Synthesis of three oligosaccharides that form part of the complex type of carbohydrate moiety of glycoproteins

Synthesis of multiantennary complex type N-glycans by use of modular building blocks

A modular set of oligosaccharide building blocks was developed for the synthesis of multiantennary N-glycans of the complex type, which are commonly found on glycoproteins. The donor building blocks were laid out for the elongation of a core trisaccharide acceptor (beta-mannosyl chitobiose) conveniently protected with a single benzylidene moiety at the beta-mannoside. Through two consecutive regio- and stereoselective couplings the donors gave N-glycans with three to five antennae in high yields. Due to the consistent protection group pattern of the donors the deprotection of the final products can be performed by using a general reaction sequence.

Synthesis and characterization of 2-O-beta-lactosylglycerol, 1,2-di-O-beta-lactosyl-(R,S)-glycerols, and 1,2,3,-tri-O-beta-lactosylglycerol

The reaction of 2,3,6,2',3',4',6'-hepta-O-acetyl-alpha-lactosyl bromide (5) and 1,3-di-O-benzylglycerol in the presence of mercury(II) cyanide in benzene-nitromethane afforded 1,3-di-O-benzyl-2-O-(2,3,6,2',3',4',6'-hepta-O-acetyl-beta-lactosyl)glyc erol (70%), which was converted into 2-O-beta-lactosylglycerol. 1,2-Di-O-beta-lactosyl-(R,S)-glycerols were obtained by way of the coupling of 5 to either 1-O-benzyl-(R,S)-glycerol or 1-O-benzyl-2-O-(2,3,6,2',3',4',6'-hepta-O-acetyl-beta-lactosyl)-(R,S)-gl ycerols. The most efficient route to 1,2, 3-tri-O-beta-lactosylglycerol (17) involved treatment of 2-O-(2,3,6,2',3',4',6'-hepta-O-acetyl-beta-lactosyl)glycerol with 3 mol. equiv. of 5 followed by removal of the blocking groups, to give 17 (47%).

Synthesis and characterization of 6-O-beta-lactosyl-alpha,beta-D-mannopyranoses and 2,6-di-O-beta-lactosyl-alpha,beta-D-mannopyranoses

The reaction of 2,3,6,2',3',4',6'-hepta-O-acetyl-alpha-lactosyl bromide (4) and benzyl 3,4-di-O-benzyl-alpha-D-mannopyranoside (3) in the presence of mercury(II) cyanide in benzene-nitromethane produced benzyl 3,4-di-O-benzyl-2,6-bis-O-(2,3,6,2',3',4',6'-hepta-O-acetyl-beta-lact osy l)-alph a D-mannopyranoside (5) and benzyl 3,4-di-O-benzyl-6-O-(2,3,6,2',3',4',6'-hepta-O-acetyl-beta-lactosyl)-alp ha-D- mannopyranoside (6), as part of a complex mixture. Column chromatography, followed by acetylation of the fraction containing 5 and 6, gave a sample of 5 and benzyl 2-O-acetyl-3,4-di-O-benzyl-6-O (2,3,6,2',3',4',6'-hepta-O-acetyl-beta-lactosyl)-alpha-D-mannopyranoside (7) in approximately 35% and 17% yields (based on 4), respectively. Deprotection of 5 and 7 afforded the target compounds, namely 2,6-di-O-beta-lactosyl-alpha,beta-D-mannopyranoses and 6-O-beta-lactosyl-alpha,beta-D-mannopyranoses, respectively. If the coupling of 4 with 3 were performed in the presence of silver trifluoromethanesulfonate and 2,4,6-trimethylpyridine, only a mixture of 3,6,2',3',4',6'-hexa-O-acetyl- alpha-lactose-1,2-[( 3,6,2',3',4',6'-hexa-O-acetyl-alpha-lactose 1,2-(benzyl 3,4-di-O-benzyl-alpha-D-mannopyranosid-6-yl orthoacetyl)-2-yl]orthoacetate) and 3,6,2',3',4',6'-hexa-O-acetyl-alpha-lactose 1,2-(benzyl 3,4-di-O-benzyl-alpha-D-mannopyranosid-6-yl orthoacetate) was obtained. The orthoacetates were characterized by n.m.r. spectroscopy. The two target materials are useful in the assessment of the binding properties of galactose-terminated ligands to the asialoglycoprotein receptor of normal rabbit and human hepatocytes.

