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Tautomeric Forms (tautomeric + form)
Selected AbstractsChemInform Abstract: Kinetics and Mechanism of Ruthenium(III) Chloride Catalyzed Oxidation of Phosphorous Acid by Thallium(III) in Acid Perchlorate Medium , Role of Tautomeric Forms of the Phosphorous Acid.CHEMINFORM, Issue 14 2001Menka Bhasin Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] Tautomeric forms of adenine: Vertical ionization energies and Dyson orbitalsINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 10 2010Raman K. Singh Abstract For the MP2/6-311++g(2df,p) optimized geometry of all the 14 adenine tautomers, the first three vertical ionization energies have been calculated using several electron propagator decouplings. The corresponding Dyson orbitals provide detailed insight into the role of structural variations in different adenine tautomers. Changes in the electron binding energies and the corresponding Dyson orbital amplitudes have been correlated with tautomeric proton shifts and changes in conjugation patterns. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [source] A model for targeted substitution mutagenesis during SOS replication of double-stranded DNA containing cis-syn cyclobutane thymine dimersENVIRONMENTAL AND MOLECULAR MUTAGENESIS, Issue 9 2006Helen A. Grebneva Abstract A model for ultraviolet mutagenesis is described that is based on the formation of rare tautomeric bases in pyrimidine dimers. It is shown that during SOS synthesis the modified DNA-polymerase inserts canonical bases opposite the dimers; the inserted bases are capable of forming hydrogen bonds with bases in the template DNA. SOS-replication of double-stranded DNA having thymine dimers, with one or both bases in a rare tautomeric conformation, results in targeted transitions, transversions, or one-nucleotide gaps. Structural analysis indicates that one type of dimer containing a single tautomeric base (TT1*, with the "*" indicating a rare tautomeric base and the subscript referring to the particular conformation) can cause A:T , G:C transition or homologous A:T , T:A transversion, while another dimer (TT2*) can cause a one-nucleotide gap. The dimers containing T4* result in A:T , C:G transversion, while TT5* dimers can cause A:T , C:G transversion or homologous A:T , T:A transversion. If both bases in the dimer are in a rare tautomeric form, then tandem mutations or double-nucleotide gaps can be formed. The dimers containing the rare tautomeric forms T1 *,, T2*,, T3*,, T4*,, and T5*, may not result in mutations. The question of whether dimers containing T4*, and T5*, result in mutations requires further investigation. Environ. Mol. Mutagen., 2006. © 2006 Wiley-Liss, Inc. [source] Crystal Structure of Garciniaphenone and Evidences on the Relationship between Keto,Enol Tautomerism and ConfigurationHELVETICA CHIMICA ACTA, Issue 7 2008Felipe Abstract Garciniaphenone (=rel- (1R,5R,7R)-3-benzoyl-4-hydroxy-8,8-dimethyl-1,7-bis(3-methylbut-2-en-1-yl)bicyclo[3.3.1]non-3-ene-2,9-dione; 1), a novel natural product, was isolated from a hexane extract of Garcinia brasiliensis fruits. The crystal structure of 1 as well as the selected geometrical and configurational features were compared with those of known related polyprenylated benzophenones. Garciniaphenone is the first representative of polyprenylated benzophenones without a prenyl substituent at C(5). Notably, the absence of a 5-prenyl substituent has an impact on the molecular geometry. The tautomeric form of 1 in the solid state was readily established by a residual-electronic-density map generated by means of a difference Fourier analysis, and there is an entirely delocalized six-membered chelate ring encompassing the keto,enol moiety. The configuration at C(7) was used to rationalize the nature of the keto,enol tautomeric form within 1. The intermolecular array in the network is maintained by nonclassical intermolecular H-bonds. [source] Synthesis of ethyl 6-aryl-4-oxo-4,6-dihydro-1(12)(13)h -pyrimido-[2,,1,:4,5][1,3,5]triazino[1,2- a]benzimidazole-3-carboxylates,JOURNAL OF HETEROCYCLIC CHEMISTRY, Issue 6 2006Anton V. Dolzhenko The