Keto-enamine Form Research Articles

(4Z)-3-Methyl-1-phenyl-4-{(phenyl)[4-(trifluoromethoxy)anilino]methylene}-1H-pyrazol-5(4H)-one

In the title compound, C24H18F3N3O2, the dihedral angles formed by the pyrazolone ring with one benzene and two phenyl rings are 26.18 (6), 74.42 (6) and 46.75 (8)°, respectively. The compound is in an enamine–keto form and its structure is stabilized by three intra­molecular (N—H⋯O, C—H⋯O and C—H⋯F) and two inter­molecular (C—H⋯O and C—H⋯F) hydrogen-bond inter­actions.

(4Z)-4-[(4-Fluorobenzylamino)(phenyl)methylene]-3-methyl-1-phenyl-1H-pyrazol-5(4H)-one

In the title compound, C24H20FN3O, the dihedral angles formed by the pyrazolone ring with the three benzene rings are 10.45 (7), 89.05 (12) and 83.90 (12)°. The compound is a ligand is in an enamine–keto form and its structure is stabilized by three intra­molecular N—H⋯O hydrogen bonds. Glide-related mol­ecules form C—H⋯O hydrogen-bonded chains along the b axis.

4-{[1-(Methoxycarbonyl)ethylamino](phenyl)methylidene}-3-methyl-1-phenyl-1H-pyrazol-5(4H)-one

The title compound, C21H21N3O3, is a neutral potentially tridentate ligand in an enamine-keto form, stabilized by an intra­molecular N—H⋯O hydrogen bond. There are two mol­ecules in the asymmetric unit.

3-{[(1Z)-(3-Methyl-5-oxo-1-phenyl-1,5-dihydropyrazol-4-ylidene)(phenyl)methyl]amino}propionic acid

In the title compound, C20H19N3O3, the pyrazolone moiety and the N atom of the 3-amino­propionic acid group are essentially coplanar. The compound is a ligand in an enamine–keto form and its structure is stabilized by two strong intramolecular N—H⋯O hydrogen bonds. Glide-related mol­ecules form O—H⋯N hydrogen-bonded chains along the c axis.

Spectroscopic analysis of recombinant rat histidine decarboxylase.

Mammalian histidine decarboxylases have not been characterized well owing to their low amounts in tissues and instability. We describe here the first spectroscopic characterization of a mammalian histidine decarboxylase, i.e. a recombinant version of the rat enzyme purified from transformed Escherichia coli cultures, with similar kinetic constants to those reported for mammalian histidine decarboxylases purified from native sources. We analyzed the absorption, fluorescence and circular dichroism spectra of the enzyme and its complexes with the substrate and substrate analogues. The pyridoxal-5'-phosphate-enzyme internal Schiff base is mainly in an enolimine tautomeric form, suggesting an apolar environment around the coenzyme. Michaelis complex formation leads to a polarized, ketoenamine form of the Schiff base. After transaldimination, the coenzyme-substrate Schiff base exists mainly as an unprotonated aldimine, like that observed for dopa decarboxylase. However, the coenzyme-substrate Schiff base suffers greater torsion than that observed in other L-amino acid decarboxylases, which may explain the relatively low catalytic efficiency of this enzyme. The active center is more resistant to the formation of substituted aldamines than the prokaryotic homologous enzyme and other L-amino acid decarboxylases. Characterization of the similarities and differences of mammalian histidine decarboxylase with respect to other homologous enzymes would open new perspectives for the development of new and more specific inhibitors with pharmacological potential.

2,3-Bifunctionalized Quinoxalines: Synthesis, DNA Interactions and Evaluation of Anticancer, Anti-tuberculosis and Antifungal Activity

A variety of 2,3-bifunctionalized quinoxalines (6-14) have been prepared by the condensation of 1,6-disubstituted-hexan-1,3,4,6-tetraones (1-4) with o-phenylenediamine, (R,R)-1,2-diaminocyclohexane and p-nitro-o-phenylenediamine. It is concluded that strong intramolecular N-H----O bonds in the favoured keto-enamine form may be responsible for the minimal biological activities observed in DNA footprinting, anti-tubercular, anti-fungal and anticancer tests with these hyper π-conjugated quinoxaline derivatives. However, subtle alteration by addition of a nitro group affecting the charge distribution confers significant improvements in biological effects and binding to DNA.

