摘要:

Past and recent experimental results on the formation or hydrolysis (or isomerization) of various acetals and ketals including α-and β-glycoside models are presented. Ab initio investigations of simple acetals are also briefly reviewed as well as recent experimental and theoretical support evidence for a synperiplanar effect. A detailed study using the semi-empirical Hamiltonian AM1 defining the reaction pathway in the hydrolysis of various acetals and ketals is reported. This overall study shows that the hydrolysis of acetals and ketals is controlled by powerful stereoelectronic effects.

ISSN:0008-4042    

DOI:10.1139/v94-258

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

The crystal structures of three tricyclic quinoxalinedione derivatives, 6-bromo-1,8-ethano-4-hydro-2,3-quinoxalinedione (1), 6-methyl-1,8-ethano-4-hydro-2,3-quinoxalinedione hydrate (2), and 6-styryl-1,8-ethano-4-hydro-2,3-quinoxalinedione (3), are reported. For1and2, the space groups areP21/nwith the unit cell parameters for1:a = 7.4003(5) Å,b = 8.5799(5) Å,c = 14.3127(9) Å, β = 90.639(6)°, and for2:a = 7.0590(2) Å,b = 10.7483(3) Å,c = 13.9509(7) Å, β = 103.290(3)°. For3, the space group isP21/c, witha = 19.3683(10) Å,b = 8.0962(16) Å,c = 19.5801(16) Å, β = 114.028(6)°. Compound3crystallizes with two molecules in the asymmetric part of the unit cell; in one of them the styryl group is disordered. The geometries of the 1,8-ethano-4-hydro-2,3-quinoxalinedione fragments are similar in all observations, with the differences mainly caused by the different nature of the substituents in the 6-position. Hydrogen bonds connect the molecules into three-dimensional networks. Head-to-tail π-stacking between molecules connected by a center of symmetry determines the packing modes in1and2but there is no π-stacking in the crystal structure of3. The crystal structures of the three quinoxaline derivative ligands for the glycine receptor suggest a mode of recognition that involves anreceptorhydrogen bond, a three-centre hydrogen bond to the neighboring carbonyl groups on the ligand, and π-stacking between ligand and receptor. This mode is consistent with the geometric constraints of the current binding site model but places greater emphasis on hydrogen-bond interactions.

ISSN:0008-4042    

DOI:10.1139/v94-259

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

The influence of 254-nm irradiation on the dissolution rate of magnetite (Fe3O4) and maghemite (γ-Fe2O3) suspended in aqueous solutions of malonic acid is analyzed and compared with the dark reaction. The effect of the addition of Fe3+, reductants such as ascorbic acid and oxidants such as Ag+or O2is described. Photochemical initiation involves the production of >FeIIby electrons photogenerated on the oxide, which triggers thermal dissolution. Experimental results are fitted by analytical and numerical solutions of the set of kinetic equations. The calculated fitting parameters reflect the lower activity of iron oxides in malonic acid compared to EDTA media.

ISSN:0008-4042    

DOI:10.1139/v94-260

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

The syntheses of Pr(ClO4)3, Pr(OTf)3(OTf = CF3SO3), Pr(ClO4)3•Cl2O6, and NO2Pr(ClO4)4are described. Vibrational spectroscopy, X-ray powder diffraction, and EXAFS (for Pr(ClO4)3) are used to characterize the structure of the complexes. [ClO4] and [OTf] appear as tridentate in Pr(ClO4)3and Pr(OTf)3and Pr is 9-coordinated. In Pr(ClO4)3•Cl2O6, chlorine trioxide is included in the lattice of Pr(ClO4)3and shows the ionic ↔ covalent equilibrium. In NO2Pr(ClO4)4, Pr(ClO4)4−retains the structure of Pr(ClO4)3, the extra [ClO4] is weakly bonded to Pr•Pr(ClO4)3crystallizes in the monoclinic system (P21/m),a = 8.608;b = 11.827;c = 7.416 Å; β = 106.29°, andV = 724.79 Å3. Distances Pr—O = 2.51 Å and Pr—Cl = 3.90 Å are determined by EXAFS. Strong fluorescence phenomena are observed when Pr(ClO4)3is excited with the 488 nm line of an ionized Ar lase

ISSN:0008-4042    

DOI:10.1139/v94-261

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

This paper reports the complete set of minimum energy (i.e. stable) conformations of For-Ala-Ala-NH2(containing three peptide bonds), computed byabinitiogeometry optimization. A more confidentstructureandrelativeenergycomparisonof the 49 different relaxed geometries was possible using the HF/3-21G method as compared to empirical (force field) methods. The analysis of the different conformers resulted in a useful structure database, incorporating 30 different β-turns, which will have some relevance to the description of peptide folding. Spectroscopic (e.g. NMR) consequences of the reportedabinitiocalculations are also discussed, and a 3D-structure assignment method for proteins is provided.

ISSN:0008-4042    

DOI:10.1139/v94-262

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

Transacetalization equilibrium constants can be measured in methanol solution. This allows a ladder of equilibrium constants to be constructed from acetophenone, for which the equilibrium constant for acetal formation has been measured in methanol, to methyl formate, for which it has not. This is the first direct measurement of an equilibrium constant for formation of an acetal of an acyclic ester. We have also determined equilibrium constants for acetal formation from benzophenone and pinacolone. The value for benzophenone is inconsistent with the value reported for the diethyl acetal by Pfeiffer and Adkins (G.J. Pfeiffer and H. Adkins. J. Am Chem. Soc.53, 1043 (1931)). We must conclude that their analytical method was subject to systematic errors.