synthesis of ethyl 6-aryl-4-oxo-4,6-dihdro-1(12)(13)H -pyrimido[2,,1,:4,5][1,3,5]triazino[1,2- a]-benzimidazole-3-carboxylates (4a-p) was described via pyrimidine ring annulation to 4-aryl-3,4-dihydro[1,3,5]triazino[1,2- a]benzimidazole-2-amines (2a-p) which were obtained from 2-guanidinobenzimidazole (1). Tautomerism in the prepared compounds was investigated using nmr spectroscopy. Compounds 2a-p were found to be present in dimethyl sulfoxide solution predominantly as 3,4-dihyhydro tautomeric form. Compounds 4a-p existed in dynamic equilibrium of 1-, 12- and 13H -forms. It was found that methylation of 4a-d led to 13-methyl substituted derivatives 9a-d exclusively. [source] Mechanism of 4-methyl-1,2,4-triazol-3-thione reaction with formaldehydeJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 5 2008Monika Wujec Abstract We have recently described theoretically the mechanism of the reaction between 4-methyl-1,2,4-triazol-3-thiol and formaldehyde that leads to the N- substituted product, N1 -hydroxymethyl-4-methyl-1,2,4-triazol-3-thione. New experimental findings indicate that the thione tautomeric form in reaction with formaldehyde also yields this product. This observation could not be explained on the basis of previous calculations, which predicted that the thione tautomer undergoes nucleophilic substitution at the sulfur atom, leading to the S- substituted product. We present theoretical explanation of the observed reactivity. We show that under experimental conditions this reaction proceeds with the intervention of the anionic form of the triazole with the Gibbs free energy of activation of only 1.8,kcal/mol. Copyright © 2008 John Wiley & Sons, Ltd. [source] NMR and DFT investigations of the substituent and solvent effect on amino,imino tautomerism in acridin-9-amines substituted at the exocyclic nitrogen atom,JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 8 2005Youssif Ebead Abstract The 1H and 13C NMR spectra of 9-(methoxyamino)acridine (1) and 9-hydrazinoacridine (2) show that these compounds exist principally in the imino tautomeric form in CDCl3, acetone- d6, CD3CN, DMSO- d6 and Py- d5, all solvents with different polarities and abilities to participate in specific interactions. The spectra of the other two compounds investigated,N -(2-chloroethyl)acridin-9-amine (3) and N -(5-methylpyridin-2-yl)acridin-9-amine (4),indicate that they coexist in the amino and imino forms. The amino tautomer of compound 3 predominates in CDCl3, CD3CN and Py- d5 and that of compound 4 in CDCl3 and Py- d5. On the other hand, the amino and imino forms of compound 3 coexist in acetone- d6 and probably DMSO- d6, whereas those of compound 4 coexist in acetone- d6 and DMSO- d6. The positions of the signals in the NMR spectra compare qualitatively with those predicted computationally at the GIAO/DFT level of theory. The equilibrium constants predicted by the DFT(PCM) method are in agreement with the results of NMR spectral analysis. In general, both the data predicted at the DFT level of theory and x-ray structural data show that the imino tautomers display a ,butterfly'-type geometry, whereas the amino forms are characterized by an almost flat acridine moiety. Electron-attracting substituents at the exocyclic N atom improve the stability of the imino form, and electron-withdrawing substituents do likewise for the amino form. The importance of tautomeric phenomena in the context of the ability of acridin-9-amines to participate in specific interactions is outlined in brief, as are the possible applications of these compounds as probes of environmental properties. Copyright © 2005 John Wiley & Sons, Ltd. [source] Photochemical syn,anti Isomerization Reaction in N4 -Hydroxycytosine.PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 2 2001An Experimental Matrix Isolation, Theoretical Study ABSTRACT Infrared spectra of N4 -hydroxycytosine isolated in argon and nitrogen low-temperature matrixes are reported. The compound was found to adopt the syn structure of the imino-oxo tautomeric form exclusively. A photoreaction (, > 250 nm) converting this form into the anti isomer was observed. The reaction is reversible and leads to a photostationary point. The initial infrared spectrum and the spectrum of the photoproduct were assigned to the syn and anti isomers, respectively. This assignment is based on a close agreement between the experimental spectra and the spectra theoretically simulated at the DFT(B3LYP)/6-31++G** level of theory. [source] Structural investigations of isomeric oxidised forms of hyperforin by HPLC-NMR and HPLC-MSnPHYTOCHEMICAL ANALYSIS, Issue 5 2003J.