2-Substituted methyl alpha-D-galactopyranosides: synthesis and binding affinity for the A and B subunits of the Griffonia simplicifolia I isolectins

Synthesis, self-assembly, and immunological activity of α-galactose-functionalized dendron-lipid amphiphiles

Nanoassemblies presenting multivalent displays of biologically active carbohydrates are of significant interest for a wide array of biomedical applications ranging from drug delivery to immunotherapy. In this study, glycodendron-lipid hybrids were developed as a new and tunable class of dendritic amphiphiles. A modular synthesis was used to prepare dendron-lipid hybrids comprising distearylglycerol and 0 through 4th generation polyester dendrons with peripheral protected amines. Following deprotection of the amines, an isothiocyanate derivative of C-linked α-galactose (α-Gal) was conjugated to the dendron peripheries, affording amphiphiles with 1 to 16 α-Gal moieties. Self-assembly in water through a solvent exchange process resulted in vesicles for the 0 through 2^nd generation systems and micelles for the 3^rd and 4^th generation systems. The critical aggregation concentrations decreased with increasing dendron generation, suggesting that the effects of increasing molar mass dominated over the effects of increasing the hydrophilic weight fraction. The binding of the assemblies to Griffonia simplicifolia Lectin I (GSL 1), a protein with specificity for α-Gal was studied by quantifying the binding of fluorescently labeled assemblies to GSL 1-coated beads. It was found that binding was enhanced for amphiphiles containing higher generation dendrons. Despite their substantial structural differences with the natural ligands for the CD1d receptor, the glycodendron-lipid hybrids were capable of stimulating invariant natural killer T (iNKT) cells, a class of innate-like T cells that recognize lipid and glycolipid antigens presented by CD1d and that are implicated in a wide range of diseases and conditions including but not limited to infectious diseases, diabetes and cancer.

A highly α-stereoselective synthesis of oligosaccharide fragments of the Vi antigen from Salmonella typhi and their antigenic activities

In this paper, a convenient approach to the synthesis of the repeating α-(1→4)-linked N-acetyl galactosaminuronic acid units from the capsular polysaccharide of Salmonella typhi is reported. The exclusively α-stereoselective glycosylation reactions were achieved by using oxazolidinone-protected glycosides as building blocks based on a pre-activation protocol. Di-, tri-, and tetrasaccharides were prepared by this short and efficient approach in high yields. The enzyme-linked immunosorbent assay experiments show that our synthetic tri- and tetrasaccharide had much higher antigenic activities than previously reported ones in the inhibition of antibody binding by the native polysaccharide. The results demonstrate that the antigenic activities of saccharides can be strengthened greatly by increasing the number of acetyl groups present.

Quantifying the electronic effects of carbohydrate hydroxy groups by using aminosugar models

Methyl amino-deoxy-glycosides with α- and β-gluco, α-galacto, or α-manno stereochemistry with the amino functionality in each of the four possible non-anomeric positions have been synthesized and their pK(a) values determined by titration. These model compounds were chosen because they are the amino derivatives of the most common glycosyl acceptors. From this study it was possible to evaluate the electron density at each of the given positions in the carbohydrate and compare them. Some general trends were observed: The basicity of the amino groups decreases in the order 6-NH(2)>3-NH(2)>2-NH(2)>4-NH(2) (referring to the position). The basicity of a of an amino-deoxy-sugar generally increases when one or more substituents on the sugar ring are axial. The basicity decreases when the amine is antiperiplanar to an oxygen atom. These findings are in agreement with the observations obtained from glycosylation chemistry and the regioselective protection of sugars.

Isolectins I-A and I-B of Griffonia (Bandeiraea) simplicifolia. Crystal structure of metal-free GS I-B(4) and molecular basis for metal binding and monosaccharide specificity

Seeds from the African legume shrub Griffonia simplicifolia contain several lectins. Among them the tetrameric lectin GS I-B(4) has strict specificity for terminal alpha Gal residues, whereas the closely related lectin GS I-A(4) can also bind to alpha GalNAc. These two lectins are commonly used as markers in histology or for research in xenotransplantation. To elucidate the basis for the fine difference in specificity, the amino acid sequences of both lectins have been determined and show 89% identity. The crystal structure of GS I-B(4), determined at 2.5-A resolution, reveals a new quaternary structure that has never been observed in other legume lectins. An unexpected loss of both Ca(2+) and Mn(2+) ions, which are necessary for carbohydrate binding in legume lectins, may be related to a particular amino acid sequence Pro-Glu-Pro in the metal binding loop. Comparison with demetallized concanavalin A reveals a different process for the loss of metal ions and for the subsequent loss of carbohydrate binding activity. The GS I-A x alpha GalNAc and GS I-B x alpha Gal complexes were constructed using homology modeling and docking approaches. The unusual presence of an aromatic amino acid at position 47 (Tyr in I-A and Trp in I-B) explains the strong preference for alpha-anomeric sugars in both isolectins. Alteration at one amino acid position, Ala(106) in I-A versus Glu(106) in I-B, is the basis for the observed specificities toward alpha GalNAc and alpha Gal.

