Mechanisms on electrical breakdown strength increment of polyethylene by acetophenone and its analogues addition: a theoretical study

A theoretical investigation is completed on the mechanism of electrical breakdown strength increment of polyethylene. It is shown that it is one of the most important factors for increasing electrical breakdown strength of polyethylene through keto-enol isomerization of acetophenone and its analogues at the ground state S₀ and the lowest triplet state T₁. The minimum structures and transition states of the keto- and the enol-tautomer of acetophenone and its analogues at the S₀ and T₁ states are obtained at the B3LYP/6-311+G(d,p) level, as well as the harmonic vibration frequencies of the equilibrium geometries and the minimum energy path (MEP) by the intrinsic reaction coordinate (IRC) theory at the same level. The two C–C bond cleavage reaction channels have been identified in acetophenone. The calculated results show that the energy barriers of keto-enol isomerization of acetophenone and its analogues at S₀ and T₁ states are much smaller than the average C-C bond energy of polyethylene, and the acetophenone doping or bond linked into polyethylene can increase the electrical breakdown strength and inhibit polyethylene electrical tree initiation and aging.

Theoretical investigation of tautomerism in N-hydroxy amidines

A DFT study with QST3 approach method is used to calculate kinetic, thermodynamic, spectral and structural data of tautomers and transition state structures of some N-hydroxy amidines. All tautomers and transition states are optimized at the B3LYP/6-311++g** and B3LYP/aug-cc-pvtz level, with good agreement in energetic result with energies obtained from CBS-QB3, a complete basis set composite energy method. The result shows that the tautomer a (amide oxime) is more stable than the tautomer b (imino hydroxylamine) as is reported in the literature. In addition, our finding shows that, the energy difference between two tautomers is only in about 4–10 kcal/mol but the barrier energy found in traversing each tautomer to another one is in the range of 33–71 kcal/mol. Therefore, it is impossible to convert these two tautomers to each other at room temperature. Additionally, transition state theory is applied to estimate the barrier energy and reaction rate constants of the hydrogen exchange between tautomers in presence of 1–3 molecules of water. The computed activation barrier shows us that the barrier energy of solvent assisted tautomerism is about 9–20 kcal/mol and lower than simple tautomerism and this water-assisted tautomerism is much faster than simple tautomerism, especially with the assisting two molecules of water.     

Mismatch discrimination of base-modified nucleic acids and their constituents: non-Watson-Crick base pairing induced by tautomerization

The effect of tautomerism on the mismatch discrimination was studied on 7-deazapurine and 8-aza-7-deazapurine analogues of isoguanosine. 7-Halogenated 7-deaza-2'-deoxyisoguanosines show better base pair discrimination than 2'-deoxyisoguanosine due to the more favored keto tautomer formation. 8-Aza-7-deazaisoguanosine and its 7-halogeno derivatives also show higher keto tautomer population than that of isoguanosine, but the 7-halogens do not bias the tautomeric equilibrium significantly as it is observed for the 7-deaza-2'-deoxyisoguanosine derivatives.     

Optical stability of gossypol

The optical stability of gossypol [1,1',6,6',7,7'-hexahydroxy-3,3'-dimetyl-5,5'-bis(1-methylethyl)-2,2'-binaphthalene-8,8'-dicarboxaldehyde], a natural product exhibiting profoundly enantiospecific antitumor and male antifertility action, was investigated by means of computational methods and thermal racemization experiments. The calculations on gossypol and several derivatives and model compounds were carried out using the MM2 force field; energies and geometries of minimum energy conformations, as well as structures along various inversion pathways, were calculated. According to the calculations, gossypol (the dialdehyde form) and its simple analogues are not thermally racemizable (energy barriers for rotational inversion above 50 kcal/mol). By contrast, the calculations suggest that the acetal tautomer of gossypol and its dehydration product (anhydrogossypol) are thermally racemizable, although the energy barriers are still relatively high (35–40 kcal/mol). Optically pure (+)-anhydrogossypol was prepared and characterized; its racemization became rapid only at high temperatures (180–200°C). When dehydration of gossypol was hindered (in aqueous solution), no racemization of gossypol was observed after prolonged heating at 90°C. © 1992 Wiley-Liss, Inc.     