-L. Wolfender Abstract The prenylated phloroglucinol hyperforin, thought to be an essential component for the anti-depressant activity of St. John's Wort (Hypericum perforatum), is unstable. The facile oxidative degradation of hyperforin poses serious problems for standardisation, and may also dramatically affect the pharmacological activity of the extracts. Hyperforin was dissolved in hexane and stored at room temperature for 3 days and yielded various closely related degradation products which, although dif,cult to isolate on the preparative scale, have been analysed by on-,ow and stop-,ow HPLC-NMR and HPLC-MS/MS. From on-line spectroscopic data, and with the aid of complementary in-mixture standard NMR two-dimensional correlation experiments, the different oxidised forms of hyperforin were found to be phloroglucinol derivatives in which a hydroxy-dihydrofuran ring is formed involving the enol OH at C-7 or C-9 (tautomeric form) and the prenyl chain at C-8 of the core nucleus of hyperforin. The strategy followed for the on-line identi,cation of these constituents is discussed. Copyright © 2003 John Wiley & Sons, Ltd. [source] (E)-2-[(4-Chlorophenyl)iminomethyl]-5-methoxyphenol and (E)-2-[(2-chlorophenyl)iminomethyl]-5-methoxyphenol: X-ray and DFT-calculated structuresACTA CRYSTALLOGRAPHICA SECTION C, Issue 10 2009ak Ko The crystal structures of the title 4-chlorophenyl, (I), and 2-chlorophenyl, (II), compounds, both C14H12ClNO2, have been determined using X-ray diffraction techniques and the molecular structures have also been optimized at the B3LYP/6-31 G(d,p) level using density functional theory (DFT). The X-ray study shows that the title compounds both have strong intramolecular O,H...N hydrogen bonds and that the crystal networks are primarily determined by weak C,H..., and van der Waals interactions. The strong intramolecular O,H...N hydrogen bond is evidence of the preference for the phenol,imine tautomeric form in the solid state. The IR spectra of the compounds were recorded experimentally and also calculated for comparison. The results from both the experiment and theoretical calculations are compared in this study. [source] The 1:1 complex of cytosine and 5-fluorouracil monohydrate revisitedACTA CRYSTALLOGRAPHICA SECTION C, Issue 7 2007Gustavo Portalone The monohydrated molecular adduct cytosine,5-fluorouracil,water (1/1/1) (denoted CytFur) [systematic name: 4-aminopyrimidin-2(1H)-one,5-fluoropyrimidine-2,4(1H,3H)-dione,water (1/1/1)], C4H5N3O·C4H3FN2O2·H2O, was determined some 40 years ago [Voet & Rich (1969). J. Am. Chem. Soc.91, 3069,3075] and is widely cited as the first example of an intermolecular complex between two pyrimidinic nucleobases. In view of the importance of these base associations, CytFur has been reinvestigated with modern laboratory equipment to higher precision and with the location and free refinement of the H atoms. The new experiment reaffirms the results of the original and clarifies the tautomeric form exhibited by the compounds. The asymmetric unit comprises a hydrogen-bonded adduct of the canonical amino,oxo tautomers in an exact 1:1 ratio and a water molecule of crystallization. This cyclic dimer forms a layered structure approximately parallel to the bc plane by joining through hydrogen bonds other such cyclic dimers. Disordered water molecules run through tunnels formed by surrounding molecular adducts along the a axis. [source] 3-Hydroxy-6-[(4-hydroxyphenylamino)methylene]cyclohexa-2,4-dienone and 2-hydroxy-6-[(4-hydroxyphenylamino)methylene]cyclohexa-2,4-dienoneACTA CRYSTALLOGRAPHICA SECTION C, Issue 6 2004ak Ko The title compounds, both C13H11NO3, exist as the keto,amine tautomers, and the formal