A theoretical examination of the acid-catalyzed and noncatalyzed ring-opening reaction of an oxirane by nucleophilic addition of acetate. Implications to epoxide hydrolases

Ab initio and density functional calculations have been performed to gain a better understanding of the epoxide ring-opening reaction catalyzed by epoxide hydrolase. The S(N)2 reaction of acetate with 1S,2S-trans-2-methylstyrene oxide to provide the corresponding diol acetate ester was studied with and without general-acid catalysis. MP2 and DFT (B3LYP) calculations predict, for the noncatalyzed reaction, a central barrier of approximately 20-21 kcal/mol separating the reactants from products depending on which carbon center in the epoxide is undergoing attack. From these gas-phase reactions the immediate alkoxide products are not energetically far below their associated transition states such that the reaction is predicted to be endothermic. Inclusion of aqueous solvation effects via a polarizable continuum model predicts the activation barrier to increase by almost 10 kcal/mol due to the solvation of the acetate ion nucleophile. The activation barrier for the epoxide ring-opening reaction is reduced to approximately 10 kcal/mol when phenol, as the general-acid catalyst, is included in the gas-phase calculations. This is due to the immediate product being the neutral ester rather than the corresponding alkoxide. The transition state in the general-acid-catalyzed reaction is earlier than that for the noncatalyzed reaction and the reaction is highly exothermic. Molecular mechanics calculations of 1S,2S-trans-2-methylstyrene oxide in the active site of murine epoxide hydrolase show two possible binding conformations. Both conformers have the epoxide oxygen forming hydrogen bonds with the acidic hydrogens of the catalytic tyrosines (Tyr381 and Tyr465). These two conformations likely lead to different products since the nucleophile (Asp333-CO(2)(-)) is positioned to react with either carbon center in the epoxide.     

Mathematical Modeling of Kinetics of Adenosine 5"-Triphosphate Hydrolysis Catalyzed by the Zn2+ Ion in the pH Range 7.4–8.3

Kinetic data on adenosine triphosphate (ATP) hydrolysis catalyzed by the Zn²⁺ ion in the pH range 7.4–8.3 are analyzed by the method of numerical simulation. The rates of forward and reverse reactions of isomeric conversion of the open conformation of ZnATP^2– (Op), which is inactive in hydrolysis to ADP, to the active cyclic conformation ZnATP^2– (Cy) in the specified range of pH are proportional to the concentration of H₃O⁺ and characterized by the same rate constants as in the range of pH above 8.5. The mechanism of the isomeric conversion Op Cy involves the formation of a pentacovalent state at -P, pseudorotation, and the abstraction of OH^– from -P of the pentacovalent intermediate with the participation of H₃O⁺ in a slow step. The sequence of steps for the formation and transformation of intermediates, which was established earlier for the ZnATP^2– associates in the pH range 7.1–7.4, is applicable to this range of pH as well. In the analyzed range of pH, the contributions from the pH-independent channel of hydrolysis of the ZnATP^2– associates and the pH-dependent channel of CyOH^– and Op(OH^–)₂ species, which determine the formation of ADP and AMP at pH > 8.5, are comparable. Changes in the concentrations of intermediate products (monomeric and associates) in the course of hydrolysis are described. General base catalysis by a nitrogen base in the steps of formation of active centers for hydrolysis, the general acid catalysis of a coordinated water molecule, the exchange of medium OH^– with OH of -phosphate, the catalysis of conversion of the inactive conformation ZnATP^2– to the active one by a proton, and a change of the rate-limiting stage of hydrolysis with a change in pH indicate the enzyme-like mechanism of the reaction.     

Hypochromic and hyperchromic effects on the vibronic structure of molecular crystal spectra. The dimer model

It is shown on model calculations that crystal-field mixing of Frenkel excition states in molecular crystals lead not only to a redistribution of their relative intensities but also to changes in the vibrational structure of weak exciton bands.     

The microdetermination of stannous tin with N-bromosuccinimide

A new titrimetric method for the microdetermination of stannous tin, e.g., stannous chloride is proposed. The mechanism of the reaction in the presence of dilute hydrochloric acid is discussed. The determination of stannous tin is done on concentrations ranging from 10 mg to 100 μg. The experimental error does not exceed ±2%. Comparative analysis of tin sheets, rods, and foils by the proposed method and the iodine method is reported. The method has been shown to be suitable for the determination of tin in canned foods.     

Acetylenic/cyanoacetylenic complexes: simulation of the Titan’s atmosphere chemistry

The structures and energies of the 1:1 acetylene/cyanoacetylene, acetylene/dicyanoacetylene and cyanoacetylene/dicyanoacetylene complexes in solid argon matrices have been investigated using FT-IR spectroscopy and ab initio calculations, at the B3LYP/6-31G** level of theory. For the three complexes, predicted frequency shifts for the L shaped structures, characterized by a hydrogen bond between the nitrogen of the cyano group and the acetylenic proton, were found to be in good agreement with those experimental. Only in the case of acetylene/cyanoacetylene complex, we obtained a second minimum with a T shaped structure characterized by an interaction between the proton of cyanoacetylene and the Π system of acetylene. It appears clearly that HC₃N acts as an electrophile or as a nucleophile in these complexes.     

Furan acetal compounds

A method was developed for the preparation of amino derivatives of 1,3-dioxanes of the furan series by amalgam reduction of the corresponding nitro compounds. This method makes it possible to obtain amino compounds without destruction of the furan and dioxane rings. 2--Aminofuryl-5-alkyl-5-methylol- and 5--aminofuryl-5,5-bis(chloromethyl)-1,3-dioxanes and their azomethines were synthesized for the first time.See [5] for communication III.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 440–442, April, 1971.     

Spin correlations in the resonance reactionsāa→ Ψ(Z) → bb

A study is made of the processes of annihilation of an arbitrarily polarized e^–e⁺ pair into a pair of particles of spin 0, 1/2, 1, and 3/2 in the (Z)-resonance region.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 53–59, October, 1978.     

Effect of nature of support and impregnating agent on lipophilicity determination for nonionic surfactants by reversed-phase thin-layer chromatography

Summary The retention of 9 nonylphenyl ethylene oxide oligomers was determined in 15 reversed-phase chromatographic systems using silicone oils of various molecular mass as impregnating agents and silica, cellulose and alumina supports. The data were evaluated with principal component analysis carried out on the covariance and the correlation matrices. The R_M values did not follow the additivity rule and they did not change linearly with increasing length of the ethylene oxide chain. This is probably due to the folded state of the chain in the eluent. The molecular mass of the silicone oils had a negligible effect on the retention whereas the retention increased with increasing level of impregnation. The support considerably influenced the lipophilicity values of these nonionic surfactants and cellulose seemed to be the most appropriate support. Calculations proved that the application of a correlation matrix may cause data distortion; therefore, the use of a covariance matrix is strongly proposed.