Spectroscopic and kinetic analyses reveal the pyridoxal 5'-phosphate binding mode and the catalytic features of Treponema denticola cystalysin.

To obtain insight into the functional properties of Treponema denticola cystalysin, we have analyzed the pH- and ligand-induced spectral transitions, the pH dependence of the kinetic parameters, and the substrate specificity of the purified enzyme. The absorption spectrum of cystalysin has maxima at 418 and 320 nm. The 320 nm band increases at high pH, while the 418 nm band decreases; the apparent pK(spec) of this spectral transition is about 8.4. Cystalysin emitted fluorescence at 367 and 504 nm upon excitation at 320 and 418 nm, respectively. The pH profile for the 367 nm emission intensity increases above a single pK of approximately 8.4. On this basis, the 418 and 320 nm absorbances have been attributed to the ketoenamine and substituted aldamine, respectively. The pH dependence of both log k(cat) and log k(cat)/K(m) for alpha,beta-elimination reaction indicates that a single ionizing group with a pK value of approximately 6.6 must be unprotonated to achieve maximum velocity. This implies that cystalysin is more catalytically competent in alkaline solution where a remarkable portion of its coenzyme exists as inactive aldamine structure. Binding of substrates or substrate analogues to the enzyme over the pH range 6-9.5 converts both the 418 and 320 nm bands into an absorbing band at 429 nm, assigned to the external aldimine in the ketoenamine form. All these data suggest that the equilibrium from the inactive aldamine form of the coenzyme shifts to the active ketoenamine form on substrate binding. In addition, reinvestigation of the substrate spectrum of alpha,beta-elimination indicates that cystalysin is a cyst(e)ine C-S lyase rather than a cysteine desulfhydrase as claimed previously.

Detection of intermediates in reactions catalyzed by PLP-dependent enzymes: O-acetylserine sulfhydrylase and serine-glyoxalate aminotransferase

All pyridoxal 5'-phosphate (PLP)-dependent enzymes catalyze their reaction via the initial formation of an external Schiff base intermediate. The internal and external Schiff bases exist in several different tautomeric forms. The chapter illustrates two predominant forms, the enolimine and ketoenamine tautomers, along with the ranges of their respective λ max values. It shows ketoenamine form in the case of the external Schiff base. The external Schiff base is formed via the intermediacy of geminal-diamine intermediates. Once the external Schiff base is formed, the reaction pathway is determined by the kind of reaction catalyzed, and this is determined by the identity of the functional group on the a-carbon that is orthogonal to the PLP ring. O-acetylserine sulfhydrylase (OASS) catalyzes the final step in the de novo synthesis of L-cysteine in enteric bacteria, that is, the conversion of O-acetyl-L-serine (OAS) and bisulfide to L-cysteine and acetate. The cysK gene, encoding for OASS-A, has been cloned and sequenced from Salmonella typhimurium , and the enzyme overexpressed using the wild-type cys K promoter.

Solution, solid state structure and fluorescence studies of 2,3-functionalized quinoxalines: evidence for a π-delocalized keto-enamine form with N–H···O intramolecular hydrogen bonds

Three quinoxaline derivatives 3, 4 and 5 were prepared by condensation of tetraones RC(O)–CH2–C(O)– (O)–CH2–C(O)R [1, R = Ph; 2, R = neo-Pen] with o-phenylenediamine or (R,R)-1,2-diaminocyclohexane. 1H, 13C and 15N NMR studies show that these derivatives are best described as their keto-enamine form with N–H···O intramolecular hydrogen bonds. This preferred tautomeric form is confirmed by X-ray structural studies of 3 and 5. Compounds 3 and 4, containing an extended delocalized π-system, exhibit fluorescence properties. In contrast, fluorescence is inhibited in 5, when the central aromatic part is replaced by a cyclohexyl group.

Activation of the coenzyme at the early step of the catalytic cycle of tryptophanase.

Tryptophanase is catalytically more competent in alkaline pH even though the coenzyme exists as an inactive aldamine structure in this pH region. The binding of a substrate analog, 3-indolepropionate to the enzyme shifts the equilibrium from the substituted aldamine to the ketoenamine form in the entire pH region studied. The resultant ketoenamine form is favorable for transaldimination with the substrate amino group, a prerequisite for subsequent catalysis. This implies that the binding of tryptophan in alkaline pH, where the enzyme shows maximum activity, converts the inactive aldamine form of the coenzyme to the active ketoenamine form, which is favorable for undergoing the next step of the catalytic process.