hydroxyl H atoms, which display strong intramolecular hydrogen bonds, are located on the N atoms. This is a verification of the preference for the keto,amine tautomeric form in the solid state. The 2-hydroxy isomer has two independent molecules, with the molecules linked by intramolecular N,H,O and O,H,O and intermolecular O,H,O hydrogen bonds into three-dimensional networks. [source] C.I. Pigment Red 266ACTA CRYSTALLOGRAPHICA SECTION C, Issue 10 2003Chia-Hsien Chang C.I. Pigment Red 266, or 4-{[4-(aminocarbonyl)phenyl]hydrazono}- N -(2-methoxyphenyl)-3-oxo-3,4-dihydronaphthalene-2-carboxamide, C25H20N4O4, adopts the keto,hydrazone tautomeric form with significant intramolecular hydrogen bonding. The molecules pack to form layers involving an extensive network of intermolecular hydrogen bonds, in which the primary amide group plays a prominent role. The good technical performance of this pigment in application may be attributed principally to the pattern of intra- and intermolecular hydrogen bonding. [source] Adsorption of 6-mercaptopurine and 6-mercaptopurine-ribosideon silver colloid: A pH-dependent surface-enhanced Raman spectroscopy and density functional theory study.BIOPOLYMERS, Issue 6 2005Abstract Surface-enhanced Raman spectroscopy (SERS) has been applied to characterize the interaction of 6-mercaptopurine-ribose (6MPR), an active drug used in chemotherapy of acute lymphoblastic leukemia, with a model biological substrate at therapeutic concentrations and as function of the pH value. Therefore, a detailed vibrational analysis of crystalline and solvated (6MPR) based on Density Functional Theory (DFT) calculations of the thion and thiol tautomers has been performed. 6MPR adopts the thion tautomeric form in the polycrystalline state. The SERS spectra of 6MPR and 6-mercaptopurine (6MP) recorded on silver colloid provided evidence that the ribose derivative shows different adsorption behavior compared with the free base. Under acidic conditions, the adsorption of 6MPR on the metal surface via the N7 and possibly S atoms was proposed to have a perpendicular orientation, while 6MP is probably adsorbed through the N9 and N3 atoms. Under basic conditions both molecules are adsorbed through the N1 and possibly S atoms, but 6MP has a more tilted orientation on the silver colloidal surface while 6MPR adopts a perpendicular orientation. The reorientation of the 6MPR molecule on the surface starts at pH 8 while in the case of 6MP the reorientation starts around pH 6. Under basic conditions, the presence of the anionic molecular species for both molecules is suggested. The deprotonation of 6MP is completed at pH 8 while the deprotonation of the riboside is finished at pH 10. For low drug concentrations under neutral conditions and for pH values 8 and 9, 6MPR interacts with the substrate through both N7 and N1 atoms, possibly forming two differently adsorbed species, while for 6MP only one species adsorbed via N1 was evidenced. © 2005 Wiley Periodicals, Inc. Biopolymers 78: 298,310, 2005 This article was originally published online as an accepted preprint. The "Published Online" date corresponds to the preprint version. You can request a copy of the preprint by emailing the Biopolymers editorial office at biopolymers@wiley.com [source] Metal-Induced Tautomerization of p - to o -Quinone Compounds: Experimental Evidence from CuI and ReI Complexes of Azophenine and DFT StudiesCHEMISTRY - A EUROPEAN JOURNAL, Issue 1 2004Stéphanie Frantz Abstract Azophenine (7,8-diphenyl-2,5-bis(phenylamino)- p -quinonediimine, Lp) reacts with [Cu(PPh3)4](BF4) or [Re(CO)5Cl] to yield the (Ph3P)2Cu+ or [(OC)3ClRe] complex of the tautomeric form 7,8-diphenyl-4,5-bis(phenylamino)- o -quinonediimine, Lo, as evident from structure determinations and from very intense metal-to-ligand charge transfer (MLCT) transitions in the visible region. Time-dependent DFT (TD-DFT) calculations on model complexes [(N,N)Re(CO)3Cl] confirm the spectroscopic results, showing considerably higher oscillator strengths of the MLCT transition for the o -quinonediimine complexes in comparison to compounds with N,N=1,4-dialkyl-1,4-diazabutadiene. The