ISSN:0008-4042    

DOI:10.1139/v94-263

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

The water-promoted pathway of peptide cleavage by carboxypeptidase A has been studied by semiempirical (AM1) quantum mechanical calculations. A relatively large model for the CPA-active site elements plus substrate has been designed, using two imidazoles and one acetate as the Zn2+ligands, acetate as the proton acceptor (simulating Glu-270), andN-ethylacetamide as the peptide-like substrate. This model, although simpler than the natural one, is one of the largest used for theoretical calculations on CPA catalytic mechanisms. To ensure that this model is able to mimic the natural system, it has been compared with the structure of the (Gly)3-L-Tyr + water + CPA complex resulting from several molecular dynamics/energy minimization simulations. Among the different steps involved in the water-promoted pathway proposed by Lipscomb's group, the attack of the oxygen atom that comes from the activated water molecule to the carbon atom of the peptide bond of the substrate has been found to be the rate-determining step, with a high enthalpy barrier of 37.9 kcal/mol. However, this enthalpy barrier is dramatically decreased when a positive charge, simulating Arg-127, is included near the scissile carbonyl. The reported results seem to favour the occurrence of the mechanism studied and indicate the limitations of using simple elements for the theoretical analysis of enzyme-catalyzed reactions.

ISSN:0008-4042    

DOI:10.1139/v94-264

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

As models for the ionization of orthoesters, a series of 1,3-dioxan-2-ylium salts have been prepared and characterized. The properties of 1,3-dioxan-2-ylium hexachloroantimonate1, 5-ethoxymethyl-1,3-dioxan-2-ylium hexachloroantimonate2, 2-methyl-5-ethoxymethyl-1,3-dioxan-2-ylium hexachloroantimonate3, and 2-phenyl-5-ethoxymethyl-1,3-dioxan-2-ylium hexachloroantimonate4, have been studied by1H and13C NMR spectroscopy. The structures of3and4have been determined using X-ray crystallography. Salts3and4both crystallize in monoclinic crystal systems. For3, the space group isC2/c, witha = 30.192(4) Å,b = 9.613(2) Å,c = 12.742(2) Å, β = 109.71(1)°,V = 3482(1) Å3, and Z = 8. Complex4crystallizes in the space groupP21/n, witha = 10.638(2) Å,b = 11.849(2) Å,c = 16.970(3) Å, β = 106.84(1)°,V = 2047(1) Å3, and Z = 4. The geometries of3and4are similar with the ether oxygen disposed over the dioxan-2-ylium ring. However, the interaction between the ether oxygen and the cationic centre is not significant due to the large internuclear distance observed. The cation5, in which the carbon chain length between the ether oxygen and C(5) was increased, was prepared. This ion was found to undergo a reversible, intramolecular rearrangement resulting in the formation of 1-methyl-4-acetoxymethyl-tetrahydrofuranyl oxonium hexachloroantimonate11.

ISSN:0008-4042    

DOI:10.1139/v94-265

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

The complexation ofN-acetylcysteine (RSH) with hydrogen peroxide has been studied experimentally and theoretically. Experimentally we have measured the evolution of RSH, H2O2, and RSSR (N-acetylcystine) as a function of time. Surprisingly, H2O2decays by a biphasic process, which is not the case for RSH and RSSR. In the first stage of the kinetics, H2O2disappears without oxidizing the thiol function of RSH. By analogy with glutathione (GSH), the formation of a complex between RSH and H2O2has been proposed. The thermodynamic equilibrium constant of complex formation has been determined. Theoretical calculations were performed within the SIBFA method to pinpoint the sites of complexation in isolated and hydrated states. A mixed "discrete–continuum" model was used to evaluate the solvent effect. The two stable complexes found in isolated state have different behaviour under the influence of the solvent. Comparison with complexed GSH is discussed.

ISSN:0008-4042    

DOI:10.1139/v94-266

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

The oxidation kinetics ofN-acetylcysteine (RSH) by hydrogen peroxide has been studied at neutral pH at different concentration ratios from 0.2 to 20 (4 × 10−4 mol L−1 ≤ [RSH]0 ≤ 2 × 10−2 mol L−1, 10−4 mol L−1 ≤ [H2O2]0 ≤ 10−2 mol L−1). In all the cases studied,N-acetylcystine (RSSR) is the only oxidized product formed. Our kinetic data have focused on the importance of the concentration ratio to reach the stoichiometric oxidation ofN-acetylcysteine by hydrogen peroxide. Indeed non-stoichiometric oxidation of RSH occurs at relatively low concentration ratios (R < 2.5) whereas stoichiometric oxidation is observed whenR > 2.5. Moreover, it has been shown that in the first minutes of the reaction there is the formation of a complex between RSH and H2O2, the stoichiometry of the complex being RSH concentration-dependent for a givenR(R > 2.5). Reaction mechanisms have been quantitatively established and thekvalues of each step determined.

ISSN:0008-4042    

DOI:10.1139/v94-267

出版商:NRC Research Press    

年代:1994

数据来源: NRC