Definition of the phage G13 receptor as structural domains of trisaccharides in Salmonella and Escherichia coli core oligosaccharides

The interaction between phage G13 and different bacterial and synthetic oligosaccharides has been studied using equilibrium dialysis inhibition. The results, and conformational analysis of the oligosaccharides, make us conclude that the phage G13 carbohydrate receptor is a conformational domain involving three sugar residues. The following trisaccharide elements contain the domain: alpha-D-Galp-(1----3)-[alpha-D-Galp-(1----6)]-alpha-D-Glcp, alpha-D-Manp-(1----3)-[alpha-D-Manp-(1----6)-alpha-D-Manp , and alpha-D-Glcp-(1----3)-[L-gly-alpha-D-man-Hepp-(1----7)]-L-gly-alph a-D- man-Hepp. Thus two structures, either a hexose substituted with alpha-D-glycopyranosyl groups in the 3- and 6-positions, or a heptose substituted with such groups in the 3- and 7-positions are functional G13 binding sites. Such domains are present in several cores of lipopolysaccharides from Salmonella and Escherichia coli species. Some cores, e.g. those from S. typhimurium chemotypes Ra, Rb1 and Rb2, contain two such domains. The identification of two G13 receptor domains within different core saccharides could explain the broad host range of this phage.

Carbohydrate binding studies on the lectin from Datura stramonium seeds

The carbohydrate-binding properties of the Datura stramonium seed lectin were studied by equilibrium dialysis, quantitative precipitation of natural and synthetic glycoproteins, and hapten inhibition of precipitation. The dimeric lectin (Mr = 86,000) possesses two carbohydrate-binding sites for N,N'N",N"'- tetraacetylchitotetritol /mol protein, with an apparent Ka = 8.7 X 10(3) M-1 at 4 degrees C. Whereas fetuin and orosomucoid reacted poorly with the Datura lectin, the asialo derivatives of these glycoproteins gave strong precipitation with the lectin. Carcinoembryonic antigen, type 14 pneumococcal capsular polysaccharide, and bovine serum albumin, highly substituted with N,N'- diacetylchitobiose units, also precipitated the lectin. Of the homologous series of chitin oligosaccharides tested, N,N',N"'- triacetylchitotriose was over 6-fold more potent than the disaccharide (N,N'- diacetylchitobiose ) which, in turn, was 90 times more reactive than N-acetyl-D-glucosamine. N-Acetyllactosamine [beta-D-Gal-(1----4)-D-GlcNAc] was also a potent inhibitor of Datura lectin being equivalent to N,N'- diacetylchitobiose . The requirement for an N-acetyl-D-glucosaminyl unit linked at the C-4 position was established. The biantennary pentasaccharide (penta-2,6) was a 500-fold more potent inhibitor than N-acetyllactosamine, suggesting that it might interact with both saccharide-binding sites of the Datura lectin simultaneously.

Synthesis of a nona- and an undeca-saccharide that form part of the complex type of carbohydrate moiety of glycoproteins

Silver trifluoromethanesulfonate-promoted condensation of 3,6-di-O-acetyl-2-deoxy-2-phthalimido-4-O-(2,3,4,6-tetra-O-acetyl-beta-D- galactopyranosyl)-beta-D-glucopyranosyl bromide with benzyl 2,4,-di-O-benzyl-6-O-(3,4-di-O-benzyl-alpha-D-mannopyranosyl)-3-O-(3,6-di-O- benzyl-alpha-D-mannopyranosyl)-alpha-D-mannopyranoside gave nonasaccharide and undecasaccharide derivatives. The nonasaccharide 2 and the undecasaccharide 3 were obtained by removal of the protecting groups followed by N-acetylation. (formula; see text).

Mitogenic leukoagglutinin from Phaseolus vulgaris binds to a pentasaccharide unit in N-acetyllactosamine-type glycoprotein glycans

The carbohydrate binding specificity of leukoagglutinin (La; Phaseolus vulgaris isolectin L4) was studied by using quantitative precipitation and precipitation-inhibition. A series of purified glycopeptides and synthetic oligosaccharides were used as inhibitors. The minimal structural unit required for La binding was the disaccharide GlcNac(1 leads to beta 2)Man. Additions for this basic unit of different sugar residues gave a positive or negative contribution to binding. The most complementary structure was the pentasaccharide (formula: see text). This pentasaccharide units occurs in tetraantennary N-acetyllactosamine-type glycoprotein glycans. Glycoproteins containing such structures were accordingly precipitated by La. Selected glycopeptides and oligosaccharides were also tested as inhibitors of La-induced DNA synthesis in human lymphocytes. The pattern of inhibition was essentially the same as that obtained by precipitation-inhibition, indicating that binding to lymphocytes via the carbohydrate binding site of the lectin is an essential step in the activation process.