Analysis of the pH- and ligand-induced spectral transitions of tryptophanase: activation of the coenzyme at the early steps of the catalytic cycle.

Tryptophanase has an absorption maximum at 338 nm at high pH and 422 nm at low pH. The 422-nm absorption species has been considered to be the catalytically competent ketoenamine form of the Schiff base of pyridoxal 5'-phosphate with a lysine residue. The 338-nm absorption band showed an intense fluorescence band at 390 nm and not around 500 nm, indicating that the 338-nm absorption species is the substituted aldamine rather than an enolimine form of the Schiff base which has been suggested previously. To explore the mechanism of the enzyme that can exert its catalytic ability at high pH where most of its coenzyme exists as the catalytically incompetent aldamine structure, the reaction of tryptophanase with 3-indolepropionate, a substrate analogue that stops the reaction at the step of the Michaelis complex, was studied at various pH values and analogue concentrations. Kinetic analysis was done based on a scheme involving eight forms of the enzyme, i.e., the liganded and unliganded forms of the ketoenamine, the substituted aldamine structures, and their protonated and deprotonated forms. Kinetic parameters were obtained for each interconversion step. The results showed that the binding of 3-indolepropionate to tryptophanase shifts the equilibrium from the substituted aldamine to the ketoenamine structure over the entire pH region studied. This implies that in the reaction of tryptophanase with tryptophan at high pH, where the enzyme shows maximum activity, the binding of the substrate to the enzyme converts the inactive aldamine form of the coenzyme to the active ketoenamine form. Mechanisms for the activation process, in which a nucleophile is expelled from the aldamine either by steric hindrance of the nucleophile with the ligand or by the negative charge of the ligand alpha-carboxylate group that stabilizes the aldimine structure, were discussed.

Photocyclization of N,N ′-diphenyl-1-hydroxy-9,10-anthraquinone diimines via excited-state intramolecular proton transfer (ESIPT)

Photolysis of 1,5-dihydroxy- and 1-hydroxy-substituted N,N′-diphenyl-9,10-anthraquinone diimines (1a and 1b) produces acridine-condensed ring systems due to oxidative cyclization. In contrast, anthraquinone diimines which bear no hydroxy group on the C1-position show no photoreactivity, providing chemical evidence for participation of excited-state intramolecular proton transfer (ESIPT) in the photocyclization. The ground-state of 1a includes only a small fraction (ca. 4%) of the keto enamine form in fast equilibrium with the enol imine form, as proved by temperature dependent 1H and 13C NMR spectroscopy. An X-ray crystal analysis of 1a reveals that the molecule exists in the crystalline state as the enol imine form with a butterfly conformation of the anthraquinone ring. The involvement of ESIPT in photoexcited 1a has been revealed by means of sub-microsecond time-resolved IR spectroscopy. Upon irradiation of 1a an increase of the ground-state keto form (B) is detected and its lifetime is deduced to be 1.3 ms. Another transient species (A) with a lifetime of 80 µs is also observed, which is assumed to be the intramolecular cycloadduct as a precursor of the photoproduct.

Crystal structure of the keto-enamine form of (+) (1S,2S)-2-(2-hydroxybenzylidene)amine-1-phenyl-1,3-propanediol

The title compound crystallizes in the orthorhombic space group P212121, withZ=4,a=6.068(1)A,b=10.922(1)A, andc=21.713(2)A. The compound is the chiral ligand of a copper complex used as an enantioselective catalyst. It crystallizes from methanol in the keto-enamine form, though the enol-imine isomer predominates in the solution. Most N-salicylideneamines studied by X-ray are enol-imines. The two tautomeric forms may interchange through anintramolecular hydrogen bond and the distances between non-H atoms in the resulting cyclic −O−H...N=C−C=C- or −C=O...H−N−C=C- fragment may be misleading, so that H atom position is the crucial factor for determination of the proper tautomeric form.

