Imino Form Research Articles

Vitamin B1-based thiazol-2-ylidene-Ru(II) complexes: recyclable transfer hydrogenation catalysts in water.

The [(p-cymene)RuCl(κ2C,N-{CNHC-NH})]+X- (CNHC = thiazol-2-ylidene) complexes with a bidentate ligand (2: X = Cl and 3: X = PF6) were prepared by a one-pot reaction of vitamin B1 (VB1, 1), Ag2O and [(p-cymene)RuCl2]2. In the complexes, VB1 coordinates through C2 and the exocyclic N in the imino form with the κ2-(C,N) coordination mode. The complexes 2 and 3 are stable in the solid state, but slowly release p-cymene in solution. Furthermore, upon heating in polar solvents, 2 or 3 can be converted by ligand exchange reactions to produce [(L)3RuCl(κ2C,N-{CNHC-NH})]+PF6- (4: L = py).Robustness was improved remarkably for 4. The complex 4 is stable in the solid state and in solution. The complexes 2-4 have been identified by 1H and 13C{1H} 2D NMR spectroscopy and 2 and 4 were studied by X-ray crystallography. In an effort to develop a recyclable catalyst in water, 2-4 were evaluated for TH of ketones and aldehydes with an azeotropic mixture of HCOOH/Et3N in water. The complexes 3 and 4 exhibited very good catalytic activity and 4 could be reused nine times without significant loss of activity, giving a high turnover frequency (TOF50%(h-1) = 1286).

The Tautomeric State of N4-Hydroxycytidine within Base-Paired RNA.

Antiviral nucleoside analogues (e.g., Molnupiravir, Remdesivir) played key roles in the treatment of COVID-19 by targeting SARS-CoV-2 RNA-dependent RNA polymerase (RdRp). The nucleoside of Molnupiravir, N4-hydroxycytidine (NHC), exists in two tautomeric forms that pair either with G or A within the RdRp active site, causing an accumulation of viral RNA mutations during replication. Detailed insights into the tautomeric states within base pairs and the structural influence of NHC in RNA are still missing. In this study, we investigate the properties of NHC:G and NHC:A base pairs in a self-complementary RNA duplex by UV thermal melting and NMR spectroscopy using atom-specifically 15N-labeled versions of NHC that were incorporated into oligonucleotides by solid-phase synthesis. NMR analysis revealed that NHC forms a Watson-Crick base pair with G via its amino form, whereas two equally populated conformations were detected for the NHC:A base pair: a weakly hydrogen-bonded Watson-Crick base pair with NHC in the imino form and another conformation with A shifted toward the minor groove. Moreover, we found a variable influence of NHC:G and NHC:A base pairs on the neighboring duplex environment. This study provides conclusive experimental evidence for the existence of two tautomeric forms of NHC within RNA base pairs.

Interaction mechanism studies of 1H-tetrazole derivatives and nitrocellulose. Part 1: introduction of amino groups on C and N atoms

The interaction analysis: the addition of aminotetrazole (5-ATZ and DATZ) mainly affected the autocatalytic reaction at the initial stage of the pyrolysis process of NC. The imino form of 5-ATZ or DATZ played an important role in this process.

Comparison Study of HOMO-LUMO Energy Gaps for Tautomerism of Triazoles in Different Solvents Using Theoretical Calculations

The phenomena of the tautomerism forms of the substituted triazoles compounds which are named [1-A] (5-phenyl-1H-1,2,4-triazol-3-amine), [1-B] (3-phenyl-1H-1,2,4-triazol-5-amine), [2-A] (5-(pyridin-2-yl)-1H-1,2,4-triazol-3-amine), [2-B] (3-(pyridin-2-yl)-1H-1,2,4-triazol-5-amine), [3-A] (5-(pyridin-3-yl)-1H-1,2,4-triazol-3-amine), [3-B] (3-(pyridin-3-yl)-1H-1,2,4-triazol-5-amine), [4-A] (5-(pyridin-4-yl)-1H-1,2,4-triazol-3-amine) and [4-B] (3-(pyridin-4-yl)-1H-1,2,4-triazol-5-amine) have been evaluated by Hartree-Fock (HF) method at the basis set (6-31G) in the both gas phase and at different type of solvents. This study of solvents effect was containing polar and non-polar solvent like water, acetone, acetonitrile, benzene and CCl4. The physico-chemical data's were evaluated for eigenvalues at different kinds of energy levels (HOMO, HOMO-1, HOMO-2, HOMO-3, LUMO, LUMO+1, LUMO+2 and LUMO+3). The difference of the energy gap (LUMO-HOMO) values for structures showed that the forms (1A, 2A, 3A and 4A) are more stable compare with (1B, 2B, 3B and 4B). From the results, we can showed that there is a difference between the two forms depends on the parameters. So, the imino form is not stable compare with the other structure.

