Proton transfer at the carboxylic sites of amino acids: A single water molecule catalyzed process

Ab initio calculations at MP2 level of theory were used to study the proton transfer at the carboxylic sites of amino acids, in the isolated, mono‐ and di‐hydrated forms. In the case of water dimer, two interaction modes with glycine neutral structures (see Fig. 3 ) were explored, corresponding to the concerted and stepwise reaction pathways. Their transition states can be described as (H₂O? H? OH₂)⁺ [Fig. 4 (a)] and (H₂O‐‐‐H? OH₂)⁺ [Fig. 4 (b)], respectively. The energy analysis indicated that the concerted pathway is preferred. In the isolated, mono‐ and di‐hydrated glycine complexes, the activation barriers of the proton transfer at the carboxylic sites were calculated to be 34.49, 16.59, and 13.36 kcal mol^?1, respectively. It was thus shown that the proton transfer is significantly assisted and catalyzed by water monomer so that it can take place at room temperature. Instead, the further addition of water molecules plays solvent effects rather than catalytic effects to this proton transfer process. The above results obtained with discrete water molecules were supported by the solvent continuum calculated data. It was also observed that the heavy dependence of the solvent continuum models on dipole moments may produce misleading results. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Selective preparation of terminal alkenes from aliphatic carboxylic acids by a palladium-catalysed decarbonylation-elimination reaction

Trialkylamines were used as additives in the decarbonylation-elimination reaction catalysed by the combination of palladium(II) chloride and DPE-Phos. Aliphatic carboxylic acids were transformed at relatively low temperature into terminal alkenes in high yield and high selectivity, without the need for distillation, thereby avoiding isomerisation.     

A catalyst system for the reaction of carboxylic acids with aliphatic isocyanates

Catalysts have been found for the selective reaction of aliphatic isocyanates with carboxylic acids giving amides after carbon dioxide extrusion. Magnesium and calcium salts lead to a dramatic increase in reaction rates while improving the selectivity when sterically hindered isocyanates and/or carboxylic acids are used.     

用从头算研究Shilov反应的机理

Shilov反应在CH~4活化中占有中心地位,它有氧化加成和σ迁移两种可能的机理。本文用较大基组的从头算研究了这两种机理的反应过程,认为Shilov反应应按氧化加成机理进行。     

Ab initio study of ice catalyzation of HOCl + HCl reaction

Experiments proved that the reaction of HOCl + HCl was very slow in the gas phase, but on ice surface it was rapid. In this work the ice catalysis of HOCl + HCl reaction was investigated by using ab initio molecular orbital theory. We applied the Hartree–Fock self‐consistent field and the second‐order Møller–Plesset perturbation theory with the basis sets of 6‐31G* to the model system. The complexes and transition state were obtained along the reaction with and without the presence of ice surface. By comparing the results, a possible catalyzation mechanism of ice on the reaction is proposed. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 78: 281–284, 2000     

Selective esterification of non-conjugated carboxylic acids in the presence of conjugated or aromatic carboxylic acids over active carbon supported methanesulfonic acid

Non-conjugated carboxylic acids are selectively esterified in good yields in the presence of conjugated or aromatic carboxylic acids by stirring over active carbon supported methanesulfonic acid in di-chloromethane at room temperature.     

Possible involvement of collective domain movement in the catalytic reaction of soluble epoxide hydrolase

The hydrolysis of 1S,2S‐trans‐methylstyrene oxide by soluble epoxide hydrolases is studied by a 4‐ns molecular dynamics (MD) simulation. An analysis of the extent of correlated motions in the active site was carried out. Based on the calculated cross correlation coefficients form the covariance matrix, a new correlation parameter, termed the supercorrelation coefficient, is introduced. The supercorrelation coefficient indicates the extent to which two amino acid residues move in a correlated manner with respect to all other residues in the protein. The resulting map of the supercorrelation coefficients was used to identify segments of the protein that may show collective domain movements. Interestingly, an anti‐correlated motion is located across the active site, involving the catalytic triad and the tyrosines. This may suggest that if a link exists between enzyme dynamics and catalysis, it may be through an anti‐correlated collective domain movement that compresses the active site, thus initiating the conversion of E–NAC to E–TS. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004     