Intramolecular hydrogen bonding and tautomerism of acylpyran-2,4-diones, -2,4,6-triones and acylpyridinediones and benzannelated derivatives. Deuterium isotope effects on 13C NMR chemical shifts

The structures of acylpyran-diones, -triones and acylpyridinediones have been studied primarily by deuterium isotope effects on 13C chemical shifts. The 3,5-diacetyltetrahydropyran-2,4,6-trione forms a double tautomeric system involving one of the carbonyl carbons of the anhydride moiety. This compound also exists as a minor symmetrical isomer with two intramolecular hydrogen bonds to the same acceptor. This isomer shows isotopic perturbation of the OH proton resonance upon deuteriation. A similar situation is found for 1,5-diphenylpentane-1,3,5-trione.The 3-acetyl-6-methyl-2H-pyran-2,4(3H)-dione is found to be tautomeric and mainly in the 4-hydroxy form. The corresponding 5-acetyl derivative forms a very weak hydrogen bond as is also found in the 5-ethoxycarbonyl-6-methylpyridine-2,4(3H)-dione. The same pattern is found for 3- and 5-acetyl-6-methylpyridine-2,4(3H)-dione. This difference in the two-bond deuterium isotope effect is related to the bond orders of the bonds linking the hydrogen bond donors and acceptors and reflects the strength of the intramolecular hydrogen bonds. The 3-acetyl-4-hydroxy-2(1H)-quinolones are tautomeric in a similar fashion.The formal hydroxypyridines are shown by isotope effects to be of the 2-pyridone form.The formal imines of most of the above compounds have also been studied and are shown to exist in their keto-enamine forms. In the case of 3-(1-amino)ethylidenequinoline-2,4(1 H,3H)-diones and 2-(1amino)ethylidene-6,7-dihydro-5H-benzo[ij]quinolizine-1,3(2H)-diones two different forms with hydrogen bonds to either the carbonyl at C-4 or the amide carbonyl group at C-2 are observed. Deuterium isotope effects on chemical shifts again turned out to be crucial in the structure elucidation.

Synthesis, characterization and electrochemical properties of β-diketone complexes of ruthenium(III)

The reactions of ruthenium(III) chloride (1 mol) with acetylacetonylidene-4-aminoantipyrine (HL 1), monobenzoylacetylacetonylidene-4-aminoantipyrine (HL 2), dibenzoylmethanylidene-4-aminoantipyrine (HL 3) and antipyrine-4-azo-β-ethylacetoacetate (HL 4) (1 mol) produce complexes of the general formula RuHLCl 3. The ligand antipyrine-4-azo-β-acetylacetone (HL 5) (1 mol) reacts with RuCl 3·3H 2O to produce RuL 5 Cl 2(H 2O)·H 2O. The ligands HL 1-HL 3 react as neutral bidentates in the ketoenamine form, whereas HL 4 reacts as a neutral bidentate in the hydrazo form. HL 5 reacts as a monobasic tridentate in the azo form. The complexes were characterized using a variety of analytical, spectral, magnetic and thermal measurements. The electrochemical redox properties of complexes I–V have been studied by cyclic voltammetry in acetonitrile. The chloro-bridged dimer complexes I–IV showed two reversible diffusion-controlled oxidation peaks. The first was attributed to the oxidation of the ruthenium(III) to the corresponding mixed-valence complex and the second to the ruthenium(IV) complex. The redox properties of complexes I–IV are dependent on the nature of ligand. The monomeric complex V has quite different properties.

A hydrogen-bonding network modulating enzyme function: asparagine-194 and tyrosine-225 of Escherichia coli aspartate aminotransferase.

The electron distribution within the coenzyme or coenzyme-substrate conjugate needs to be properly regulated during the catalytic process of aspartate aminotransferase (AspAT). Asn194 and Tyr225 may function in regulating the electron distribution through hydrogen-bonding to O(3') of the coenzyme, pyridoxal 5'-phosphate (PLP) or pyridoxamine 5'-phosphate (PMP). The roles of Tyr225 have already been explored by site-directed mutagenesis (Inoue et al., 1991; Goldberg et al., 1991). In the present studies, the mutant enzymes Asn194-->Ala and Asn194-->Ala + Tyr225-->Phe were analyzed kinetically and spectroscopically and were compared with the wild-type and Tyr225-->Phe enzymes. The kinetic studies showed that Asn194 is not essential for AspAT catalysis, although the Kd values for the substrates were increased by 10- to 50-fold upon the replacement of Asn194. The measurements of the absorption and fluorescence excitation spectra revealed that the ratio of an enolimine to a ketoenamine form was considerably increased as a tautomeric form of the protonated PLP in the active site of the double mutant enzyme. The pH-pKd relationship for the binding of maleate to AspAT could be explained by a simple thermodynamic cycle where only one ionizing group (the imine nitrogen of the internal aldimine bond) affects the binding of maleate. The analyses of the pH-pKd curves for the wild-type and mutant enzymes showed that (i) the hydrogen bond between O(3') of PLP and Asn194 is weakened by the binding of maleate to AspAT, while the hydrogen bond between O(3') and Tyr225 is not changed, and that (ii) the replacement of Asn194 causes some effect hampering the binding of maleate.(ABSTRACT TRUNCATED AT 250 WORDS)