Phototransformations of 2-aminonicotinic acid resolved with matrix isolation infrared spectroscopy and ab initio calculations

The structure and phototransformations of 2-aminonicotinic acid (ANA) induced by tunable UV laser were studied in low-temperature argon matrices with the use of Fourier-transform infrared (FTIR) spectroscopy. After the deposition of the argon matrix, two amino isomers (ANA-1 and ANA-2) were detected, with populations of 96 % and 4 %, respectively. Electronic structure calculations confirm these to be the two most stable isomers of ANA. The irradiation of the amino isomers gives rise to an intricate system of photoinduced reactions, which involves amino-imino phototautomerism, and rotations of the carboxylic group. Following irradiation at 350 nm, a rotation of the carboxylic group of ANA-1 was detected leading to increase of the ANA-2 form. In turn, at 310 nm two new imino forms (INA-1 and INA-2) of 2-aminonicotinic acid were generated indicating that UV-induced hydrogen atom transfer between ANA and INA tautomeric forms took place. The corresponding reverse photorotamerizations were also detected at 390 nm, resulting in the recovery of the initial ANA-1 and ANA-2 isomers. On the basis of electronic structure calculations, we hypothesize that ANA-INA phototautomerization takes place in the vibrationally hot electronic ground state. Its occurrence appears to be contingent on the molecule being able to access a conical intersection between the lowest singlet excited state and the ground state.

Platinum(II) Complexes of Tridentate ‐Coordinating Ligands Based on Imides, Amides, and Hydrazides: Synthesis and Luminescence Properties

AbstractFive Pt(II) complexes are described in which the metal ion is bound to anionic ‐coordinating ligands. The central, deprotonated N atom is derived from an imide Ar−C(=O)−NH−C(=O)−Ar {PtL1–2Cl; Ar=pyridine or pyrimidine}, an amide py−C(=O)−NH−CH2−py {PtL3Cl}, or a hydrazide py−C(=O)−NH−N=CH−py {PtL4Cl}. The imide complexes PtL1–2Cl show no significant emission in solution but are modestly bright green/yellow phosphors in the solid state. PtL3Cl is weakly phosphorescent. PtL4Cl is formed as a mixture of isomers, bound through either the amido or imino nitrogen, the latter converting to the former upon absorption of light. Remarkably, the imino form displays fluorescence in solution, λ0,0=535 nm, whereas the amido shows phosphorescence, λ0,0=624 nm, τ=440 ns. It is highly unusual for two isomeric compounds to display emission from states of different spin multiplicity. The amido‐bound PtL4Cl can act as a bidentate ‐coordinating ligand, demonstrated by the formation of bimetallic complexes with iridium(III) or ruthenium(II).

On the chemiluminescence emission of luminol: protic and aprotic solvents and encapsulation to improve the properties in aqueous solution.

Luminol is a popular molecule that is currently gaining further interest due to its potential role for non-invasive cancer treatments. Design of more efficient derivatives in this context would benefit from a clear knowledge on the origin of the distinct intensity and spectroscopic properties in protic and aprotic solvents observed experimentally, which are still not rationalized. By efficiently combining molecular dynamics, quantum methodologies based on density functional theory and multiconfigurational quantum chemistry and hybrid approaches, and developing herein a computational approach for accurately determining "molar negative extinction (or gain) coefficients of emission", we firstly demonstrate that the amino and imino forms of the 3-aminophthalate dianion are responsible for the chemiluminescence in protic and aprotic medium, respectively. Secondly, we show that the coupling between the adjacent amino and carboxylate groups of luminol existing in aprotic solvents must be kept in aqueous solution to increase the chemiexcitation and emission intensity. Thirdly, modifications of luminol are proposed and simulated showing improved performances as compared to the parent molecule (stronger emission electronic transition and longer emission wavelengths) under the physiological conditions of relevance in biological and medical applications.