Theoretical study of the full reaction mechanism of human soluble epoxide hydrolase

The complete reaction mechanism of soluble epoxide hydrolase (sEH) has been investigated by using the B3LYP density functional theory method. Epoxide hydrolases catalyze the conversion of epoxides to their corresponding vicinal diols. In our theoretical study, the sEH active site is represented by quantum-chemical models that are based on the X-ray crystal structure of human soluble epoxide hydrolase. The trans-substituted epoxide (1S,2S)-beta-methylstyrene oxide has been used as a substrate in the theoretical investigation of the sEH reaction mechanism. Both the alkylation and the hydrolytic half-reactions have been studied in detail. We present the energetics of the reaction mechanism as well as the optimized intermediates and transition-state structures. Full potential energy curves for the reactions involving nucleophilic attack at either the benzylic or the homo-benzylic carbon atom of (1S,2S)-beta-methylstyrene oxide have been computed. The regioselectivity of epoxide opening has been addressed for the two substrates (1S,2S)-beta-methylstyrene oxide and (S)-styrene oxide.     

Ab initio reaction path for cisplatin interaction with L‐cysteine and L‐methionine

Ab initio calculations are used to track the reaction pathway of interaction between cisplatin and the sulfur‐containing amino acids cysteine (Cys) and methionine (Met). Structures of all reactive species as well as thermodynamic and kinetic properties were calculated and discussed based on the role played by the level of theory. Twenty‐three different levels of theory were examined including HF, DFT, and perturbation theory at MP2 and MP4(SDQ) orders. The rate constant for a second‐order associative ligand exchange mechanism (k₂) was calculated by means of transition state theory. This quantity is quite sensitive to small fluctuation of activation free energy, therefore is a good benchmark to assess the performance of different methods of calculations. The k₂ values predicted by DFT methods were in best agreement with experiment, found equal to (10²k₂ in M^?1 s^?1) 3.42 for Met (PBE1PBE) and 1.90 for Cys (B3P86). The experimental values are 3.6 and 2.2 for Met and Cys, respectively. The solvent effect plays a primary role to the kinetic properties, accounting for ～30% of the activation Gibbs free energy. The outcomes from the present study promptly show the adequacy of distinct theoretical approaches to describe the reactivity of cisplatin, thus might be useful for further studies involving other Pt(II) complexes. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Retention modelling of electrostatic and adsorption effects of aliphatic and aromatic carboxylic acids in ion-exclusion chromatography

The retention mechanism of aliphatic and aromatic carboxylic acids in ion-exclusion chromatography has been investigated with consideration of simultaneous electrostatic repulsion effects and hydrophobic adsorption effects. A mathematical relationship between the retention factor of the analyte and the mobile-phase composition (sulfuric acid concentration and percentage of methanol), the type of analyte (pK_a and hydrophobicity) and some physical characteristics of the stationary phase has been derived. Thirteen carboxylic acids (comprising mono- and divalent, aliphatic and aromatic acids) were chosen and used to acquire retention data on three different cation-exchange stationary phases (in which the sulfonate functional groups are bound to polystyrene–divinylbenzene, polymethacrylate or silica) using 14 mobile-phase compositions of varying pH and percent methanol. These retention data were used to derive the parameters necessary to solve the retention model using non-linear regression. In this way, a quantitative measure of the effects of adsorption phenomena on analyte retention were obtained. The model was then used to optimise the separation of nine carboxylic acids.     

