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Change of Prototropic Equilibria for Uracil when Going from Neutral Molecule to Charged Radicals. Quantum-Chemical Studies in the Gas Phase

Raczyńska E. D., Zientara K., Kolczyńska K., Stępniewski T. M.

Polish Journal of Chemistry

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2009

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Vol. 83, nr 5

821-834

EN

To understand various substitution, oxidation or degradation reactions for uracil, quantum-chem i calcalculations were performed for two classes of charged radicals (cations and anions) of uracil (U+ź and U-ź) and model compounds: phenol (P+ź and P-ź) and hydroxyazines (HP1-4+ź and HP1+-4-ź). In calculations, all possible eighteen prototropic tautomers-rotamers of U and all possible five prototropic tautomers-rotamers of P and HP1-4 were considered. Stabilities, internal effects and aromatic character, estimated for charged radicals, were compared with those observed previously for neutral molecules. The great est changes of the tautomeric preferences take place for radical anions. One-electron reduction stabilizes the keto and amide functions in studied compounds, whereas one-electron oxidation favors the enol function for ph nol and the amide function for uracil and hydroxyazines.

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Consequences of Covalent and Non-Covalent Interactions on Conformational Preferences and pi-Electron Delocalization for the Substrate (Pyruvate) and Inhibitor (Oxamate) of Lactate Dehydrogenase - DFT Studies

Raczyńska E. D., Hallmann M., Duczmal K.

Polish Journal of Chemistry

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2008

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Vol. 82, nr 5

1077-1090

EN

Conformational preferences and pi-electron delocalization for the isolated substrate (pyruvate) and inhibitor (oxamate) of lactate dehydrogenase (LDH), their protonated forms, adducts with alkali metal cations, and complexes with imidazole and/or the guanidinium cation (models for His195 and Arg171 of the LDH pocket, respectively) have been investigated at the DFT(B3LYP)/6-31++G(d,p) level. Covalent and non covalent bonds in acids, adducts and complexes cause greater conformational changes for pyruvate than for oxamate. Delocalization of pi electrons for the OCO frag ments is similar for similar structures. For isolated anions, adducts and complexes, the HOMED(OCO) values are between 0.9 and 1.0. For protonated forms, the HOMED(OCO) val ues are re duced to 0.55-0.75. Delocalization of pi electrons for the CCO fragment in pyruvate and for the NCO fragment in oxamate in creases when the anions form covalent and non-covalent bonds with other ions and molecules. The HOMED(CCO) values vary from 0.1 to 0.5 forsigma-pi conjugated pyruvate, whereas the HOMED(NCO) values vary from 0.9 to 1.0 for n-pi conjugated oxamate.

3

Tautomeric Equilibria and pi-Electron Delocalization for the Substrate (Pyruvate) and Inhibitor (Oxamate) of Lactate Dehydrogenase - DFT Studies

Raczyńska E. D., Duczmal K., Hallmann M.

Polish Journal of Chemistry

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2007

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Vol. 81, nr 9

1655-1666

EN

Keto-enol tautomerism for the substrate (pyruvate) of lactate dehydrogenase (LDH) and amideiminol tautomerism for its inhibitor (oxamate) were studied at the DFT(B3LYP)/6-31++G(d,p) level. Both anions (and also both radicals) prefer the C=O forms, i.e. the keto and amide form, respectively. The OH forms (enolpyruvate and iminoloxamate) have higher Gibbs free energies. Their amounts in the tautomeric mixtures are larger than 0.01% for anions and lower than 0.001% for radicals. pi-Electron delocalization for OCO fragments is greater than that for XCO fragments for both anions and radicals.

4

Tautomeric Equlibria (1,5-Proton Shift) in Simple Heteroconjugated Systems XH-A=B-C=Y X=A-B-CYH in Relation to Electron Delocalization

Ośmiałowski B., Raczyńska E. D., Gawinecki R.

Polish Journal of Chemistry

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2005

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Vol. 79 / nr 6

1093-1097

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Variations of Tautomeric Preferences in Histamine Monocation-Ab initio Studies for "Essential" and "Scorpio" Conformations from the Gas Phase to Aqueous Solution

Darowska M., Raczyńska E. D.

Polish Journal of Chemistry

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2002

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Vol. 76 / nr 7

1027-1035

EN

The polarizable continuum model (PCM) for geometries optimized at the RHF/6-31G* was applied to study the variations of the tautomeric preferences in the histamine monocation from the gas phase to aqueous solution. Seven solvents of different polarities (from cyclohexane to water) were chosen and calculations performed. A change of the tautomeric preference takes place already in apolar solvents containing heteroatoms. The ring N-aza protonated form (ImH+) is only favoured in the gas phase and cyclohexane,oe benzene, CCl4. The chain N-amino protonated form (AmH+-T1) predominates in other solvents: CHCl3, THF, acetone, water.

6

Wewnętrzna solwatacja a reakcje przeniesienia protonu w fazie gazowej dla związków azotowych

Raczyńska E.D.

