On the influence of solvent on the stereoselectivity of glycosylation reactions.

Methodology development in carbohydrate chemistry entails the stereoselective formation of C-O bonds as a key step in the synthesis of oligo- and polysaccharides. The anomeric selectivity of a glycosylation reaction is affected by a multitude of parameters, such as the nature of the donor and acceptor, activator/promotor system, temperature and solvent. The influence of different solvents on the stereoselective outcome of glycosylation reactions employing thioglucopyranosides as glycosyl donors with a non-participating protecting group at position 2 has been studied.

Reducing the Toxicity of Designer Aminoglycosides as Nonsense Mutation Readthrough Agents for Therapeutic Targets.

A significant proportion of genetic disease cases arise from truncation of proteins caused by premature termination codons. In eukaryotic cells some aminoglycosides cause readthrough of premature termination codons during protein translation. Inducing readthrough of these codons can potentially be of therapeutic value in the treatment of numerous genetic diseases.

Total Biosynthesis of Legionaminic Acid, a Bacterial Sialic Acid Analogue.

Legionaminic acid, Leg5,7Ac2 , a nonulosonic acid like 5-acetamido neuraminic acid (Neu5Ac, sialic acid), is found in cell surface glycoconjugates of bacteria including the pathogens Campylobacter jejuni, Acinetobacter baumanii and Legionella pneumophila. The presence of Leg5,7Ac2 has been correlated with virulence in humans by mechanisms that likely involve subversion of the host's immune system or interactions with host cell surfaces due to its similarity to Neu5Ac. Investigation into its role in bacterial physiology and pathogenicity is limited as there are no effective sources of it.

General low-temperature reaction pathway from precursors to monomers before nucleation of compound semiconductor nanocrystals.

Little is known about the molecular pathway to monomers of semiconductor nanocrystals. Here we report a general reaction pathway, which is based on hydrogen-mediated ligand loss for the precursor conversion to 'monomers' at low temperature before nucleation. We apply (31)P nuclear magnetic resonance spectroscopy to monitor the key phosphorous-containing products that evolve from MXn+E=PPh2H+HY mixtures, where MXn, E=PPh2H, and HY are metal precursors, chalcogenide precursors, and additives, respectively.

Insights into the Mechanistic Role of Diphenylphosphine Selenide, Diphenylphosphine, and Primary Amines in the Formation of CdSe Monomers.

The formation mechanism of CdSe monomers from the reaction of cadmium oleate (Cd(OA)2) and SePPh2H in the presence of HPPh2 and RNH2 was studied systematically at the M06//B3LYP/6-31++G(d,p),SDD level in 1-octadecene solution. Herein, SePPh2H, HPPh2, and RNH2 act as hydrogen/proton donors with a decreased capacity, leading to the release of oleic acid (RCOOH). The longer the radius of the coordinated atom is, the larger the size of the cyclic transition state is, which lowers the activation strain and the Gibbs free energy of activation for the release of RCOOH.

Utilizing CMP-Sialic Acid Analogs to Unravel Neisseria gonorrhoeae Lipooligosaccharide-Mediated Complement Resistance and Design Novel Therapeutics.

Neisseria gonorrhoeae deploys a novel immune evasion strategy wherein the lacto-N-neotetraose (LNnT) structure of lipooligosaccharide (LOS) is capped by the bacterial sialyltransferase, using host cytidine-5'-monophosphate (CMP)-activated forms of the nine-carbon nonulosonate (NulO) sugar N-acetyl-neuraminic acid (Neu5Ac), a sialic acid (Sia) abundant in humans. This allows evasion of complement-mediated killing by recruiting factor H (FH), an inhibitor of the alternative complement pathway, and by limiting classical pathway activation ("serum-resistance"). We utilized CMP salts of six additional natural or synthetic NulOs, Neu5Gc, Neu5Gc8Me, Neu5Ac9Ac, Neu5Ac9Az, legionaminic acid (Leg5Ac7Ac) and pseudaminic acid (Pse5Ac7Ac), to define structural requirements of Sia-mediated serum-resistance.

In a glycosylation reaction how does a hydroxylic nucleophile find the activated anomeric carbon?

The mechanism by which nucleophilic hydroxyls are attracted to activated glycopyranosyl donors is not known. Besides the intrinsic attraction of oxygen centred negative dipoles towards the developing electron deficiency at the anomeric carbon only a few suggestions have been given in the literature. By studying the effect on Density Functional Theory (DFT) modelled glycosylation reactions on the presence of polar additives as tested with acetonitrile two possible effects have been identified.

Mechanistic study of the role of primary amines in precursor conversions to semiconductor nanocrystals at low temperature.

Primary alkyl amines (RNH2) have been empirically used to engineer various colloidal semiconductor nanocrystals (NCs). Here, we present a general mechanism in which the amine acts as a hydrogen/proton donor in the precursor conversion to nanocrystals at low temperature, which was assisted by the presence of a secondary phosphine. Our findings introduce the strategy of using a secondary phosphine together with a primary amine as new routes to prepare high-quality NCs at low reaction temperatures but with high particle yields and reproducibility and thus, potentially, low production costs.