Application of zirconium-modified silica gel as a stationary phase in the ion-exclusion chromatography of carboxylic acids II. Separation of aliphatic carboxylic acids with pyromellitic acid as eluent and with suppressed conductimetric detection

The application of zirconium-modified silica gels (Zr-Silica) as stationary phases for ion-exclusion chromatography with conductimetric detection (IEC–CD) for C₁–C₈ aliphatic carboxylic acids (formic, acetic, propionic, butyric, valeric, caproic, heptanoic and caprylic acids) was carried out using pyromellitic acid as the eluent. Zr-Silicas were prepared by the reaction of the silanol group on the surface of silica gel with zirconium tetrabutoxide [Zr(OCH₂CH₂CH₂CH₃)₄] in ethanol solution. An ASRS-Ultra anion self-regenerating suppressor in the K⁺ form was used for the enhancement of conductimetric detector response of these aliphatic carboxylic acids. A Zr-Silica adsorbed on 10 mg zirconium g⁻¹ silica gel was the most suitable stationary phase in IEC–CD for the separation of these aliphatic carboxylic acids. Excellently simultaneous separation and highly sensitive detection for these aliphatic carboxylic acids were achieved in 25 min by IEC–CD with the Zr-Silica column (250×4.6 mm I.D.) and a 0.2 mM pyromellitic acid containing 0.15% heptanol as the eluent.     

An ab initio MO study on the direct and migratory reaction mechanism for the formation of HF in H+ClF system

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Computational study of the reaction of the methylsulfonyl radical,CH3S(O)2, with NO2

The mechanism of the reaction between the methylsulfonyl radical, CH₃S(O)₂, and NO₂ is examined using density functional theory and ab initio calculations. Two stable association intermediates, CH₃SNO₂ and CH₃S(O)ONO, may be formed through the attack of the nitrogen or the oxygen atom of NO₂ radical to the S atom. Interisomerization and decomposition of these intermediates are investigated using high level energy methods and specifically, CCSD(T), CBS‐QB3, and G3//B3LYP. The computational investigation indicates that the lowest energy reaction pathway leads to the products CH₃S(O)₃ + NO, through the decomposition of the most stable association adduct CH₃S(O)ONO. This result fully supports the relevant assumption of Ray et al. (Ray et al., J. Phys. Chem. 1996, 100, 8895], on which the experimental evaluation of the rate constant was based, namely that CH₃S(O)₃ + NO are the most probable products of the reaction CH₃S(O)₂ + NO₂. © 2014 Wiley Periodicals, Inc.     

离子排斥色谱用于厌氧消化处理水中脂肪族羧酸的分析（英文）

The analysis of seven aliphatic carboxylic acids(formic,acetic,propionic,iso-butyric,n-butyric,iso-valeric and n-valeric acid) in anaerobic digestion process waters for biogas production was examined by ion-exclusion chromatography with dilute acidic eluents(benzoic acid,perfluorobutyric acid(PFBA) and sulfuric acid) and non-suppressed conductivity/ultraviolet(UV) detection.The columns used were a styrene/divinylbenzene-based strongly acidic cation-exchange resin column(TSKgel SCX) and a polymethacrylate-based weakly acidic cation-exchange resin column(TSKgel Super IC-A/C).Good separation was performed on the TSKgel SCX in shorter retention times.For the TSKgel Super IC-A/C,peak shape of the acids was sharp and symmetrical in spite of longer retention times.In addition,the mutual separation of the acids was good except for iso-and n-butyric acids.The better separation and good detection was achieved by using the two columns(TSKgel SCX and TSKgel Super IC-A/C connected in series),lower concentrations of PFBA and sulfuric acid as eluents,non-suppressed conductivity detection and UV detection at 210 nm.This analysis was applied to anaerobic digestion process waters.The chromatograms with conductivity detection were relatively simpler compared with those of UV detection.The use of two columns with different selectivities for the aliphatic carboxylic acids and the two detection modes was effective for the determination and identification of the analytes in anaerobic digestion process waters containing complex matrices.     

