Tautomeric Research Articles

Potential Push-Pull Carbon Superbases Based on Methyl Substitution of Rare Tautomers of Imines

Push-pull imines with strong electron donor group(s) display exceptional basicity in the gas phase. Most of them do not exhibit prototropic tautomerism, and gas-phase acid-base equilibria have been already well described and reviewed. Some questions remain for tautomeric systems, particularly for their uncommon forms. As shown by quantum-chemical calculations, some often-neglected tautomers display higher basicity than the thermodynamically favored forms. However, their participation in tautomeric mixtures being in equilibrium is negligible, and their basicity can be impossible to measure in the gas phase by the equilibrium method. During this work, we examined the gas-phase proton basicity for some acyclic and cyclic push-pull organic bases containing the tautomeric amidine or guanidine group. By quantum-chemical calculations, we confirmed the existence of very low amounts of rare tautomeric forms, in particular, those bearing a methylidene (=CH2) group. We also demonstrated that the alkyl derivatives of rare tautomers, being free of prototropy, can be good candidates as very strong push-pull C bases, i.e., bases protonated on the =CH2 group.

A hydrazine-hydroxy-pyran-2-one derivatives as a potential anticancer and antibacterial agent: synthesis, spectroscopic, SC-XRD, DFT/TD-DFT, Hirshfeld surface analysis, in silico molecular docking and nonlinear optical responses studies

This study presents the synthesis and thorough characterisation of a hydrazine-hydroxy-pyran-2-one derivative referred to as compound (L1 : 3, 3′- [(1E.2E)-hydrazine-1, 2-diylidene-dieth-1-yl-1-ylidene] bis(4-hydroxy-6-methyl-2H-pyran-2-one). Structural elucidation using UV-vis, FT-IR and single crystal X-ray diffraction (SC-XRD) confirmed tautomeric forms of L1 . The SC-XRD analysis revealed a bi-zwitterionic form (BZL1 ), stabilised by the intramolecular proton transfer between two enol units and azine tautomerism. The planar molecular structure, supported by XRD, DFT and Hirshfeld surface analysis, showed charge-assisted hydrogen bonds [N+−H…O−] forming S(6) ring motifs. Crystals were stabilised by C-H…O hydrogen bonds and π-π stacking, analysed through Hirshfeld surfaces and 2D fingerprint plots. Compound L2 : 3-[(1E)-1-(9H-fluoren-9-ylidenehydrazinylidene)ethyl]-4-hydroxy-6-methyl-2H-pyran-2-one), is illustrated in this paper to compare two derivatives of dehydroacetic acid (DHA) and to enrich this study. Molecular docking revealed binding affinities of hybrid compounds with target proteins. DFT and TD-DFT analyses examined structural, electronic and optical properties, aligning well with experimental data. BZL1 exhibited higher electronic hardness (3.632 eV) than L2 (3.506 eV). Variations in α, Δα and ρ values were observed, with L1 showing stronger γ(0; 0, 0, 0) and γ(−ω; ω, 0, 0) while L2 excelled in γ(−2ω; ω, ω, 0). Consequently, these results highlight the potential of these compounds as promising candidates for use in third-order nonlinear optical applications.

Polarization Change via Spin Crossover and Valence Tautomerism Behavior in [FeGa] Complexes

Polarization Change via Spin Crossover and Valence Tautomerism Behavior in [FeGa] Complexes

Stability and Properties of Ultraviolet Filter Avobenzone under Its Diketo/Enol Tautomerization Induced by Molecular Encapsulation with β-Cyclodextrin.