Synthesis and Analgesic and Anti-Inflammatory Activities of (3,3-Dipropyl-6,7-Dimethoxy-3,4-Dihydroisoquinolin-1(2H)-Ylidene)-Acetamide Hydrochlorides

Cyclocondensation of 4-(3,4-dimethoxybenzyl)heptan-4-ol with N-substituted cyanoacetamide was used to synthesize (3,3-dipropyl-6,7-dimethoxy-3,4-dihydroisoquinolin-2(2H)-ylidene)acetamides. The hydrochlorides of the resulting enaminoamides exist in the imino form. All hydrochlorides displayed analgesic effects in the hotplate test at the level of sodium metamizole, and the most active were the amide with the N-substituted 2-(3,4-dimethyoxyphenyl)ethyl radical, which had greater levels of analgesic activity than metamizole and nimesulide. Many of the resulting amides displayed anti-inflammatory effects in a carragheenan model which were comparable in magnitude to those of sodium metamizole.

Proton tautomerism in 2-nitramino-C-nitropyridine derivatives - Experimental and quantum chemical study

The structures of 2-nitramino-3-nitropyridine and 2-nitramino-5-nitropyridine have been characterized by X-ray diffraction and Density Functional Theory (DFT) studies. In the crystals, both compounds exist as the imino forms. The DFT calculations were performed in order to explore the amino-imino tautomerism of the studied compounds in the gas phase and the influence of solvent polarity on the tautomeric equilibrium. The Harmonic Oscillator Model of Aromaticity index (HOMA) and Nucleus Independent Chemical Shift (NICS) calculated for the pyridine rings of the studied systems, demonstrated a noticeable decrease in aromaticity of the imino forms. This study showed also that the higher stability of the tautomeric states could be observed for the systems with the intramolecular hydrogen bonds, which contribute to the formation of six-membered quasi-rings.

Синтез, анальгетическая и противовоспалительная активность гидрохлоридов (3,3-дипропил-6,7-диметокси-3,4-дигидроизохинолин-1(2<I>H</I>)-илиден)ацетамидов

Циклоконденсацией 4(3,4-диметоксибензил)гептан-4-ола с N-замещёнными цианацетамидами синтезированы (3,3-дипропил-6,7-диметокси-3,4-дигидроизохинолин-1(2H)-илиден)ацетамиды. Гидрохлориды полученных енаминоамидов существуют в иминоформе. Все гидрохлориды показали анальгетический эффект в тесте «горячей пластинки» на уровне метамизола натрия, наиболее активным оказался амид, N-замещённый 2(3,4-диметоксифенил)этильным радикалом, превосходящий по уровню анальгетической активности метамизол натрия и нимесулид. Большая часть полученных амидов показали противовоспалительный эффект на каррагениновой модели, сравнимый по уровню с метамизолом натрия.

DFT study of adenine–cytosine mismatch in quaternary systems involving DNA bases

In the present work, the adenine–cytosine mismatch is theoretically investigated in quaternary systems involving DNA bases. The calculations, in gas phase, are performed at M06-2X/6-311++G(d,p) level. The results indicate that the quaternary systems involving G–C base pair are more stable than those containing A–T. Moreover, in the presence of A–T or G–C base pairs, the absolute values of formation energy in quaternary systems involving imino tautomer of adenine (A*) are higher than those including imino form of cytosine (C*). A comparison between systems having A* indicates that the absolute value of formation energy in quaternary systems increases when A* stacks with pyrimidine bases (C or T). In the case of systems containing C*, this behavior is observed when C* stacks with purine bases (G or A). Similar to binary systems, the quaternary systems involving A*–C thermodynamically prefer to transform their geometries into systems containing A–C*. The base pairs are stabilized by hydrogen bonds. The strength of hydrogen bonds in binary and quaternary systems is also examined using atoms in molecules (AIM) and natural bond orbital (NBO) analyses.