锗烯与乙烯环加成反应的理论研究

用RHF/6-31G^*解析梯度方法研究了单重态锗烯与乙烯环加成反应的机理,用二级微扰方法对各构型的能量进行了相关能校正,并用统计热力学方法和过渡态理论计算了该反应在不同温度下的热力学函数的变化和动力学性质。结果表明,此反应历程由两步组成:1)锗烯与乙烯生成了一中间配合物,是一无势垒的放热反应,2)中间配合物异构化为产物锗杂环丙烷,此步势垒经零点能校正后为26.9kJ.mol^-^1(MP2/6-31G^*//6-31G^*);从热力学和动力学的综合角度考虑,该反应在200-300K温度下进行为宜,如此,反应既有较大的自发趋势和平衡常数,又具有较快的反应速率。     

Derivatization of carboxylic acids with 9-bromomethylacridine using micellar phase-transfer catalysis

Summary Micellar phase-transfer catalysis (MPTC) offers the opportunity to derivatize carboxylic acids directly in an aqueous matrix without prior extraction of the acids into a suitable aprotic solvent. The currently developed MPTC system consists of a non-ionic surfactant, Arkopal N-130, an ion-pair agent, tetrakis-(decyl)-ammonium bromide, and a novel fluorescence reagent, 9-bromomethylacridine. The MPTC system can be applied to the derivatization of many types of carboxylic acids. The reaction rate is affected by the lipophilicity of the acid and by the presence of other functional groups. For lipophilic carboxylic acids the reaction is complete within 5 min at 60°C and pH 7.0.     

The dimerization of radical anions of aromatic carboxylic acids competing with the self-protonation reaction

Quantum-chemical calculations (CNDO/2) of the theoretical relationship between the rate constants for the dimerization and self-protonation of radical anions show that dimer formation in the one-electron electroreduction of aromatic carboxylic acids (benzoic (1), 1-naphthoic (2), and 9-anthroic (3) is most probable for1. It is established that during the constant potential electrolysis (CPE) of1 a mixture of “head-to-tail” dimers is formed in the presence of 0.1M Bu₄NClO₄ (DMF). Their ratio depends on the amount of electricity passed through the solution. The CPE of 2 in the presence of 20 % H₂O affords 1,4-dihydro-1-naphthoic acid in up to 70 % yield. The high yield (∼70 %) of 9,10-dihydro-9-anthroic acid during the CPE of 3 can be accounted for by the decomposition of the dimeric product followed by protonation of the anionic species. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1735–1738, October, 1993.     

硅烯与甲醛环加成反应的理论研究

本文用RHF/6-31G^*解析梯度方法研究了单重态硅烯与甲醛环加成反应的机理,用二级微扰方法对各构型的能量进行了相关能校正,并用统计热力学方法和过渡态理论计算了该反应在不同温度下的热力学函数的变化和动力学性质。结果表明,此反应历程由两步组成:1)硅烯与甲醛生成一中间配合物,是一无势垒的放热反应,2)中间配合物异构化为产物,此步势垒经零点能校正后只有51.4kJ·mol^-^1(MP2/6-31G^*//6-31G^*);从热力学和动力学的综合角度考虑,该反应在300～400K温度下进行为宜,如此,反应既有较大的自发趋势和平衡常数,又具有较快的反应速率。     

Studies on the Kinetics of the FO CO Reaction

IntroductionAproposedcycleinvolvingtheFOradicalsisl:ThecontributionofthiscycletothecatalyticdestrUctionofozoneisdeterndnedbythefirststep.Thereaction(l)isalsothoughttobeapossiblesinkforatInosphericCO2.Baueretal.3gavearateconstantofl.24.lO-i3cm3/sforreaction(l),whichwasbelievedtobecorrectwithinafactorof2at9(X)-l4(X)K.Theactivationenergywasestimatedtobe46.OkJ/mol.Bedzhanyanetal.4reportedk<4xlO-"and<5xlO-l6cm3/sat3ooand55OK,respectively.Alowerbarrierofl3.8kJ/molcanbeextrapoIated.Francisco…