Synthesis of a hexasaccharide that relates to the arabinogalactan epitope

A hexasaccharide derivative of the arabinogalactan epitope, methyl beta-D-galactopyranosyl-(1-->6)-[alpha-L-arabinofuranosyl-(1-->3)]-beta-D-galactopyranosyl-(1-->6)-beta-D-galactopyranosyl-(1-->6)-[alpha-L-arabinofuranosyl-(1-->3)]-alpha-D-galactopyranoside, was synthesized efficiently using a 3+3 strategy. The key step is the preparation of the trisaccharide donor, isopropyl 2,3,4,6-tetra-O-benzoyl-beta-D-galactopyranosyl-(1-->6)-[2,3,5-tri-O-benzoyl-alpha-L-arabinofuranosyl-(1-->3)]-2,4-di-O-benzoyl-1-thio-beta-D-galactopyranoside, from isopropyl 1-thio-beta-D-galactopyranoside using a one-pot synthesis of a 3,6-differentially protected building block.

Synthesis and biological activities of methyl oligobiosaminide and some deoxy isomers thereof

Methyl oligobiosaminide (1) the core structure of oligostatin C, and five analogues, the 6-hydroxy-(2), 2-deoxy- (3), 2-deoxy-6-hydroxy- (4), 3-deoxy- (5), and 3-deoxy-6-hydroxy derivatives (6), were synthesized by coupling the protected pseudo-sugar epoxide 46 with suitable methyl 4-amino-4-deoxy-alpha-D-hexopyranoside derivatives. Compounds 3 and 6 showed notable inhibitory activity against alpha-D-glucosidase and alpha-D-mannosidase, respectively, whereas compound 1 had almost no activity.

Isolation and characterization of a second lectin (SNA-II) present in elderberry (Sambucus nigra L.) bark

A second lectin (SNA-II) has been isolated from elderberry (Sambucus nigra L.) bark by affinity chromatography on immobilized asialo-glycophorin. This lectin is a blood group nonspecific glycoprotein containing 7.8% carbohydrate and which is rich in asparagine/aspartic acid, glutamine/glutamic acid, glycine, valine, and leucine. Gel filtration on Superose 12 gave a single symmetrical peak corresponding to Mr, 51,000; SDS-acrylamide electrophoresis gave a single polypeptide, Mr, 30,000. Hence SNA-II appears to be a homodimer. The lectin is a Gal/GalNAc-specific lectin which is precipitated by glycoproteins containing GalNAc-terminated oligosaccharide chains (e.g., asialo-ovine submaxillary and hog gastric mucins), and by glycoproteins and polysaccharides having multiple terminal nonreducing D-galactosyl groups as occur in asialoglycophorin, asialo-laminin and Type 14 pneumococcal polysaccharide. The carbohydrate binding specificity of SNA-II was studied by sugar hapten inhibition of the asialo-glycophorin precipitation reaction. The lectin's binding site appears to be most complementary to Gal-NAc linked alpha to the C-2, C-3, or C-6 hydroxyl group of galactose. These disaccharide units are approximately 100 times more potent than melibiose, 60 times more potent than N-acetyllactosamine, and 30 times more potent than lactose. Interestingly, the blood group A-active trisaccharide containing an L-fucosyl group linked alpha 1-2 to galactose was 10-fold poorer as an inhibitor than the parent oligosaccharide (GalNAc alpha 1-3Gal), suggesting steric hindrance to binding by the alpha-L-fucosyl group; this explains the failure of the lectin to exhibit blood group A specificity.

Synthesis of D-galactosamine derivatives and binding studies using isolated rat hepatocytes

Derivatives of glycosides of D-galactosamine were prepared in order to study further the binding requirement of the Gal/GalNAc receptor in mammalian hepatocytes. These structures included N-propanoyl, N-benzoyl, and N,N-phthaloyl derivatives of 2-hydroxyethyl 2-amino-2-deoxy-beta-D-galactopyranoside, 6-amino-hex-1-yl 2-deoxy-2-(trifluoroacetamido)-beta-D-galactopyranoside, the mono- and di-O-methyl derivatives of allyl 2-acetamido-2-deoxy-beta-D-galactopyranoside, and allyl 2-acetamido-2,4-dideoxy-4-fluoro-alpha-D-galactopyranoside. The inhibition results confirmed some of our previous findings on the involvement of the hydroxyl groups, and provided new information on the involvement of the N-substituent, as well as on the requirement of hydrogen bonding of the 4-hydroxyl group in binding.

Properties of the lectin from the hog peanut (Amphicarpaea bracteata)

An N-acetyl-D-galactosamine-specific lectin has been isolated from the two seed forms of the hog peanut (Amphicarpaea bracteata) using an affinity support containing the synthetic type A blood group trisaccharide alpha-D-GalNAc-(1,3)-[alpha-L-Fuc-(1,2)]-beta-D-Gal (Synsorb A). The affinity-purified lectin appears to be identical in both seed types. Gel filtration on Sephadex G-200 gives a single symmetrical peak corresponding to Mr 135,000. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis shows four subunit forms, each of which contains carbohydrate. Limited amino terminal sequencing indicates heterogeneity in two of the first 10 residues. The lectin contains no cysteine. There are four equivalent, noninteracting GalNAc binding sites per 135,000-Da molecule, having an association constant for methyl N-acetyl-alpha-D-galactosaminide of 4.0 X 10(4) M-1. Precipitin and hapten inhibition studies show the lectin to be specific for terminal, nonreducing D-GalNAc units, with a preference for the alpha-anomer and enhanced specificity for the disaccharide, GalNAc alpha 1,3GalNAc. There is also a single adenine binding site per Mr 135,000 lectin molecule with an association constant of 1.3 X 10(6) M-1.