The influence of Cu+ binding to hypoxanthine on stabilization of mismatches involving hypoxanthine and DNA bases: a DFT study

In the present work, the influence of Cu+ binding to N3- and N7-positions of hypoxanthine on energetic, geometrical and topological properties of hypoxanthine–guanine, hypoxanthine–adenine, hypoxanthine–cytosine, hypoxanthine–thymine and hypoxanthine–hypoxanthine mismatches is theoretically investigated. The calculations, in gas phase, are performed at B3LYP/6-311++G(3df,3pd) level of theory. Unlike the other mispairs, Cu+ binding to N3-position of hypoxanthine causes the proton transfer process from enol form of hypoxanthine to imino forms of adenine and cytosine. This process also occurs in all mismatches having enol form of hypoxanthine when Cu+ binds to N7-position of hypoxanthine. The mismatches are stabilized by hydrogen bonds. The influence of Cu+ on hydrogen bonds is also examined by atoms in molecules (AIM) and natural bond orbital (NBO) analyses.Communicated by Ramaswamy H. Sarma

Synthesis, potent anti-TB activity against M. tuberculosis ATTC 27294, crystal structures and complex formation of selected 2-arylidenehydrazinylbenzothiazole derivatives

The synthesis, anti-TB activity, crystal structures and ligand ability of a number of 2-arylidene-benzylidenehydrazinylbenzothiazole derivatives, 1, have been investigated. The compounds 1 were obtained from 2-hydrazinobenzothiazole and substituted benzaldehydes in refluxing methanol in yields, after recrystallisation, of 55–75%. The crystal structure determination of compounds, 1e (aryl = 4-MeOC6H4), 1h (aryl = pyridin-2-yl), 1f (aryl = 2-HO-5-MeC6H3) revealed amino forms, whereas an imino form was found for 1f (aryl = 2-HO-4-MeOC6H3). Despite the different tautomeric forms of 1e and 1h, the two compounds have similar cell dimensions and furthermore their intermolecular interactions combine to form similar sub-structures. Pairs of NH⋯N hydrogen bonds and π···dπ stacking interactions produce dimers in all compound. The compounds with the best anti-mycobacterial activity were found to be 1c (aryl = 2-O2NC6H4), 1d, (aryl = 2-HOC6H4), 1e and 1h, all having superior activities to that of the standard drug, ethambutol. Moreover two of these compounds have the capacity to act as tridentate ligands, namely 1d [a potential ONN chelator] and 1h [a potential NNN chelator]. Compound 1d (HL) acts as a tridentate O,N,N-donor to Cu(II) in forming the dimeric octahedral complex{[(L) (H2O)Cu][ O3SCF3]}2, 3, from Cu(II) (O3SCF3)2 in moist MeOH. The monomeric square-pyramidal [(L) (H2O)Cu][ O3SCF3], with an axial OS(O2)CF3 ligand and equatorial H2O ligand, dimerizes through extensive π···π interactions involving two complete planar L-Cu fragments. The dimers are linked into a three dimensional array by OH⋯O and NH⋯O hydrogen bonds and by further π···π interactions. Complex 3, while still very active, has only about 40% of the activity of its ligand against M. tuberculosis ATTC 27294.

Supramolecular solid-state structure, potential energy surfaces and evaluation of antiproliferative effect of 2-benzothiazolylhydrazone derivatives in vitro