Inclusion complexation of the sunscreen ingredient avobenzone (AVB) with β-cyclodextrin (β-CD) was investigated to improve its aqueous solubility and photostability; another ultraviolet (UV) filter, oxybenzone (OXB), and the phytochemical antioxidant curcumin (CUR) served as a comparison. In this study, the 1-octanol/water partition coefficients, acid dissociation constants, phase-solubility diagrams with β-CD, and ultraviolet-visible (UV-vis) spectral changes induced by UVA1 (365 nm) irradiation were evaluated. β-CD at concentrations 50-100 times that of AVB most effectively protected the photostability of AVB. Additionally, an UVA1-insensitive species with a diketo tautomer, which has an UVC-absorbing band and the potential to cause photodegradation, was stored in the inclusion complex. Acetonitrile-water mixtures at various volume ratios were screened to mimic the internal cavity of β-CD for the AVB tautomeric species using nuclear magnetic resonance (NMR) spectral integrals for the components. The results indicated that β-CD provides a hydrophobic environment similar to that of a 40-50% acetonitrile aqueous solution and enhances the photostability of AVB. However, excess β-CD induced a hyperchromic effect on the diketo tautomer. Aggregation of the AVB/β-CD inclusion complexes at β-CD concentrations of ≥2 mM enhances UVC band absorption. To avoid excess β-CD, a molar ratio of 50-100 of β-CD to AVB is recommended as the optimal composition. This study newly exhibited that the cavity of β-CD mitigates the reactivity of UVA1 toward AVB by inducing the diketo tautomer form of AVB within the cavity.

Identification and characterization of shifted G•U wobble pairs resulting from alternative protonation of RNA.

RNA can serve as an enzyme, small molecule sensor, and vaccine, and it may have been a conduit for the origin of life. Despite these profound functions, RNA is thought to have quite limited molecular diversity. A pressing question, therefore, is whether RNA can adopt novel molecular states that enhance its function. Covalent modifications of RNA have been demonstrated to augment biological function, but much less is known about non-covalent alterations such as novel protonated or tautomeric forms. Conventionally, a G•U wobble has the U shifted into the major groove. We used a cheminformatic approach to identify four structural families of shifted G•U wobbles in which the G instead resides in the major groove of RNA, which requires alternative tautomeric states of either base, or an anionic state of the U. We provide experimental support for these shifted G•U wobbles via the potent, and unconventional, in vivo reactivity of the U with dimethylsulfate (DMS) in three organisms. These shifted wobbles may play important functional roles and could serve as drug targets. Our cheminformatics approach is general and can be applied to identify alternative protonation states in other RNA motifs, as well as in DNA and proteins.

Differently-charged tautomeric forms of three related hexacoordinated complex compounds of Fe(II)

Differently-charged tautomeric forms of three related hexacoordinated complex compounds of Fe(II)

Pd(0)/Pd(II) Electromerism Triggered by Lewis Base Coordination to a Redox-Active Silicon Z-Type Ligand.

Electromerism (aka. valence tautomerism) corresponds to the switching of electronic distributions between redox-active ligands and central elements. While this phenomenon is well established for several transition metals, the Pd(0)/Pd(II) couple could not yet be involved due to the high energy of the Pd(0) state. In this study, we present Pd(0)/Pd(II) electromerism by using a redox-active bis(phosphanyl-amidophenolato)silane. Strong silicon-based Z-type interaction stabilizes the Pd(0) state with an oxidized dicationic open-shell singlet ligand while binding of a Lewis base at the Lewis acidic silicon results in the loss of Z-type interaction and the intramolecular rearrangement to the Pd(II)/reduced ligand electromer. It introduces a novel concept for spin state switching by combining ligand ambiphilicity and redox activity.

Pd(0)/Pd(II) Electromerism Triggered by Lewis Base Coordination to a Redox‐Active Silicon Z‐Type Ligand

Electromerism (aka. valence tautomerism) corresponds to the switching of electronic distributions between redox‐active ligands and central elements. While this phenomenon is well established for several transition metals, the Pd(0)/Pd(II) couple could not yet be involved due to the high energy of the Pd(0) state. In this study, we present Pd(0)/Pd(II) electromerism by using a redox‐active bis(phosphanyl‐amidophenolato)silane. Strong silicon‐based Z‐type interaction stabilizes the Pd(0) state with an oxidized dicationic open‐shell singlet ligand while binding of a Lewis base at the Lewis acidic silicon results in the loss of Z‐type interaction and rearrangement to the Pd(II)/reduced ligand electromer. It introduces a novel concept for spin state switching by combining ligand ambiphilicity and redox activity.

