Theoretical foundation for the presence of oxacarbenium ions in chemical glycoside synthesis.

Glycoside formation in organic synthesis is believed to occur along a reaction path involving an activated glycosyl donor with a covalent bond between the glycosyl moiety and the leaving group, followed by formation of contact ion pairs with the glycosyl moiety loosely bound to the leaving group, and eventually solvent-separated ion pairs with the glycosyl moiety and the leaving group being separated by solvent molecules. However, these ion pairs have never been experimentally observed. This study investigates the formation of the ion pairs from a covalent intermediate, 2,3,4,6-tetra-O-methyl-α-d-glucopyranosyl triflate, by means of computational chemistry. Geometry optimization of the ion pairs without solvent molecules resulted in re-formation of the covalent α- and ß-triflates but was successful when four solvent (dichloromethane) molecules were taken into account. The DFT(M06-2X) computations indicated interconversion between the α- and ß-covalent intermediates via the α- and ß-contact ion pairs and the solvent-separated ion pairs. The calculated activation Gibbs energy of this interconversion was quite small (10.4-13.5 kcal/mol). Conformational analyses of the ion pairs indicated that the oxacarbenium ion adopts (4)H3, (2)H3/E3, (2)H3/(2)S0, E3, (2,5)B, and B2,5 pyranosyl ring conformations, with the stability of the conformers being strongly dependent on the relative location of the counteranion.

Chlorotrinitromethane and its exceptionally short carbon-chlorine bond.

Compounds that deviate from the normal range of bonding can help to assess the strengths and weaknesses of the models currently used to describe chemical bonds. Furthermore, computer simulations of molecules require experimental data to describe accurately the energies and forces between interacting molecules. Compounds that contain the trinitromethyl group, with three nitro groups bonded to one carbon atom, show remarkable inter- and intramolecular effects. In this paper, we report the structural features of chlorotrinitromethane in the solid state and present the first reliable solid-state geometry parameters of an α-halogen derivative of the trinitromethyl pseudohalogen. We found several intriguing geometrical features in terms of intra- and intermolecular interactions, as well as an exceptionally short carbon-chlorine bond (1.694(1) Å). Using a combined crystallographic and computational approach, we show that these effects can be described in terms of the computed electrostatic potential of the molecular surface.

Anion exchange silica monolith for capillary liquid chromatography.

An anion exchange monolithic silica capillary column was prepared by surface modification of a hybrid monolithic silica capillary column prepared from a mixture of tetramethoxysilane (TMOS) and methyltrimethoxysilane (MTMS). The surface modification was carried out by on-column copolymerization of N-[3-(dimethylamino)propyl]acrylamide methyl chloride-quaternary salt (DMAPAA-Q) with 3-methacryloxypropyl moieties bonded as an anchor to the silica surface to form a strong anion exchange stationary phase. The columns were examined for their performance in liquid chromatography (LC) and capillary electrochromatography (CEC) separations of common anions. The ions were separated using 50 mM phosphate buffer at pH 6.6. Evaluation by LC produced an average of 30,000 theoretical plates (33 cm column length) for the inorganic anions and nucleotides. Evaluation by CEC, using the same buffer, produced enhanced chromatographic performance of up to ca. 90,000 theoretical plates and a theoretical plate height of ca. 4 mum. Although reduced efficiency was observed for inorganic anions that were retained a long time, the results of this study highlight the potential utility of the DMAPAA-Q stationary phase for anion separations.

13C NMR study of how the oxyanion pKa values of subtilisin and chymotrypsin tetrahedral adducts are affected by different amino acid residues binding in enzyme subsites S1-S4.

A range of substrate-derived chloromethane inhibitors have been synthesized which have one to four amino acid residues. These have been used to inhibit both subtilisin and chymotrypsin. Using 13C NMR, we have shown that all except one of these inhibitors forms a tetrahedral adduct with chymotrypsin, subtilisin, and trypsin. From the pH-dependent changes in the chemical shift of the hemiketal carbon of the tetrahedral adduct, we are able to determine the oxyanion pKa in the different inhibitor derivatives. Our results suggest that in both the subtilisin and chymotrypsin chloromethane derivatives the oxyanion pKa is largely determined by the type of amino acid residue occupying the S1, subsite while binding in the S2-S4 subsites only has minor effects on oxyanion pKa values. Using free energy relationships, we determine that the different R groups of the amino acid residues binding in the S1 subsite only have minor effects on the oxyanion pKa values. We propose that the lower polarity of the chymotrypsin active site relative to that of the subtilisin active site explains why the oxyanion pKa is higher and more sensitive to the type of chloromethane inhibitor used in the chymotrypsin derivatives than in the subtilisin derivatives.