Biomedical subjects
Publications and source records attributed to Alexander Wei.
Au surfaces are functionalized by stable dithiocarbamate ligands when exposed to carbon disulfide and secondary amines. The adsorbed dithiocarbamates are robust under a wide pH range and can resist displacement by other chemisorptive surfactants, providing an attractive method for conjugating sensitive molecules onto metal surfaces.
Several 6-C-substituted 2-acetamido-2-deoxy-beta-D-glucopyranosides (beta-D-GlcNAc monosaccharides 1a-3a and 1,4-linked disaccharides 1b-3b) were studied by solution NMR spectroscopy. Conformational analysis of the (6S)- and (6R)-C-methyl-substituted beta-d-GlcNAc monosaccharides indicates that the stereodefined methyl groups impose predictable conformational biases on the exocyclic C-5-C-6 bond, as determined by (1)H-(1)H and (13)C-(1)H coupling constants. Variable-temperature NMR experiments in methanol-d(4) were performed to determine DeltaDeltaH and DeltaDeltaS values derived from the two lowest energy conformers. These indicate that while the influence of 6-C-methyl substitution on conformational enthalpy is in accord with the classic principles of steric interactions, conformational preference in solution can also be strongly affected by other factors such as solvent-solute interactions and solvent reorganization.
The regioselective ring opening of pyranosidic 4,6-p-methoxybenzylidene acetals with BH(3)/Bu(2)BOTf in THF can be tuned by adjusting the reaction temperature and reagent concentrations. Reductive cleavage at 0 degrees C resulted in the exclusive formation of 4-O-p-methoxybenzyl (PMB) ethers, whereas reaction at -78 degrees C produced 6-O-PMB ethers in high yields. The latter condition was observed to be compatible with a variety of acid-sensitive functional groups, including allyl and enol ethers. The presence of water does not interfere with reductive ring opening and may contribute toward in situ generation of H(+) as a catalyst for 6-O-PMB ether formation. Reductive cleavage under rigorously aprotic conditions is greatly decelerated, and yields only the 4-O-PMB ether. The temperature-dependent reductive cleavage of the 4,6-acetal can be described in terms of kinetic versus thermodynamic control: Lewis-acid coordination of the more accessible O-6 is favored at higher temperatures, whereas protonation of the more basic but sterically encumbered O-4 predominates at low temperatures.
A protocol for cluster size distribution analysis was developed in order to parametrize local two-dimensional (2D) order in a quantitative manner, using mean cluster sizes and fractional hcp cluster formation (fhcp). Cluster size analysis was performed on 2D arrays of Au nanoparticles encapsulated in resorcinarene tetrathiol, which were organized into close-packed films at aqueous interfaces. The degree of monolayer formation and 2D order within the self-assembled nanoparticle arrays was observed to be strongly dependent on the amount and type of electrolyte (chloride and/or citrate) adsorbed on the nanoparticle surface, prior to encapsulation and extraction to the solvent interface. Increasing the concentration of adsorbed electrolyte could promote monoparticulate film formation but had a variable effect on local 2D order.
Methyl D-glucoside and d-glucose pentaacetate are transformed, respectively, into methyl alpha-O-glucuronide 3 and hydroxymethyl beta-C-glucuronide 9, which undergo decarboxylative elimination efficiently to produce 4-deoxypentenoside 4 and L-glucal 10. These unsaturated pyranosides provide an expeditious entry into mirror-image oligosaccharides, as demonstrated in the synthesis of the unnatural enantiomer of the H-type II blood group determinant trisaccharide (D-Fuc-(alpha1-->2)-L-Gal-(beta1-->4)-L-GlcNAc-beta-OMe). This work illustrates that D-glucose, a common starting material in the synthesis of naturally occurring carbohydrates, can also be used to prepare their mirror-image analogues.
Submicron-sized Au particles and Au/SiO(2) nanocomposites (superparticles) as large as 670 nm have been introduced into tsA201 cells with minimal cell trauma by cationic transfection systems. Successful implantations can be characterized by the expression of co-transfected DNA.
[reaction: see text] L-Glycals can be prepared in seven steps from readily available D-glucals, enabling the facile construction of mirror-image carbohydrates such as the L-lactosamine derivative shown above.
A selection of recent results illustrating the application of off-axis electron holography to the study of magnetic microstructure in closely-spaced nanoparticles and nanowires is reviewed. Examples are taken from the characterization of FeNi nanoparticle chains, Co nanoparticle rings, two-dimensional arrays of naturally occurring magnetite crystals in minerals, and single crystalline Co nanowires. Approaches that can be used to separate the magnetic signal of interest from the mean inner potential contribution to the measured holographic phase shift are described, and the spatial and phase resolution that can be achieved are discussed.
Frozen-solution conformational analysis (FrSCA) can be performed on organic compounds using REDOR spectroscopy. REDOR measurements on frozen aqueous solutions of 13C-methyl beta-15N-aminoglucoside indicate a bimodal distribution of conformations in a 68:32 ratio, with 13C-15N distances of 4.31 and 3.55 A, respectively. The high resolution and straightforward sample preparation make FrSCA an attractive alternative to solution-based NMR methods of conformational analysis.
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Orthogonally protected chitosan tetrasaccharides were synthesized in a convergent fashion by trichloroacetimidate activation. The anomeric substituent at the reducing end of the disaccharide acceptor has a remarkably strong influence on glycosidic coupling; a thiophenyl-substituted disaccharide was observed to be an unusually poor glycosyl acceptor in comparison with the corresponding allyloxy-substituted disaccharide.
Core-shell ensembles of citrate-stabilized gold nanoparticles (20-80 nm) on submicron silica cores (330-550 nm) have been prepared by electrostatic self-assembly with shell packing densities as high as phi = 0.55.
Weakly ferromagnetic cobalt nanoparticles can assemble spontaneously into nanosized "bracelets" when dispersed in organic solvents containing resorcinarenes as surfactants. Bracelet self-assembly occurs in solution and is directed by magnetic dipolar interactions, whereas nanoparticle rings with larger diameters are produced by evaporation-driven flow on wetted surfaces.
[reaction: see text] 4-Deoxypentenosides, which are readily derived from D-sugars, resemble glycals in structure and reactivity and can undergo stereoselective epoxidation and S(N)2 nucleophilic addition to produce L-sugars in pyranosidic form.
Introduction of a stereodefined methyl group at the C-6 position of N-acetylglucosamine mono- and disaccharides creates a strong and predictable orientational bias on the geminal C-6 hydroxyl in solution, as determined by (1)H-(1)H and (13)C-(1)H NMR coupling constants. The conformational directing effect is more pronounced in the disaccharides because of the greater steric demand imposed by the neighboring glycosidic unit.