Studies on tautomeric forms of Guanine-Cytosine base pairs of nucleic acids and their interactions with water molecules

The relative stabilities of Guanine-Cytosine (G-C) DNA bare base pairs, its tautomeric forms and microhydrated base pairs are theoretically investigated with a focus on the keto-enol tautomerism as well as on the cis-trans isomerism using ab initio and density functional theory methods. The stabilities of the G-C bare base pairs, its tautomeric forms and microhydrated base pairs were affected by various factors including keto-enol tautomerization, cis-trans enol isomerization, and steric hindrance between the base pair and water molecules. The Atoms in Molecules theory (AIM) is employed to investigate H-bonding patterns both in bare and microhydrated base pairs. From the above topological results, an excellent linear correlation is shown between electron density [rho(r)], and its Laplacian [V2rho(r)] at the bond critical points. NBO analysis has been carried out to study the charge transfer between proton acceptor to the antibonding orbital of the X-H bond both in bare and microhydrated base pairs.     

Quantum chemical investigation of intramolecular thione-thiol tautomerism of 1,2,4-triazole-3-thione and its disubstituted derivatives

The one step intramolecular thione-thiol tautomerism of 1,2,4-triazole-3-thione and its disubstituted derivatives has been studied through the use of electronic structure methods. Due to the absence of experimental data for the parent molecule of 1,2,4-triazole-3-thione the structure and energetics of aforementioned tautomers were derived using various basis sets and levels including HF, B3LYP, and MP2 methods. The gas phase results show that in all different levels of theory the most stable tautomer is the thione form. It has also been revealed that B3LYP/6-31G(d,p) level is quite well suited and reliable to investigate these kinds of tautomerism. To account the influence of substituents on the mentioned tautomerization, the tautomerism and conformational properties as well as vibrational analysis of 20 halophenyl and isopyridyl derivatives were investigated using B3LYP/6-31G(d,p) calculations. In all cases the calculations indicate that substituents have no considerable effects on relative stabilities and energy barriers for the thione-thiol proton transfer and the thione forms are the predominant species in the gas phase. In order to figure out the relative stabilities of the species involved in the tautomerism, geometrical and natural bond orbital (NBO) analyses have been employed. It has also been shown that the computed vibrational frequencies of tautomers with different scaling factors could be used to interpret the vibrational frequencies in IR spectrum of similar species.     

Computational study of dynamic motions and possibility of crossing between surfaces in 3-hydroxy-5-(1H-pyrrol-2-yl)-2H-pyrrol-2-one

Potential energy surfaces (PESs) for tautomerism and two dynamic motions of 3-hydroxy-5-(1H-pyrrol-2-yl)-2H-pyrrol-2-one and its tautomer were calculated using density functional theory. Calculated energies confirm that T1 is 10.95 kJ mol^? 1 more stable than T2. Dynamic study of possible motions shows the high energy-level transition state at D8 = 90° for ring rotation and at D15 = 80° and 90° for rotation of OH bond, respectively, in T1 and T2. In addition, calculated rate constant for conversion of T1 to T2 (tautomerism) is 116 M^? 1 s^? 1, for relative rotations of rings in T1 and T2 are, respectively, 5.62 × 10^? 2 and 1.53 M^? 1 s^? 1 and for rotations of OH bond in T1 and T2 are, respectively, 3.56 × 10⁵ and 6.15 × 10⁴ M^? 1 s^? 1. In the next part of the study, orbital occupancies, natural bond orbitals (NBO) charges and hybridisation in relative rotation of rings and internal reaction coordinate (IRC) steps have been extracted to study the possibility of level crossing. These data show that PESs for IRC and ring rotation have different symmetries. So that these two potential curves cannot have effective non-adiabatic level crossing. However, the levels are weakly avoided or the possibility of level crossing is higher than related system because the energy difference between their barrier energies is not very high.     

Ionization behaviour and tautomerism-dependent lipophilicity of pyridine-2(1H)-one cardiotonic agents

The acid-base properties of pyridine-2(1H)-one derivatives, analogues of the cardiotonic agent milrinone, were studied by capillary zone electrophoresis (CZE). Electrophoretic mobility and pH data were fitted to equilibrium expressions and apparent dissociation constants (pKa) calculated by non-linear regression. Compared with the ultraviolet (UV) spectrophotometric method and potentiometric titrations, the CZE technique showed advantages, such as rapidity and applicability to compounds that are sparingly soluble in water. Based on the pKa values, intramolecular electronic interactions were assessed. The lipophilicity of a number of derivatives was also examined, by determining their n-octanol/water distribution coefficients over a wide pH range, and found to be significantly affected by 2-pyridone/2-hydroxypyridine tautomerism. As revealed by a comparison between experimental and calculated log P values, electron withdrawing substituents, especially at the C(6) position of 2-pyridone, favour the less polar hydroxypyridine tautomers both in water and octanol. Our results indicate that the positive inotropism of milrinone-related compounds could be explained taking ionization and tautomerism into account.     

