure oxides what includes even cyclic adsorption and instant release of H2 below 100 °C in the case of gold–ceria system. Morphology and structure characterisation by transmission electron microscopy (TEM) and powder X-ray diffraction (PXRD) confirms that, with the obtained Au loadings, we achieved excellent dispersion of AuNPs while maintaining their small size, preferably below 5 nm, even though the Au/CeO2 catalyst contained broad distribution of AuNPs sizes.

Kinetic vs. thermodynamic control of β-functionalized cyclic ketones: a theoretical investigation of regioselective formation of enolates

The reported theoretical study sheds light on the regioselective formation of enolates in β-functionalized ketones, highlighting the importance of non-covalent interactions.

Gas-phase identification of (Z)-1,2-ethenediol, a key prebiotic intermediate in the formose reaction.

Prebiotic sugars are thought to be formed on primitive Earth by the formose reaction. However, their formation is not fully understood and it is plausible that key intermediates could have formed in extraterrestrial environments and subsequently delivered on early Earth by cometary bodies. 1,2-Ethenediol, the enol form of glycolaldehyde, represents a highly reactive intermediate of the formose reaction and is likely detectable in the interstellar medium. Here, we report the identification and first characterization of (Z)-1,2-ethenediol by means of rotational spectroscopy. The title compound has been produced in the gas phase by flash vacuum pyrolysis of bis-exo-5-norbornene-2,3-diol at 750 °C, through a retro-Diels-Alder reaction. The spectral analysis was guided by high-level quantum-chemical calculations, which predicted spectroscopic parameters in very good agreement with the experiment. Our study provides accurate spectral data to be used for searches of (Z)-1,2-ethenediol in the interstellar space.