The in situ condensation reaction of 2-hydrazinobenzothiazole with salicylaldehyde, 3,4-dihydroxybenzaldehyde, 2,4-dihydroxybenzaldehyde, 2,5-dihydroxybenzaldehyde, 2,3-dihydroxybenzaldehyde, 2-hydroxy-1-naphthaldehyde, 2-methoxy-1-naphthaldehyde, 4-methoxy-1-naphthaldehyde and 6-methoxy-2-naphthaldehyde produced 9 hydrazone Schiff bases (L1–L9, respectively) which were identified and characterized by elemental analysis, IR and NMR spectroscopy. The crystal and molecular structures of four Schiff bases (L1, L7–L9) have been determined by the single-crystal X-ray diffraction method confirming the imino form of L1 and the amino tautomeric form of L7–L9 compounds. Molecular structure analysis also confirmed that reported compounds are E-isomers relative to exo C = N imino bond. The Nhydrazino–H group of amino tautomers forms Nhydrazino–H···Nthiazolyl intermolecular hydrogen bonds shaping molecules into R22(8) rings, while imino tautomer of L1 forms C(4) infinite helical chains via Nthiazolyl–H···Nhydrazino type of intermolecular hydrogen bond. The methoxy group (L7–L9) further shaped these primary supramolecular synthons into different supramolecular arrangements via C–H···O, C–H···N and C–H···S intermolecular hydrogen bonds. The role of aryl substituents in the shaping and stabilization of supramolecular architectures of L1, L7–L9 is supported by quantum chemical calculations. Strong antiproliferative effects on tumor cells and cytotoxic effects on fibroblasts are shown for all ligands L1–L9 with exception of L6 and L7 that had no effect on fibroblast cells.

Solvent-dependent tautomerism of the inverted isomer of meso-tetraphenylporphine: Effect of the polarity of the medium

It is found that aza-imino tautomerism (a ⇆ b) of the inverted porphyrinoids and its mechanism are, along with the stability of tautomeric forms in the Solv–B system, determined by the nature of a base B and the polarity of a solvent Solv. It is shown that the transition from the C6H6–B system to MeCN–B is characterized by an approximate doubling of stability constants KT of the imine form (b), and by a change of the number of molecules B involved in the process (from two to one). According to quantum-chemical data (DFT, B3LYP, CC-pVDZ) and the results from spectral measurements (electronic absorption spectra, EAS), the stability of tautomer b (imino form) falls in the series of solvents DMF > Py ~ Et2NH > MeCN > Me2CO, and tautomer a is to a lesser extent stabilized in the given media by electron donors through the formation of hydrogen bonds (except for Me2CO: DMF > Py ≫ Me2CO ≫ MeCN, Et2NH).

Investigation of tautomeric behavior of 3-amino-4-[4-(dimethylamino)phenyl]-4,5-dihydro-1,2,5-thiadiazole 1,1-dioxide using Fourier Transform infrared and nuclear magnetic resonance spectroscopic methods: A density functional theory supported study

Sulfonamide derivatives have been widely incorporated in different types of studies, particularly in bioorganics and medicinal chemistry. Molecular conformation or tautomeric forms of molecules are directly related to their pharmaceutical and biological activities. In the scope of this work two possible tautomeric forms of 3-amino-4-[4-(dimethylamino)phenyl]-4,5-dihydro-1,2,5-thiadiazole 1,1-dioxide (C10H14N4O2S) molecule were tried to be identified by employing infrared and nuclear magnetic resonance spectroscopic methods. Obtained spectroscopic results suggest that 3-amino-4-[4-(dimethylamino)phenyl]-4,5-dihydro-1,2,5-thiadiazole 1,1-dioxide in its powder form shows the traces of both conformers (amino and imino) while in its liquid state in deuterated dimethyl sulfoxide it is mainly in imino form.

Effects of ionization on stability of 1-methylcytosine - DFT and PCM studies.

Consequences of ionization were studied by quantum-chemical methods (DFT and PCM) for 1-methylcytosine (MC)—a model of the nucleobase cytosine (C) connected with sugar in DNA. For calculations, three prototropic tautomers (one amino and two imino forms) and two imino zwitterions were considered, including conformational or configurational isomerism of exo heterogroups. Ionization and interactions between neighboring groups affect intramolecular proton-transfers, geometric and thermodynamic parameters, and electron delocalization for individual isomers. We discovered that an imino isomer is present in the isomeric mixture in the highest amount for positively ionized MC. Its contribution in neutral and negatively ionized MC is considerably smaller. Acid-base parameters for selected radical ions were estimated in the gas phase and compared to those of neutral MC. Gas-phase acidity of radical cations is close to that of the conjugate acid of MC, and gas-phase basicity of radical anions is close to that of the conjugate base of MC. Various routes of amino-imino conversion between neutral and ionized isomers were considered. Energetic-barrier for intramolecular proton-transfer in MC is close to that in the parent system—formamidine.Electronic supplementary materialThe online version of this article (doi:10.1007/s00894-016-3020-2) contains supplementary material, which is available to authorized users.