Surprising structural lability of a cysteine-S-conjugate precursor of 4-methyl-4-sulfanylpentan-2-one, a varietal aroma in wine of Vitis vinifera L. cv. Sauvignon blanc

4-Methyl-4-sulfanylpentan-2-one (1; 4MSP) provides a characteristic aroma compound of wines made from Vitis vinifera L. cv. Sauvignon blanc. 4MSP has a strong box-tree odor with a very low perception threshold and is derived from the cysteinylated precursor S-(1,1-dimethyl-3-oxobutyl)cysteine (4; P-4MSP). P-4MSP is transformed into 4MSP during alcoholic fermentation and is an excellent marker of varietal aroma potential. An improved synthesis of P-4MSP as well as of its deuterium-labeled analogue [D(6)]-P-4MSP is described. Several analytical methods (NMR, IR, LSI-MS, GC/MS, ESI-MS(n)) were combined to elucidate spontaneous reversible structural changes of P-4MSP at different pH values. At low pH, P-4MSP has a linear keto form. The keto-enol tautomerism was observed at neutral pH. At pH 8, the formation of N-substituted intramolecular hemiaminal was characterized by ESI-MS and ESI-MS(n) experiments. The hemiaminal loses H(2)O at high pH to produce a cycloimine, which is easily opened by acid hydrolysis. The keto-enol tautomerism explained the incorporation of only six D-atoms during the preparation of the P-4MSP deuterated standard even if [D(10)]mesityl oxide was used. Derivatization conditions for GC/MS analysis strongly affected the ratio of the monosilylated intramolecular cyclic form and the disilylated linear form of P-4MSP. The structural changes of P-4MSP may have a considerable impact on the development of methods of measuring varietal aroma potential.     

A predictive thermodynamic model for the bioreduction of acetophenone to phenethyl alcohol using resting cells of Saccharomyces cerevisiae.

Equilibrium conversions were observed in the range of 60.2-76.0% with different initial compositions of reaction media for the bioreduction of acetophenone using resting cells of Saccharomyces cerevisiae in aqueous solutions at 30 degrees C. The reduction of acetophenone in the cells under anaerobic conditions is considered to be coupled with the oxidation of ethanol to acetate in the cytoplasm. A biphasic thermodynamic model is proposed which includes a nonuniform distribution of reagents across the cell membrane, a transmembrane pH gradient, ideal and nonideal solution models, and a basic reaction stoichiometry (ACP + (1/2) EtOH + (1/2)H2O <--> PEA + (1/2)Ac- + (1/2)H+). The intracellular activity coefficients were based on the Lewis-Randall rule for acetophenone, phenethyl alcohol, and H2O and Henry's law for ethanol, acetate anion, and H+. The overall standard Gibbs free energy was estimated to be -0.11 kcal/mol at a pH 7, 25 degrees C, and 1 atm. The intracellular thermodynamic activity coefficients of acetophenone and phenethyl alcohol were predicted to be 471.2 and 866.4, respectively, using the measured initial distribution coefficients and calculated extracellular activity coefficients. The model reflected a zero Gibbs free energy change at calculated conversions within 4% of the measured equilibrium conversions. The analysis verified the effect of the concentration ratio of the substrate acetophenone to the co-substrate ethanol on the conversion efficiency and suggested that the intracellular pH and the pH gradient across the cell transmembrane significantly affect the predicted equilibrium conversion. The intracellular pH of resting, viable cells of Bakers' yeast at the bioconversion conditions was determined experimentally to be 5.77.     