REGIOISOMERIC N-ALKYLATION OF SOME INDAZOLES

Objective: Indazole scaffold have two interconvertible tautomeric forms. Based on our previous studies, regioisomeric N-alkylation of some indazole analogs was synthesized in this study and their structures were elucidated by 2D NMR methods. Material and Method: Regioisomers were resolved for N-benzylations and alkylation of some non-substituted and substituted indazoles, under basic conditions (K2CO3) in DMF. Result and Discussion: It was observed that, their occurrence ratios of N1 : N2 is almost equal (50%). Their structures were established by combination of 1H-1H NOE (Nuclear Overhauser Effect Spectroscopy, NOESY) and HMBC (Heteronuclear Multiple Bond Correlation) NMR methods.

Conflicting findings in pyrrolo[1,2-d]tetrazoles: a computational study

Two structural problems concerning the prototropic and ring-chain tautomerism of pyrrolo[1,2- d]tetrazoles were clarified by means of DFT calculations. The first one concerns the carboxyethyl ester structure of the compound obtained by treating 7-methylpyrrolotetrazolide with ethyl chloroformate and the second one the structure of a precursor of a dye that was not a tetrazole as reported but 2-azido-5-chloro-1 H-pyrrole-3,4-dicarbonitrile. Although the conclusions to the two problems have already been published elsewhere by one of the present authors (DM), the new NMR data of the aforementioned compound together with GIAO/B3LYP/6-311++G(d,p) calculations establish those conclusions on a solid basis. However, the most important thing about the present work is to establish that 1 H-pyrrole is a peculiar azole, while the pyrrolide anion behaves like a classic azole.

Proton Occupancies in Histidine Side Chains of Carbonic Anhydrase II by Neutron Crystallography and NMR - Differences, Similarities and Opportunities.

Histidine is a key amino-acid residues in proteins that can exist in three different protonation states: two different neutral tautomeric forms and a protonated, positively charged one. It can act as both donor and acceptor of hydrogen bonds, coordinate metal ions, and engage in acid/base catalysis. Human Carbonic Anhydrase II (HCA II) is a pivotal enzyme catalyzing the reversible hydration of carbon dioxide. It contains 12 histidine residues: sixsurface exposed, two buried, three active site zinc ion ligands, and one is a proton shuttle. Comparing results from NMR spectroscopy with previously determined neutron protein crystal structures enabled a side-by-side investigation of the proton occupancies and preferred tautomeric states of the histidine residues in HCA II. Buried and zinc coordinating histidines remain in one neutral tautomeric state across the entire pH range studied, as validated by both methods. In contrast, solvent-exposed histidines display high variability in proton occupancies. While the data were overall remarkably consistent between methods, some discrepancies were observed, shedding light on the limitations of each technique. Therefore, combining these methods with full awareness of the advantages and drawbacks of each, provides insights into the dynamic protonation landscape of HCA II histidines, crucial for elucidating enzyme catalytic mechanisms.

Unexpected Reaction of Dialkyl α-Hydroxy-benzylphosphonates with Dialkyl Phosphites and a Few Related Reactions.