Raman and infrared spectra, normal coordinate analysis and ab initio calculations of 4-Amino-2-chloropyrimidine-5-carbonitrile

The present work reports an experimental and theoretical study of molecular structure of 4-Amino-2-chloropyrimidine-5-carbonitrile (ACPC) in solid phase. Raman and infrared (IR) spectra of ACPC have been recorded in the region of 3600–100 cm−1 and 4000–200 cm−1, respectively. Calculations with the methods of B3LYP and Møller–Plesset second perturbation (MP2) were carried out for structural and vibrational predictions. The computational approaches were tested and adapted by comparing the predicted spectra to results obtained experimentally. In order to assign the calculated frequencies for the recorded ones, a normal coordinate analysis has been performed for vibrations with respect to the nuclei displacements for all the fundamental frequencies. By computing and analyzing, in detail, the infrared and Raman spectra of ACPC, the changes in the vibrational features were probed by identifying two tautomers of ACPC; (i) amino ACPC which could exist in two conformational structures, and (ii) imino ACPC which could exist in four conformational structures. Both computational and spectral results were in favor of the amino tautomer with no evidence for the existence of interconversion between amino and imino forms. Moreover, only one of the amino ACPC conformers was found to be the most stable one where the hybridization of amino group was found to be sp3. The theoretical predictions agree well with the available experimental data, accounting for the interconversion process in the amino/imino functional groups. We demonstrate that vibrational spectroscopy constitutes a powerful tool for studying molecular structure due to its high sensitivity to changes in molecular geometry.

Clonidine complexation with hydroxypropyl-beta-cyclodextrin: From physico-chemical characterization to in vivo adjuvant effect in local anesthesia

Clonidine (CND), an alpha-2-adrenergic agonist, is used as an adjuvant with local anesthetics. In this work, we describe the preparation and characterization of an inclusion complex of clonidine in hydroxypropyl-beta-cyclodextrin (HP-β-CD), as revealed by experimental (UV–vis absorption, SEM, X-ray diffraction, DOSY- and ROESY-NMR) and theoretical (molecular dynamics) approaches. CND was found to bind to HP-β-CD (Ka=20M−1) in 1:1 stoichiometry. X-ray diffractograms and SEM images provided evidence of inclusion complex formation, which was associated with changes in the diffraction patterns of the pure compounds. NMR experiments revealed changes in the chemical shift of H3HP-β-CD hydrogens (Δ=0.026ppm) that were compatible with the insertion of CND in the hydrophobic cavity of the cyclodextrin. Molecular dynamics simulation with the three CND species that exist at pH 7.4 revealed the formation of intermolecular hydrogen bonds, especially for the neutral imino form of CND, which favored its insertion in the HP-β-CD cavity. In vitro assays revealed that complexation retarded drug diffusion without changing the intrinsic toxicity of clonidine, while in vivo tests in rats showed enhanced sensory blockade after the administration of 0.15% CND, with the effect decreasing in the order: CND:HP-β-CD+bupivacaine>CND+bupivacaine>bupivacaine>CND:HP-β-CD>clonidine. The findings demonstrated the suitability of the complex for use as a drug delivery system for clinical use in antinociceptive procedures, in association with local anesthetics.

The effects of tautomerization and protonation on the adenine–cytosine mismatches: a density functional theory study

In the present work, we demonstrate the results of a theoretical study concerned with the question how tautomerization and protonation of adenine affect the various properties of adenine–cytosine mismatches. The calculations, in gas phase and in water, are performed at B3LYP/6-311++G(d,p) level. In gas phase, it is observed that any tautomeric form of investigated mismatches is more stabilized when adenine is protonated. As for the neutral mismatches, the mismatches containing amino form of cytosine and imino form of protonated adenine are more stable. The role of aromaticity on the stability of tautomeric forms of mismatches is investigated by NICS(1)ZZ index. The stability of mispairs decreases by going from gas phase to water. It can be explained using dipole moment parameter. The influence of hydrogen bonds on the stability of mismatches is examined by atoms in molecules and natural bond orbital analyses. In addition to geometrical parameters and binding energies, the study of the topological properties of electron charge density aids in better understanding of these mispairs.