Mechanisms on electrical breakdown strength increment of polyethylene by aromatic carbonyl compounds addition: a theoretical study

A theoretical investigation is accomplished on the mechanisms of electrical breakdown strength increment of polyethylene at the atomic and molecular levels. It is found that the addition of aromatic carbonyl compounds as voltage stabilizers is one of the important factors for increasing electrical breakdown strength of polyethylene, as the additives can trap hot electrons, obtain energy of hot electrons, and transform the aliphatic cation to relatively stable aromatic cation to prevent the degradation of the polyethylene matrix. The HOMO-LUMO energy gaps (E _g), the ionization potentials (IPs), and electron affinities (EAs) at the ground states of a series of aromatic carbonyl compounds are obtained at the B3LYP/6-311+G(d,p) level. The theoretical results are in good agreement with the available experimental findings, show that 2,4-dioctyloxybenzophenone (Bzo) and 4,4'-didodecyloxybenzil (Bd) molecules can effectively increase the electrical breakdown strength when they are doped into polyethylene because of their much smaller E _g values than all the other studied aromatic carbonyl molecules and excellent compatibility with polymers matrix.     

The base-catalyzed keto-enol tautomerism of chrysophanol anthrone. A DFT investigation of the base-catalyzed reaction

The keto-enol tautomerization of chrysophanol anthrone was systematically investigated via a series of density functional theory computations using the M06-2X/6-31?+?G(d,p) level of theory. Bulk solvent effects were taken into account implicitly using the polarisable continuum model and explicitly with one pyridine molecule. Moreover, the tautomeric equilibrium was estimated by calculating the equilibrium constant. The pyridine-assisted tautomerism was found to be the energetically preferred pathway, where the pyridine acts as a proton carrier through a two-step reaction, in which the first one corresponds to the deprotonation of the chrysophanol anthrone and the second step to the protonation yielding the enol form. The results show, in accordance with experimental reports, that in the process of dissociation of the protonated pyridine and the aryloxylate anion, the ion is quite susceptible to oxidation. The highly dipolar structures of the transition states and intermediates are stabilised by pyridine up to 7.0?kcal/mol. The non-covalent interactions stabilising the transition states and the intermediates were analysed. Additionally, an intermolecular hydrogen bonding between the enol form of chrysophanol anthrone and the pyridine is formed at the last stage of the reaction. The keto-enol equilibrium analysis shows that the enol form of chrysophanol anthrone must be present in small amounts in basic solvents.     

Enantioselective reduction of acetophenone and its derivatives with a new yeast isolate Candida tropicalis PBR-2 MTCC 5158

The enantioselective bioreduction of acetophenone and its various analogues has been carried out using a new yeast strain, Candida tropicalis MTCC 5158, to obtain the corresponding (S)-aryl ethanols with good yield and almost absolute enantioselectivity. The catalytic ability of this microbial strain for acetophenone reduction has been examined and also various parameters of the bioreduction reaction have been optimized. Studies on the catalytic performance showed that this microorganism is capable of carrying out the reduction in a broad range of pH (3-10) and temperature (25-40 degrees C), making it a more versatile biocatalyst. The preparative scale bioreduction of acetophenone using resting cells of Candida tropicalis yielded S-(-)-1-phenyl ethanol with 43% yield and >99% enantiomeric excess.     

An explanation of the induction of mutations by 2-aminopurine from an ab initio molecular orbital study

The molecular mechanism of induction of mutations by 2-aminopurine (AP) was studied by an ab initio molecular orbital method. Cytosine (C) is converted to its disfavored imino tautomer more easily than AP, judging from the calculated total energies of the bases and the base analogue. This suggests that a stable AP:C base mispair via two hydrogen bonds can be formed with the imino tautomer of C. These results stress the importance of the imino form of C in AP-induced mutagenesis and support the 'trigger mechanism', in which formation of one hydrogen bond between AP and C is considered to stimulate the tautomeric shift of AP or C. The calculated relative stabilities of various base pairs and mispairs were in good agreement with experimental findings.     

Consequences of one-electron oxidation and one-electron reduction for 4-aminopyrimidine—DFT studies

The consequences of one-electron oxidation and one-electron reduction were studied for 4-aminopyrimidine (4APM), which displays prototropic tautomerism. Since experimental techniques are incapable of detecting less than 0.1% of minor tautomers, quantum-chemical calculations [DFT(B3LYP)/6-311+G(d,p)] were carried out for all possible tautomers of neutral 4AMP and its redox forms, 4APM (+ ?) and 4APM (- ?). Four tautomers were considered: one amine and three imine tautomers (two NH and one CH form). Geometric isomerism of the exo?=?NH group was also taken into account. One-electron oxidation (4APM - e → 4APM (+??)) has no significant effect on the tautomeric preferences; it influences solely the composition of the tautomeric mixture. The amine tautomer is favored for both 4APM (+??) and 4APM. An interesting change in the tautomeric preference occurs for 4APM (- ?). One-electron reduction (4APM?+?e → 4APM (- ?)) favors the C5 atom for the labile proton. The preference of the imine CH tautomer in the tautomeric mixture of 4APM (- ?) may partially explain the origin of CH tautomers in nucleobases.     