The condensation of dialkyl α-hydroxy-benzylphosphonates with dialkyl phosphites and that of α-hydroxybenzyl-diphenylphosphine oxide with diphenylphosphine oxide unexpectedly gave the corresponding phosphorylated α-hydroxy derivatives. This new reaction proved to be general. The formation of the two products may be similar and involves the attack of the hydroxy group of the α-hydroxyphosphonate or α-hydroxyphosphine oxide on the phosphorus atom of the trivalent tautomer form (Y2POH) of the Y2P(O)H reagent (Y= MeO, EtO, or Ph) going with the elimination of an alcohol and water molecule, respectively. The mechanism was supported by DFT computations at the M062X/6-31G (d,p) level of theory, including suitable proton transfer networks. The condensations discussed are typical autocatalytic reactions promoted by the alcohol or water molecules formed. The initial promoters are the traces of water inevitably present in the mixture. In the reaction of α-hydroxyphosphonates with dialkyl phosphites, the -P(O)(OR)H derivative is the primary product that is partially hydrolyzed to the -P(O)(OH)H species by the traces of water under the conditions of the reaction. Arbuzov reaction of diethyl α-bromobenzylphosphonate with ethyl diphenyphosphinite afforded the target-like phenylmethylene-phosphine oxide─phosphonate derivative.

CYP3A4-Mediated Bioactivation of the β-Amyloid Precursor Protein-Cleaving Enzyme 1 Inhibitor JNJ-54861911 Results in Redox-Neutral Addition of Glutathione via Catalysis by Glutathione S-Transferase α1, Identified as the Major Target Protein in Human Hepatocytes.

The β-amyloid precursor protein-cleaving enzyme 1 (BACE1) inhibitor JNJ-54861911, a candidate for the treatment of Alzheimer's disease, was withdrawn from clinical trials due to drug-induced liver injury (DILI). This paper describes our investigation of the metabolism of JNJ-54861911 to understand the potential contribution to the observed DILI. In human hepatocytes, JNJ-54861911 is metabolized by CYP450 3A4 to a reactive intermediate (RI), which undergoes glutathione (GSH) addition at C6 of the 2-amino-4-methyl-1,3-thiazin-4-yl moiety via glutathione S-transferase α1 (GSTA1) catalysis. Despite the preponderant role of CYP3A4 as an enabler, the adduct has the same level of oxidation as that of JNJ-54861911. The exact mechanism of RI formation might involve a sulfoxide (with further reduction) or tautomeric forms of JNJ-54861911 bearing a reactive thiazinium cation activating both the C2 and C6 positions. The cell pellet from the human hepatocyte incubated with 14C-JNJ-54861911 was analyzed via gel electrophoresis, resulting in the identification of a major protein adduct on GSTA1, a cross-link resulting from the addition of GSH and lysine 120 to JNJ-54861911, most likely on position C6 and on the nitrile, respectively. Ultimately, this major adduct might only account for 15-25% of the total covalent binding (CVB). Other important contributors to CVB might exist, like the bioactivation of the major diaminothiazine metabolite (DIAT). The level of covalent binding (CVB) burden (fraction of the dose resulting in CVB) is clearly below 1 mg/day, with a low daily dose of 25 mg. Despite this limited magnitude of CVB, it could still contribute to the liver enzyme elevations observed in approximately 20% of the patients and to the few cases of severe immune-mediated DILI. The latter could occur through a haptenization phenomenon and/or by inducing stress in hepatocytes. Such stress may activate an innate immune response, which, in turn, promotes the adaptive immune response.

Determination of the structure of a new potentially active pharmaceutical substance