Low-temperature approach to highly emissive copper indium sulfide colloidal nanocrystals and their bioimaging applications.

We report our newly developed low-temperature synthesis of colloidal photoluminescent (PL) CuInS2 nanocrystals (NCs) and their in vitro and in vivo imaging applications. With diphenylphosphine sulphide (SDPP) as a S precursor made from elemental S and diphenylphosphine, this is a noninjection based approach in 1-dodecanethiol (DDT) with excellent synthetic reproducibility and large-scale capability. For a typical synthesis with copper iodide (CuI) as a Cu source and indium acetate (In(OAc)3) as an In source, the growth temperature was as low as 160 °C and the feed molar ratios were 1Cu-to-1In-to-4S.

Synthetic archaeosome vaccines containing triglycosylarchaeols can provide additive and long-lasting immune responses that are enhanced by archaetidylserine.

The relation between archaeal lipid structures and their activity as adjuvants may be defined and explored by synthesizing novel head groups covalently linked to archaeol (2,3-diphytanyl-sn-glycerol). Saturated archaeol, that is suitably stable as a precursor for chemical synthesis, was obtained in high yield from Halobacterium salinarum. Archaeosomes consisting of the various combinations of synthesized lipids, with antigen entrapped, were used to immunize mice and subsequently determine CD8(+) and CD4(+)-T cell immune responses.

Complications of modeling glycosylation reactions: can the anomeric conformation of a donor determine the glycopyranosyl oxacarbenium ring conformation?

That the ring conformation of glycopyranosyl oxacarbenium ions can influence the stereochemical outcome of glycosylation reactions has been postulated for some time. Some new ionization calculations show that the ultimate conformation (4)H(3) or (5)S(1) of D-glucopyranosyl oxacarbenium ions depends on the initial ϕ(H) (CH-1-C-1-S(+)-SCH(3)) conformation of anomeric thiosulfonium ions. Evidence is also presented that nucleophile:electrophile hydrogen bonded complexes, 1,6-anhydro-carbenium ions and electron rich carbon nucleophile:oxacarbenium ion complexes are all probably artifacts of neglecting counter ions or nucleophiles in the DFT calculation.

Plausible Transition States for glycosylation reactions.

The Transition State (TS) for any chemical glycosylation reaction is not known with certainty. Both experimental and computational approaches have been limited due to the complexity of the problem. This work describes a preliminary computational ionization approach using density functional theory calculations to arrive at hypothetical TSs.

Could diastereoselectivity in the presence of O-2 chiral nonparticipating groups be an indicator of glycopyranosyl oxacarbenium ions in glycosylation reactions?

Although long postulated, the existence of glycopyranosyl oxacarbenium ions as intermediates or transition states (TS) in chemical glycosylation reactions has not been convincingly demonstrated experimentally. It is anticipated that elucidation of such reactive species will greatly assist synthetic chemists to control the α/β stereoselectivity by rational means. Previous density functional theory (DFT) calculations from our group found that the torsion potential about C-2-O-2 in protected glycopyranosyl donors changed from a conventional 3-fold rotor to a 2-fold rotor with a strong syn (CH-2-C-2-O-2-CPg) preference once the donor was ionized to its oxacarbenium ion.

Isopranoid- and dipalmitoyl-aminophospholipid adjuvants impact differently on longevity of CTL immune responses.

The success of lipid membranes as cytotoxic T-cell (CTL) adjuvants requires targeted uptake by antigen-presenting cells (APCs) and delivery of the antigen cargo to the cytosol for processing. To target the phosphatidylserine (PS) receptor of APCs, we prepared antigen-loaded liposomes containing dipalmitoylphosphatidylserine and archaeal lipid liposomes (archaeosomes), containing an equivalent amount of archaetidylserine, and compared their ability to promote short and long-term CTL activity in animals. CTL responses were enhanced by the incorporation of PS into phosphatidylcholine/cholesterol liposomes and, to a lesser extent, into phosphatidylglycerol/cholesterol liposomes, that correlated to the amount of surface amino groups reactive with trinitrobenzoyl sulfonate.

Development of new glycosylation methodologies for the synthesis of archaeal-derived glycolipid adjuvants.

To commercialize the production of glycolipid adjuvants, their synthesis needs to be both robust and inexpensive. Herein we describe a semi-synthetic approach where the lipid acceptor is derived from the biomass of the archaeon Halobacterium salinarum, and the glycosyl donors are chemically synthesized. This work presents some preliminary results using the promoter system N-iodosuccinimide (NIS) and a stable 0.

The CMP-legionaminic acid pathway in Campylobacter: biosynthesis involving novel GDP-linked precursors.

The sialic acid-like sugar 5,7-diacetamido-3,5,7,9-tetradeoxy-D-glycero-D-galacto-nonulosonic acid, or legion-aminic acid, is found as a virulence-associated cell-surface glycoconjugate in the Gram-negative bacteria Legionella pneumophila and Campylobacter coli. L. pneumophila serogroup 1 strains, causative agents of Legionnaire's disease, contain an alpha2,4-linked homopolymer of legionaminic acid within their lipopolysaccharide O-chains, whereas the gastrointestinal pathogen C.