complexes are additionally stabilized through hydrogen bonding between two now ortho -positioned NHPh substituents and one fluoride of the BF4, anion (Cu complex) or the chloride ligand (Re complex). DFT Calculations on the model ligands p -quinonediimine or 2,5-diamino- p -quinonediimine and their ortho -quinonoid forms with and without Li+ or Cu+ are presented to discuss the relevance for metal-dependent quinoproteins. [source] NMR Quantification of Tautomeric Populations in Biogenic Purine BasesEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 9 2009Bartl Abstract Purine bases such as purine, adenine, hypoxanthine, and mercaptopurine are known to exist in several tautomeric forms. Characterization of their tautomeric equilibria is important not only for predicting the regioselectivity of their N -alkylation reactions, but also for gaining knowledge of the patterns with which these compounds of significant biological activity form hydrogen bonds with their biological targets. The tautomeric equilibria of purine and some purine derivatives in methanol and N,N -dimethylformamide solutions were investigated by low-temperature 1H and 13C NMR spectroscopy. The N(7)H and N(9)H tautomeric forms were quantified by integrating the individual 1H NMR signals at low temperatures. The Gibbs free energy differences were calculated and the effects of substitution on the N(7)H/N(9)H ratio discussed. A previously published theoretically predicted mechanism of the tautomeric exchange is compared with our measurements in deuteriated solvents. The influence of concentration on the temperature of coalescence indicates that supramolecular clusters play a significant role in this proton transfer process. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source] Synthesis of Pyrazolyl-2-pyrazolines by Treatment of 3-(3-Aryl-3-oxopropenyl)chromen-4-ones with Hydrazine and Their Oxidation to Bis(pyrazoles)EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 22 2004Albert Lévai Abstract The synthesis of several 1-acetyl-3-aryl-5-[3-(2-hydroxyphenyl)pyrazol-4-yl]-2-pyrazolines 3a,3h has been accomplished by treatment of the 3-(3-aryl-3-oxopropenyl)chromen-4-ones 1a,h with hydrazine hydrate in hot acetic acid. The 1-acetyl-3-aryl-5-(3-chromonyl)-2-pyrazolines 2a,2f were also obtained as by-products. Oxidation of the 1-acetyl-4-pyrazolyl-2-pyrazolines 3a,3f with DDQ gave the 3(5)-aryl-5(3)-[3-(2-hydroxyphenyl)pyrazol-4-yl]pyrazoles 5a,5f. The oxidation of the 2-pyrazoline rings was accompanied by N -deacylation. The reaction mechanisms of both transformations are discussed, the first one being supported by experimental results. The structures of all new derivatives were established by NMR and the evidence of prototropic tautomerism is carefully discussed. Theoretical calculations of energies and of the 1H and 13C NMR chemical shifts of the possible tautomeric forms of 5(3)-[3-(2-hydroxyphenyl)pyrazol-4-yl]-3(5)-(4-methoxyphenyl)pyrazole (5c), by B3LYP and GIAO, showed that compounds of this type probably exist as mixtures of two tautomers. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source] Synthesis of 2,4-Diaryl-3,4-dihydro-2H - naphth[2,1- e][1,3]oxazines and Study of the Effects of the Substituents on Their Ring - Chain TautomerismEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 10 2004István Szatmári Abstract A number of 2-(,-amino-Y-substituted-benzyl)-1-naphthol hydrochlorides were prepared by a convenient Mannich-type aminoalkylation. 2,4-Diaryl-3,4-dihydro-2H -naphth[2,1- e][1,3]oxazines were prepared through the ring-closure reactions of the starting aminonaphthols with aromatic aldehydes, which proved to furnish three-component (ring1,open,ring2) tautomeric mixtures in CDCl3 at 300 K. The electronic effects of the 2-aryl groups on the ratios of the ring - chain tautomeric forms at equilibrium could be described by Equation (1). Study of the effects of substituents X and Y on the tautomeric equilibria [by the aid of the multiple linear regression analysis of Equations (2) and (3)] revealed that the trans -chain equilibrium constants are significantly influenced by the inductive effect (,F) of substituent Y on the 4-phenyl ring. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source] Structure Investigation of Bridgehead Aziridine: Synthesis, Theoretical, and Crystallographic Study of 2,4,6-Triphenyl-1,3-diazabicyclo[3.1.0]hex-3-eneHELVETICA CHIMICA ACTA, Issue 2 2006Giuseppe Bruno Abstract A one-pot three-component procedure to efficiently create the 1,3-diazabicyclo[3.1.0]hex-3-ene system is reported. The molecular structure of 2,4,6-triphenyl-1,3-diazabicyclo[3.1.0]hex-3-ene (3) was studied by X-ray diffraction and compared to ab initio and density-functional-theory (DFT) calculations restricted to the core moiety. Geometry optimizations for structural isomers and tautomeric forms of this aziridine fragment, taken as simplified models, were carried out at high calculation levels. Moreover, the same methods were utilized to evaluate the proton affinity of two crucial aziridine tautomers. [source] Intra and intermolecular hydrogen bonding in formohydroxamic acid,INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 1 2008Damanjit Kaur Abstract The presence of hydrogen bonding interactions in several tautomeric forms of formohydroxamic acid (FHA) and 1:1 association among the tautomeric forms and water-coordinated tautomeric forms of FHA is explored theoretically. Out of the seven equilibrium structures, four tautomeric forms have been selected for aggregation with single water molecule and dimer formation. Fifteen aggregates of FHA with H2O have been optimized at MP2/AUG-cc-PVDZ level and analyzed for intramolecular and intermolecular H-bond interactions. Twenty-seven dimers of the four tautomeric forms have been obtained at MP2/6-31+G* level. The stabilization energies associated with dimerization and adduct formation with water are the result of H-bond interactions and range from very weak to medium. The atomic charges and NBO analysis indicate that the electrostatic and the charge transfer are the important components favoring H-bond formation. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [source] DFT study of core-modified porphyrin isomersINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 1 2007Y. Soujanya Abstract B3LYP/6-311+G** calculations were performed systematically on 1,2 (syn) and 1,3 (anti) tautomeric forms of oxa- and thia- core-modified porphyrin isomers, which resulted in a total of 86 structures. The structural and energetic variation in all the isomers were analyzed. In corrphycene, hemiporphycene and porphycene the Z forms are more stable compared to the corresponding E forms in both the anti and syn oxa- and thiaporphyrin isomers. In contrast, in the syn isomeric forms of [3.0.1.0], [3.1.0.0] and [4.0.0.0] oxaporphyrins and in both syn and anti forms of thiaporphyrin isomers, Z forms are less stable. The HOMO and LUMO values are both negative and varied in a narrow zone, indicating no dramatic effect on the position of heteroatom substitution on the redox properties. The effect of geometric constraints due to the alteration of meso-bridge length and the hetero atom disposition in the porphyrin core on the relative stabilities of the isomers is analyzed. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [source] Gas-phase theoretical prediction of the metal affinity of copper(I) ion for DNA and RNA basesJOURNAL OF MASS SPECTROMETRY (INCORP BIOLOGICAL MASS SPECTROMETRY), Issue 3 2003Nino Russo Abstract The most stable tautomeric forms of free DNA and RNA bases were considered as substrates for the interaction of Cu+ ion. Several suitable attachment sites were selected that involved mono- and bi-coordination of the cation. B3LYP/6,311 + G(2df,2p) bond energies showed that copper ion has the major affinity for guanine and cytosine bases. The proposed values of Cu+ ion affinity are 59.9, 60.0, 80.2, 88.0 and 69.0 kcal mol,1 for uracil, thymine, cytosine, guanine and adenine, respectively. The preference for the mono- or bi-coordination depends on the particular tautomer for each base. Copyright © 2003 John Wiley & Sons, Ltd. [source] Restricted rotation/tautomeric equilibrium and determination of the site and extent of protonation in bi-imidazole nucleosides