Rare Earth(III) Complexes of a Dibasic Quadridentate Schiff Base Derived from Benzoylacetone and Sulfamethoxypyridazine

Abstract Rare earth(III) complexes of a Schiff base (H2L) derived from benzoylacetone and sulfamethoxypyridazine have been synthesized and characterized by elemental analyses, conductance measurements, solubilities, thermal analyses, and IR, 1HNMR and electronic spectral data. Elemental analyses reveal the presence of 2:3 (metal: ligand) stoichiometry and molar conductivities in DMF show the non-electrolytic nature of all the complexes. IR and 1HNMR spec-tral studies indicate that the Schiff base acts as a dibasic qua-dridentate ligand in the ketoenamine form coordinating to the rare earth(III) through the enamine N, carbonyl 0, sulfonyl 0 and pyridazinyl N atoms. The electronic spectra measured in the solid state exhibit onyl slight shifts in the visible region. The cova-lent parameters of β, δ and b½ have been calculated based on those spectra.

Characterization of Pd(II) and UO 2(VI) complexes of some schiff base ligands

Palladium(II) chloride and dioxouranium(VI) acetate complexes of some Schiff bases derived from 4-aminoantipyrine and certain carbonyl compounds such as salicylaldehyde (HL 1), 2-hydroxy-1-naphthaldehyde (HL 2), 2,4-dihydroxybenzaldehyde (HL 3) and acetylacetone (HL 4) have been prepared and characterized. Spectroscopic and other analytical studies reveal that the Schiff bases react with the metal ion as monobasic tridentates in the enolimine form, except HL 3 and HL 4 ligands coordinate with Pd(II) in the ketoenamine form as neutral tridentate and neutral bidentate respectively. IR spectra and conductance measurements of some metal complexes show the presence of coordinated chloride ion and acetate group. The IR spectra also reveal that the Pd(II) complex of ligand HL 4 has a square planar cis- configuration.

Stereochemistry and tautomeric equilibria of zinc(II) complexes of the condensation products between (1R)-3-hydroxymethylenebornan-2-one and L-amino acids

The zinc(II) complexes derived from the condensation of (1R)-3-hydroxymethylenebornan-2-one and a series of L-amino acids, [ZnL], undergo tautomeric equilibria in solution between enolimine and ketoenamine species. While the enolimine forms exhibit the expected behaviour for metal complexes of amino acid Schiff bases, the ketoenamine forms appear structurally similar to the complex derived from the cyclic amino acid L-proline, containing a chiral tetrahedral nitrogen donor. The presence of this tetrahedral nitrogen atom and the possibility for the metal atom itself to become a chiral centre can originate up to four diastereomeric ketoenamine forms of the [ZnL] complexes, while only two can be formed for the derivative of L-proline, where the nitrogen atom co-ordinates stereospecifically to the metal. The diastereomeric ketoenamine forms of the [ZnL] complexes can be interconverted through the planar enolimine form, and this tautomerization process can be followed by n.m.r. measurements, since the signals of the various isomers approach the fast exchange limit near 100 °C. For the derivative of L-proline this possibility is no longer available and the proton signals of the two diastereomeric ketoenamines are not averaged by heating the sample up to ca. 130 °C.

Vitamin B6 and derivatives. II—13C NMR study of systems formed by pyridoxal phosphate or pyridoxal with octopamine

AbstractSchiff base formation by condensation of pyridoxal phosphate or pyridoxal with octopamine in aqueous or methanolic medium was investigated by 13C NMR. Enolimine forms are favoured in methanolic solution, whereas in aqueous solution the predominant ketoenamine forms show a hydrogen bond between the iminium proton and the C‐3‐O− phenolate anion of the pyridoxal phosphate. The addition of Cu++ ions is consistent with the hypothesis of chelation of Cu++ by the nitrogen of the aldimine function and the phenolate function of pyridoxal phosphate.