Wiadomości Chemiczne

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2000

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[Z] 54, 1-2

67-86

EN

Intramolecular hydrogen bonding which can be formed in the neutral or ionic forms of polyfunctional compounds strongly influences the simple proton-transfer reactions and tautomeric equilibria in the gas phase. This effect, called internal solvation increases the proton affinities of nitrogen ligands, such as proton sponges with a rigid structure, and diamines, amidinamines, bipyridines and amidinazines with a flexible conformation by 20-100 kJ/mol in comparison to monofunctional or model compounds. In consequence diamines and amidinamines belong to superbases (PA>1000 kJ/mol). In aqueous solutions, only proton sponges are strong bases. Due to strong steric effects, water molecules cannot open the ionic NźźźH-N+ bridge in the proton sponges. Bidentate nitrogen ligands with a flexible conformation behave in solution similarly as monofunctional derivatives. External solvation by water molecules eliminates the NźźźH-N+ bridge, changes conformation of the monocation (from cyclic to linear), reduces polarizability of the chain, and thus decreases the basicity. For tautomerizing azoles, the internal solvation changes the pK(T) values by more than 2 pK(T) units in comparison to derivatives without this interaction. Application of the Taft and Topsom equation to series of 4(5)-substituted imidazoles indicates that the internal solvation in the gas phase has similar influence on the transmission of substituent effects as the external solvation in water.

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Tautomeric Preferences in Isolated 2- and 4-Hydroxypyridines: Theoretical (AM1) Analysis of Structural (Internal) Effects

Raczyńska E. D.

Polish Journal of Chemistry

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2000

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Vol. 74, nr 9

1283-1283

EN

Prototropic tautomerism in fifty monosubstituted 2- and 4-hydroxypyridines was studied by theoretical calculations. Analysis of internal effects was performed and partial structural effects are discussed.

8

Zastosowanie spektrometrii masowej cyklotronowego rezonansu jonowego do badania reakcji przeniesienia protonu w fazie gazowej

Raczyńska E.D.

Wiadomości Chemiczne

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1998

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[Z] 52, 5-6

385-403

EN

The principle of ion cyclotron resonance (ICR) reported in 1930 by Lawrence et al. [9] was firstly applied in mass spectrometry by Hipple, Sommer and Thomas in 1949 [26]. Their instrument has never been commercilized due to several problems with electronics and vacuum. Next instrument constructed by Wobschall et al. In the 1960s [27] had more chamce. It was modified by Llewellyn [28] in cooperation with Baldeschwieler et al. [29] and became a commercial Varian mass spectrometer in the late 1960s. Its drift cell contained three separated regions: ion source, analyser and ion collector. Important modification in ICRMS was proposed by McIver in 1970 [10a, b]. He introduced a one region trapped-ion analyzer cell and pulsed mode of operation. First FT-ICR experiments were carried out by Comisarow and Marshall in 1974 [11]. They also applied a one region trapped-ion analyzer cell. The pulsed mode of operation in the one region cell was combined with Fourier transform techniques in first commercial Nicolet FT-ICR mass spectrometer. From this moment the FT-ICR mass spectrometry has become more attractive and more frequently used by chemists. Since ions may be trapped for extended periods prior to detection in an ICR cell, both, ICR and FT-ICR mass spectrometry have been used to the study of gas-phase ion-molecule reactions [1-6]. First experimental gas-phase data of sufficient precision obtained by ICRMS became available from the 1970s for the proton-transfer reactions [2, 7, 8, 10, 41-43]. Relative acidity (DGA) or basicity (DGB) determinations were based on measurements of the equilibrium constants of proton exchange between two anions (A-1 and A-2) or two bases (B1 and B2) [7, 8, 13a]. These reactions have been studied in order to determine intrinsic gas-phase acidities or basicities of compounds and to establish general gas-phase acidity/basicity scale. Presently there are few thousands gas-phase data compiled in this scale by Lias et al. [50]. The upper limit of the GB scale is separated from the lower limit of the GA scale by ca 150 kJ/mol (Fig. 6), and thus spontaneous neutralization reactions between neutral acids and bases can not yet be observed in the gas phase.

9

Comparison of hydrogen bonding with gas-phase and broensted basicity of N(1),N(1)-dimethylformamidines. Substituent effects

Raczyńska E.D.

Polish Journal of Chemistry

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1998

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Vol. 72, nr 7

1215-1227

EN

Substituent effects observed for series of N(1),N(1)-dimethylformamidines in the hydrogen bonding reaction (as delta Delta G-HB with 4-fluorophenol) in non polar solvent (CCl4) have directly been compared with those found in the proton transfer reaction in the gas phase (as delta Delta GB) and polar solvent (as delta Delta G-alc in azeotropic ethanol). Significant differences between the aryl and alkyl subfamilies are observed. For aryl groups almost linear relations are found. There are only slight differences between the transmission of substituent field/inductive and resonance effects to the reaction centre in the hydrogen bonding and proton transfer reactions. For alkyl (simple alkyl, heteroalkyl and arylalkyl) groups no good relations are found. Hydrogen bonding basicity of alkyl groups depends on substituent steric, polarizability and field/inductive effects. Gas-phase basicity depends on substituent polarizability and field/inductive effects and Broensted basicity depends mainly on substituent field/inductive effect.