Synthesis of archaeal glycolipid adjuvants-what is the optimum number of sugars?

As part of a programme to optimize the use of archaeal-lipid liposomes (archaeosomes) as vaccine adjuvants, we present the synthesis and immunological testing of an oligomeric series of mannose glycolipids (Manp(1-5)). To generate the parent archaeol alcohol precursor, the polar lipids extracted from the archaeon Halobacterium salinarum were hydrolyzed to remove polar head groups, and the archaeol so generated partitioned into diethyl ether. This alcohol was then iteratively glycosylated with the donor 2-O-acetyl-3,4,6-tri-O-benzyl-alpha/beta-d-mannopyranosyl trichloroacetimidate to yield alpha-Manp-(1-->2) oligomers.

Adjuvant potential of archaeal synthetic glycolipid mimetics critically depends on the glyco head group structure.

Subunit vaccines capable of providing protective immunity against the intracellular pathogens and cancers that kill millions of people annually require an adjuvant capable of directing a sufficiently potent cytotoxic T lymphocyte response to purified antigens, without toxicity issues. Archaeosome lipid vesicles, prepared from isoprenoid lipids extracted from archaea, are one such adjuvant in development. Here, the stability of an archaeal core lipid 2,3-di-O-phytanyl-sn-glycerol (archaeol) is used to advantage to synthesize a series of disaccharide archaeols and show that subtle variations in the carbohydrate head group alters the type and potency of immune responses mounted in a mammal.

O-2 Substituted pyranosyl oxacarbenium ions are C-2-O-2 2-fold rotors with a strong syn preference.

The substituent at O-2 of glycopyranosides is known to have a pronounced effect on both the formation and the cleavage of glycosides at C-1. This is primarily attributed to stereoelectronic effects on the formation and stability of the related glycopyranosyl oxacarbenium ions. Previous QM studies of 2-O-methyl substituted manno and gluco configured pyranosyl oxacarbenium ions found a preference for the methyl carbon to be syn to the CH-2 methine.

DFT studies of the ionization of alpha and beta glycopyranosyl donors.

Current attempts at mimicking the transition states (TSs) of glycosyl processing enzymes (GPEs) that proceed through TSs with a high degree of oxacarbenium ion formation suffer from a paucity of data about the conformations of such oxacarbenium ions. Because TSs are maxima, the current models based on minimized structures may need some refinement. As part of studies directed at optimizing chemical glycosylation the ionization of 3,4,6-tri-O-acetyl-alpha/beta-D-glucopyranosyl chlorides and triflates, 2,3,4,6-tetra-O-methyl-alpha/beta-D-glucopyranosyl fluorides, chlorides and triflates, 2,3,4,6-tetra-O-methyl-alpha/beta-D-mannopyranosyl fluorides, 2,3-di-O-methyl 4,6-O-benzylidene alpha/beta-D-mannopyranosyl triflates and 2,3-di-O-methyl 4,6-O-benzylidene alpha/beta-D-glucopyranosyl triflates was studied by a prototypic density functional theory (DFT) procedure.

DFT studies of the role of C-2-O-2 bond rotation in neighboring-group glycosylation reactions.

Although the synthetic utility of the 1,2-trans relationship of the products of neighboring group participation is well established, it is still common to find glycosylation reactions where the stereochemical purity of the products is not 100%. As part of an ongoing series of density functional theory (DFT) studies of the factors that affect glycosylation reactions which are aimed at allowing synthetic chemists to achieve such selectivities, the structures of four oxacarbenium ions and eight methanol complexes of these ions were optimized for the prototypical ions 2-O-acetyl-3,4,6-tri-O-methyl-D-gluco- (1) and mannopyranos-1-yl (2). These studies corroborate the two-conformer hypothesis and further demonstrate that glycopyranosyl oxacarbenium ions exhibit facial selectivity that depends on, besides the inherent steric and Van der Waals effects, the conformational effect associated with the change from sp(2) to sp(3) hybridization at C-1 during nucleophilic attack and H-bonding between the incoming nucleophile and the electronegative atoms of the electrophile.

Investigations into the role of oxacarbenium ions in glycosylation reactions by ab initio molecular dynamics.

We present a constrained ab initio molecular dynamics method that allows the modeling of the conformational interconversions of glycopyranosyl oxacarbenium ions. The model was successfully tested by estimating the barriers to ring inversion for two 4-substituted tetrahydropyranosyl oxacarbenium ions. The model was further extended to predict the pathways that connect the (4)H(3) half-chair conformation of 2,3,4,6-tetra-O-methyl-d-glucopyranosyl cation to its inverted (5)S(1) conformation and the (4)H(3) half-chair conformation of 2,3,4,6-tetra-O-methyl-d-mannopyranosyl cation to its inverted (3)E conformation.