Conformational study of palindromic tripeptides (GPG, IPI and KPK) in HIV-1 protease--a density functional theory study

A comparative study has been carried out on three palindromic tripeptides Gly-Pro-Gly, Ile-Pro-Ile and Lys-Pro-Lys which were present in HIV protein along with their analogues applying density functional computation at B3LYP/6-31G* level of theory. Discrepancy from the structural analysis has been noted for all the systems and it was found to be more for amide capped structure at the C terminal of proline. The puckering amplitude A and Phase angle P of the pyrrolidine ring of proline in the chosen palindromic tripeptides and their analogues were calculated from the endocyclic torsion angles. The minimum energy conformers lying well within the prescribed region of proline were obtained for the derived compounds from potential energy surface scan mentioning that no role has been played by its terminal residues. This is further supported by the simulated amide bands identifying the helical structure for all three palindromic tripeptides signifying the importance of proline. The molecular properties such as stabilization energy, chemical hardness along with dipole moment were calculated and interpreted. The values of Calpha-H(s) and the peptide backbone N-Calpha-CO for all the selected conformers specify the three palindromic tripeptides to have a symmetrical achiral structure.     

Multiple resonance damping or how do trees escape dangerously large oscillations?

To further understand the mechanics of trees under dynamic loads, we recorded damped oscillations of a Douglas fir (Pseudotsuga menziesii) tree and of its stem without branches. Eigenfrequencies of the branches were calculated and compared to the oscillation frequency of the intact tree. The term eigenfrequency is used here to characterize the calculated resonance frequency of a branch fixed at the proximal end to a solid support. All large branches had nearly the same frequency as the tree. This property is a prerequisite for the distribution of mechanical energy between stem and branches and leads to an enhanced efficiency of damping. We propose that trees constitute systems of coupled oscillators tuned to allow optimal energy dissipation.     

Phloretin keto-enol tautomerism and inhibition of glucose transport in human erythrocytes (including effects of phloretin on anion transport).

Under various pH conditions phloretin demonstrates keto-enol tautomerism with a pK value of 7.26 +/- 0.06. As Wilbrandt has shown ((1950) Arch. Exp. Pathol. Pharmacol. 212, 9-29) phloretin added to erythrocytes inhibits glucose efflux, but not glucose influx. At pH 6.5 a Ki value of 0.36 and at pH 9 of 22.7 microM was measured; only the ketonic form of phloretin contributes to the inhibition of glucose efflux. This was also the case for inhibition of galactose efflux and anion exchange. The geometry optimization of a large number of conformations of the ketonic and enolic forms of phloretin demonstrates different shapes of the molecules. Only the ketonic form shows several overlapping structures with beta-D-glucopyranose. Considering surplus binding of phloretin under glucose efflux conditions as being equivalent to the number of glucose transporters, a number of about 200,000 molecules was determined. By two independent methods 210,000 and 171,000 molecules per cell were calculated. This result is in close agreement with the number of glucose transporter sites of the erythrocyte.     

Crystallographic studies on damaged DNAs. II. N(6)-methoxyadenine can present two alternate faces for Watson-Crick base-pairing, leading to pyrimidine transition mutagenesis

In a previous paper, 2'-deoxy-N(6)-methoxyadenosine (mo(6)A) was shown to form a mismatch base-pair with 2'-deoxycytidine with a Watson-Crick-type geometry. To fully understand the structural basis of genetic mutations with damaged DNA, it is necessary to examine whether the methoxylated adenine residue still has the ability to form the regular Watson-Crick pairing with a thymine residue. Therefore, a DNA dodecamer with the sequence d(CGCGmo(6)AATTCGCG) has been synthesized and its crystal structure determined. The methoxylation has no significant effect on the overall DNA conformation, which is that of a standard B-form duplex. The methoxylated adenine moieties adopt the amino tautomer with an anti conformation around the C(6)-N(6) bond to the N(1) atom, and they form a Watson-Crick base-pair with thymine residues on the opposite strand, similar to an unmodified adenine residue. It is concluded that methoxylated adenine can present two alternate faces for base-pairing, thanks to the amino<-->imino tautomerism allowed by methoxylation. Based on this property, two gene transition routes are proposed.