by multinuclear NMR and GIAO-DFT calculationsJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 3 2005Jukka Mäki Abstract The restricted rotation about the conjoining bond in a series of 4,-substituted bi-imidazole nucleosides {5-amino-4-[4,-R-imidazol-2,-yl]-1-(,- d -ribofuranosyl)-1H -imidazole; where R=H, methyl, hydroxymethyl, oxalo, formyl} due to intramolecular hydrogen bonding between N-3, and the N-6 protons concomitant with prototropic tautomerism has been examined using multinuclear (1H, 13C and 15N) experimental NMR. Substitution at the 4, position causes the interconversion, whilst still an intramolecular process, to yield complex spectra as the dynamic process consists of a two-site exchange between non-degenerate tautomeric forms (asymmetric sites). The preferred tautomers were identified experimentally in each case and compared with theoretically determined structures geometry optimized using density functional theory (DFT) at the B3LYP/6,31G(d,p) level of theory on which gauge-independent atomic orbital-DFT (GIAO-DFT) computations at the B3LYP/cc-pVTZ level of theory were applied to calculate the chemical shifts of the 1H, 13C and 15N nuclei. Both the site and the extent of protonation of the bi-imidazole nucleosides were also similarly ascertained using the same methodology. Protonation at the pyridine-type nitrogen (N-3,) of the outer imidazole ring, the principle site of protonation, effectively eliminated the barrier to rotation about the conjoining bond yielding time-averaged spectra experimentally. Copyright © 2004 John Wiley & Sons, Ltd. [source] Role of 2-oxo and 2-thioxo modifications on the proton affinity of histidine and fragmentation reactions of protonated histidine,RAPID COMMUNICATIONS IN MASS SPECTROMETRY, Issue 17 2010Adrian K. Y. Lam A combination of electrospray ionisation (ESI), multistage and high-resolution mass spectrometry experiments was used to compare the gas-phase chemistry of the amino acids histidine (1), 2-oxo-histidine (2), and 2-thioxo-histidine (3). Collision-induced dissociation (CID) of all three different proton-bound heterodimers of these amino acids led to the relative gas-phase proton affinity order of: histidine >2-thioxo-histidine >2-oxo-histidine. Density functional theory (DFT) calculations confirm this order, with the lower proton affinities of the oxidised histidine derivatives arising from their ability to adopt the more stable keto/thioketo tautomeric forms. All protonated amino acids predominately fragment via the combined loss of H2O and CO to yield a1 ions. Protonated 2 and 3 also undergo other small molecule losses including NH3 and the imine HN=CHCO2H. The observed differences in the fragmentation pathways are rationalised through DFT calculations, which reveal that while modification of histidine via the introduction of the oxygen atom in 2 or the sulfur atom in 3 does not affect the barriers against the loss of H2O+CO, barriers against the losses of NH3 and HN=CHCO2H are lowered relative to protonated histidine. Copyright © 2010 John Wiley & Sons, Ltd. [source] Cocrystallization of two tautomers: 4-(1-{[4-(dimethylamino)benzylidene]hydrazono}ethyl)benzene-1,3-diol and 6-[(E)-1-{[4-(dimethylamino)benzylidene]hydrazino}ethylidene]-3-hydroxycyclohexa-2,4-dien-1-one (1/1)ACTA CRYSTALLOGRAPHICA SECTION C, Issue 4 2009Zhen-Hua Wu Two different tautomeric forms of a new Schiff base, C17H19N3O2·C17H19N3O2, are present in the crystal in a 1:1 ratio, namely the enol,imine form 4-(1-{[4-(dimethylamino)benzylidene]hydrazono}ethyl)benzene-1,3-diol and the keto,amine form 6-[(E)-1-{[4-(dimethylamino)benzylidene]hydrazino}ethylidene]-3-hydroxycyclohexa-2,4-dien-1-one. The tautomers are formed by proton transfer between the hydroxy O atom and the imine N atom and are hydrogen bonded to each other to form a one-dimensional zigzag chain along the crystallographic b axis via intermolecular hydrogen bonds. [source] Multinuclear magnetic resonance, electrospray ionization,mass spectroscopy, and parametric method 5 studies of a new derivative of gossypol with 2-thiophenecarbohydrazide as well as its complexes with LI+, Na+, K+, RB+, and Cs+ cationsBIOPOLYMERS, Issue 3 2006Piotr Przybylski Abstract A new derivative of racemic gossypol with 2-thiophenecarbohydrazide (GHHT) and its complexes with monovalent cations have been synthesized and studied by electrospray ionization,mass spectroscopy (ESI-MS), multinuclear nuclear magnetic resonance (NMR), as well as by the Parametric Method 5 (PM5) methods. It is demonstrated that GHHT forms stable complexes of 1:1 stoichiometry with monovalent metal cations. The structures of the complexes are stabilized by three types of intramolecular hydrogen bonds. The spectroscopic methods have provided clear evidence that GHHT and its complexes exist in the DMSO-d6 solution in the N-imine,N-imine tautomeric forms. The structures of the GHHT and its complexes with Li+, Na+, K+, Rb+, and Cs+ cations are visualized and discussed in detail. © 2006 Wiley Periodicals, Inc. Biopolymers 83: 213,225, 2006 This article was originally published online as an accepted preprint. The "Published Online" date corresponds to the preprint version. You can request a copy of the preprint by emailing the Biopolymers editorial office at biopolymers@wiley.com [source] Phosphino-Aminothiazoline Platinum(II) and Platinum(II)/Gold(I) Complexes: Structural, Chemical and Vapoluminescent PropertiesCHEMISTRY - A EUROPEAN JOURNAL, Issue 36 2007Roberto Pattacini Dr. Abstract Phosphino-amino-thiazolines and -thiazoles can exist in solution in two tautomeric forms, in which the NH proton involves the endo-cyclic or exo-cyclic nitrogen atom. The two tautomers show different reactivities toward alcoholysis; the imino form degrades more rapidly. Their bischelated platinum complexes were studied in the solid state by single crystal X-ray diffraction. Thus, the unique stereoelectronic features of the [Pt(PNth)] (PNth=diphenylposphino-aminothiazoline) moiety were revealed. The complex cis -[Pt(PNth)2] reacts with gold(I) salts to yield dimetallic compounds, the molecular structures of which have been determined by X-ray diffraction. Solid cis -[Pt(PNth)2] shows vapoluminescent properties if exposed to alcohol vapors. A combined photophysical and crystallographic investigation has been carried out to clarify the unprecedented rigidochromic role of the alcohol in this phenomenon. [source] The Tautomeric Forms of Cyameluric Acid Derivatives,CHEMISTRY - A EUROPEAN JOURNAL, Issue 4 2007Nadia Abstract The tautomerism of cyameluric acid C6N7O3H3 (1,a), cyamelurates and other heptazine derivatives has recently been studied by several theoretical investigations. In this experimental study we prepared stannyl and silyl derivatives of cyameluric acid (1,a): C6N7O3[Sn(C4H9)3]3 (3,a), C6N7O3[Sn(C2H5)3]3 (3,b), and C6N7O3[Si(CH3)3]3 (4). In order to investigate the structure of 1,a the mono- and dipotassium cyamelurate hydrates K(C6N7O3H2),2,H2O (5) and K2(C6N7O3H),1,H2O (6) were synthesized by UV/Vis-controlled titration of a potassium cyamelurate solution with aqueous hydrochloric acid. Compounds 3,6 were characterized by FTIR and solid-state NMR spectroscopy as well as simultaneous thermal analysis (TGA, DTA). The single crystal X-ray structures of the salts 5 and 6 show that the hydrogen atoms in both anions are localized on the peripheral nitrogen atoms. This indicates,in combination with the solid-state NMR studies,that the most stable tautomer of solid 1,a is the triketo form with C3h symmetry. However, derivatives of both the hydroxyl and the amido tautomers may be formed depending on the substituent atoms: The spectroscopic data and single crystal structures of compounds C6N7O3[Si(CH3)3]3 (4) and the solvate C6N7O3[Sn(C2H5)3]3,C2H4Cl2 (3,b,) show that the former is derived from the symmetric trihydroxy form of 1,a, while 3,b, crystallizes as a chain-like polymer, which contains the tin atoms as multifunctional building blocks, that is, bridging pentacoordinated Et3SnO2 and Et3SnON units as well as non-bridging four-coordinated Et3SnN units. The cyameluric nucleus is part of the polymeric chains of C6N7O3[Sn(C2H5)3]3,C2H4Cl2 (3,b,), by the action of both tautomeric forms of cyameluric acid, the amide and the ester form. [source] | |||||||||||||||||||||||||