Introduction. The development of methods for analyzing new potentially active pharmaceutical substances is an important part of substance standardization. An integrated approach to confirming the structure of a substance is especially important when the substance may exist in different tautomeric forms, since the properties of the substance may change depending on tautomerism, affecting, among other things, the pharmacological activity.Aim. The aim of our study was to determine the tautomeric composition of the potential active pharmaceutical substance 5-butyl-6-hydroxy-2,3-diphenylpyrimidin-4(3H)-one in solid state and solution for the subsequent development of its dosage form and determination of bioavailability.Material and methods. The object of the study was the substance 5-butyl-6-hydroxy-2,3-diphenylpyrimidin-4(3H)-one. Spectra were taken to confirm the structure: infrared on a PerkinElmer Spectrum 3 device (PerkinElmer Inc., USA) in the frequency range from 4000 to 400 cm–1, nuclear magnetic resonance 1H and 13C on a pulsed broadband spectrometer Bruker AM-500 (400 and 100 MHz) (Bruker, Germany) in DMSO-d6 solvent, ultraviolet using SF-2000 (LLC "OKB Spektr", Russia) in the wavelength range of 250–400 nm, and high-performance liquid chromatography with tandem mass spectrometry (HPLC-MS/MS) was also carried out.Results and discussion. A feature of pyrimidine hydroxy derivatives is the presence of tautomeric forms, which can affect both the physicochemical properties of the substance and its pharmacological activity. The study found that in the solid state, 5-butyl-6-hydroxy-2,3-diphenylpyrimidin-4(3H)-one is in equilibrium between the diketo form and the enol form, and when the substance is dissolved in dimethyl sulfoxide (DMSO), the enol form is predominant. The purity was confirmed by HPLC-MS/MS.Conclusion. The structure of 5-butyl-6-hydroxy-2,3-diphenylpyrimidin-4(3H)-one was confirmed by IR and NMR spectroscopy. UV spectra were obtained, and HPLC-MS/MS was performed to exclude possible absorption of ultraviolet radiation by impurities.

Protecting Group Control of Hydroxyketone-Hemiketal Tautomeric Equilibrium Enables the Stereoselective Synthesis of a 1'-Azido C-Nucleoside.

The synthesis of 1'-azido C-nucleosides is described to expand the set of azide-functionalized nucleosides for bioorthogonal applications and as potential antiviral drugs. Lewis acid-promoted azidation of a nucleoside hemiketal resulted in the formation of a tetrazole through a Schmidt reaction manifold. Conformational control to prevent ring-chain tautomerism enabled efficient 1'-azidation with complete β-diastereoselectivity. The unique reactivity and further derivation of the 1'-azido C-nucleosides are also reported.

Unveiling differential interaction pattern for iminium and alkanolamine forms of Sanguinarine with β-Lactoglobulin protein

A comparative study on the interaction of two tautomeric forms of sanguinarine (SANG), an alkaloid with therapeutic properties, with β-lactoglobulin (β-LG) protein was explored using spectroscopic and computational methods. The spectroscopic study reveals a high binding affinity for alkanolamine to monomeric β-LG (at pH = 9) as compared to iminium to dimeric β-LG (at pH = 6.2). Temperature dependent fluorescence study provides thermodynamic parameters for the binding process. Circular dichroism spectra showed changes in the secondary structure of the protein with major conformational transition from α-helix to β-sheets. Molecular docking and MD simulation validate the stable protein-drug complex during a 200 ns simulation period. All results clearly depict the differential interactions of two forms of SANG with β-LG protein. Overall, the characterization of SANG binding interactions with the whey milk protein provides valuable insights for pharmacological research and design of novel drug carriers based on β-LG protein.

Unveiling Drug Discovery Insights Through Molecular Electrostatic Potential Analysis

ABSTRACTMolecular electrostatic potential (MESP) analysis has emerged as a pivotal tool in drug discovery, providing insights into molecular reactivity and noncovalent interactions essential for drug function. While widely used MESP‐on‐isodensity surface analysis offers interpretations of electron‐rich or deficient regions of a drug molecule, the MESP topology parameters such as spatial minimum (Vmin) and MESP at nuclei (Vn) provide a quantitative understanding. The investigation into the correlation between MESP parameters and various molecular properties such as lipophilicity, pKa (acidity/basicity), conformations, and tautomeric forms is crucial for understanding the impact on biological activity of drugs and facilitating drug design. Moreover, MESP topology analysis serves as a fundamental tool in elucidating the pharmacological behavior of compounds and optimizing their therapeutic efficacy. A quantitative study utilizing Vn parameters to assess the hydrogen bond propensity of a drug presents a novel strategy for investigating drug‐receptor interactions with increased precision. The qualitative and quantitative analysis of the MESP features of various drugs, including their applications in cancer, tuberculosis, tumors, inflammation, and infectious diseases such as malaria, bacterial infections, fungal infections, and viral infections, is conducted in this review.

