Glycosyl donors based on the 2,6-di-O-acyl-3,4-O-isopropylidene-d-galactopyranosyl-(leaving group) structure have been shown experimentally to have a high propensity for giving acyl transfer to the alcohol nucleophile as major side products in the glycosylation reaction. The corresponding cations of these relatively rigid glycosyl donors were investigated by density functional methods. The precursor cations resulting from neighboring group assistance from the 2-O-acyl group were found to be the most stable. The nucleophile methanol most favorably approaches the LUMO of such cations on the former carbonyl carbon. The resulting stable intermediate has a long C−O bond of 2.79 Å. It is suggested that such intermediates can lead to both acyl transfer and β-glycoside after passing through at least one further transition state.

We have studied the solvation of divalent copper by water and ammonia through the optimization of the structures of [Cu(H2O) n ]2+ and [Cu(NH3) n ]2+, n = 3−8, by static density functional theory and ab initio molecular dynamics simulations. We found that as the number of solvent molecules increases to more than four, the additional ligands prefer to be hydrogen-bonded to the planar tetragonal primary hydration shell of [Cu(solvent)4]2+ instead of filling the vacant axial position. The energetic preference of water is about 20−35 kJ/mol for the hydrogen bond compared to the axial position, whereas ammonia shows preference of only a few kJ/mol. Dynamical simulations were successful in reaching the lowest energy conformations. Especially remarkable is the dynamics of [Cu(H2O)8]2+, which has evolved from an eight-coordinate structure to a planar structure with four primary and four secondary solvent molecules in a short 10 ps simulation. Both [Cu(H2O)8]2+ and [Cu(NH3)8]2+ prefer a quasi-planar structure with a total of eight hydrogen bonds between the solvent molecules in the first and second solvation shells. Each secondary water and ammonia is hydrogen-bonded to two adjacent molecules in the primary solvation shell. It is remarkable that ammonia can form two hydrogen bonds with only one lone electron pair. The strong network of hydrogen bonds stabilizes the tetragonal planar primary hydration shell. These calculations indicate that the high kinetic stability of the eight-coordinate clusters in previous mass spectrometry experiments is related to the stabilization of the planar primary solvation shell by the network of hydrogen bonds. We found a correlation between experimental ion signals in the gas phase and the planarity of the first solvation shells.

Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n'arrivez pas à les repérer, communiquez avec nous à PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. Questions? Contact the NRC Publications Archive team atPublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. If you wish to email the authors directly, please see the first page of the publication for their contact information. NRC Publications Archive Archives des publications du CNRCThis publication could be one of several versions: author's original, accepted manuscript or the publisher's version. / La version de cette publication peut être l'une des suivantes : la version prépublication de l'auteur, la version acceptée du manuscrit ou la version de l'éditeur. NRC Publications Record / Notice d'Archives des publications de CNRC:http://nparc.cisti-icist.nrc-cnrc.gc.ca/eng/view/object/?id=357c916e-19e7-4437-b461-25622e976464 http://nparc.cisti-icist.nrc-cnrc.gc.ca/fra/voir/objet/?id=357c916e-19e7-4437-b461-25622e976464 Wako-shi, Saitama 351, Japan ReceiVed: April 27, 2005; In Final Form: July 12, 2005 The conformation of the six-membered ring of pyranosyl sugars has pronounced effects on the physical and chemical properties of carbohydrates. We present a method to determine key features of the potential energy surfaces, such as transition states associated with the inversion pathways of the model compounds cyclohexane, tetrahydropyran, p-dioxane, m-dioxane, s-trioxane, and 2-oxanol. Finally, we make the first determination of the pathways for inversion of penta-O-methyl-R-D-glucopyranose and penta-O-methyl--D-glucopyranose. For both anomers, a transition state with five coplanar atoms with appreciable O E character was found. The method is based on constrained Car-Parrinello ab initio molecular dynamics, as implemented in the projector augmented-wave (PAW) method. The constraints are derived from the normal modes of six-membered rings and are described in terms of the canonical conformations 1 C 4 chair, 1,4 B boat, and O S 2 skew-boat. The PAW derived trajectories are in agreement with previous suggestions in the literature that pseudorotation is an important feature of such conformational interconversions. The dynamic nature as well as the internal coordinate-based constraints provide a method which can reliably accommodate pseudorotation. To determine semiquantitative energies, we recalculate key conformations using standard quantum mechanical calculations while keeping the ring dihedral angles frozen at their values found in the dynamics. In all cases where experimental barriers are known, our results are in excellent agreement.