The Reaction of Fenchone and Camphor Hydrazones with 5-Acyl-4-Pyrones as a Method for the Synthesis of New Polycarbonyl Conjugates: Tautomeric Equilibrium and Antiviral Activity.

Selective synthesis of polycarbonyl conjugates of (+)-fenchone and (+)-camphor was carried out (44-91 % yields) via the ring-opening transformation of 5-acyl-4-pyrones with hydrazones of the corresponding monoterpenoids. A strong influence of the hydrazone fragment on the observed tautomeric equilibrium of the tricarbonyl system was shown. Although the major tautomer of the conjugates is the acyclic polycarbonyl form, the camphor-based conjugates undergo new type of ring-chain tautomerism, diketoenaminone-dihydropyridone equilibrium, and predominantly exist in the cyclic dihydropyridone form in DMSO-d6. The polyketones can undergo intramolecular cyclization to form N-amino-4-pyridones in high selectivity. In vitro screening for activity against the influenza virus H1 N1 and vaccinia virus was estimated for the obtained conjugates. The (+)-fenchone derivatives demonstrated the higher activity against vaccinia virus than camphor derivatives. The conjugate, which was prepared from diethyl isochelidonate and hydrazone (+)-fenchone, showed the highest activity against vaccinia virus (SI=17).

Synthesis, characterization, and color performance of bis-azo and bis-pyrazole derivatives for dyeing of polyester

In this study, a novel series of bis-azo dyes 3–10 were synthesized via a two-step process. Initially, benzene-1,4-bis(diazonium) chloride was reacted with several active methylene compounds, such as ethyl acetoacetate, acetyl acetone, ethyl cyanoacetate, and malononitrile. yielding the corresponding bis-azo dyes 3–6. Subsequently, these dyes were cyclized through condensation with phenylhydrazine, resulting in the formation of bis-pyrazole derivatives 7–10. Comprehensive spectroscopic characterization, employing techniques like infrared (IR), nuclear magnetic resonance (NMR), and ultraviolet-visible (UV–Vis) spectroscopy, along with elemental analysis, elucidated the structural features, and electronic transitions of the synthesized dyes. The electronic absorption properties were investigated experimentally and computationally, revealing bathochromic and hypsochromic shifts influenced by substituents and tautomeric forms. The dyes showcased excellent dyeing performance on polyester fabrics, exhibiting notable color strength, perspiration fastness, light fastness, and wash fastness. Theoretical investigations of the electronic properties were conducted utilizing Density Functional Theory (DFT) methodology, employing the B3LYP functional with the 6–31G(d,p) basis set, enabling a comprehensive understanding of the structure-performance relationships governing the dyeing behavior of these dyes on polyester.

A Spin-Labeled Derivative of Gossypol

Gossypol and its derivatives arouse interest due to their broad spectrum of biological activities. Despite its wide potential application, there is no reported example of gossypol derivatives bearing stable radical functional groups. The first gossypol nitroxide hybrid compound was prepared here via formation of a Schiff base. By this approach, synthesis of a gossypol nitroxide conjugate was performed by condensation of gossypol with a 4-amino-TEMPO (4-amino-2,2,6,6-tetramethylpiperidin-1-oxyl) free radical, which afforded the target product in high yield. Its structure was proven by a combination of NMR and EPR spectroscopy, infrared spectroscopy, mass spectrometry, and high-resolution mass spectrometry. In addition, the structure of the gossypol nitroxide was determined by single-crystal X-ray diffraction measurements. In crystals, the paramagnetic Schiff base exists in an enamine–enamine tautomeric form. The tautomer is strongly stabilized by the intra- and intermolecular hydrogen bonds promoted by the resonance of π-electrons in the aromatic system. NMR analyses of the gossypol derivative proved that in solutions, the enamine–enamine tautomeric form prevailed. The gossypol nitroxide at micromolar concentrations suppressed the growth of tumor cells; however, compared to gossypol, the cytotoxicity of the obtained conjugate was substantially lower.