Blood group antigens: synthesis of TN glycopeptide related to human glycophorin AM

Fully convergent solid phase synthesis of antifreeze glycoprotein analogues

The convergent solid phase synthesis of C-linked analogues of antifreeze glycoprotein (AFGP) has been achieved. In this approach, three to six carbohydrate residues are simultaneously coupled to a resin-bound polypeptide. Glycopeptides ranging from 1.6 to 3.0 kDa are easily prepared in 26-44% yield demonstrating the utility of this approach.

Synthetic carbohydrate vaccines: synthesis and immunogenicity of Tn antigen conjugates

A tumor-associated carbohydrate antigen, Tn antigen (GalNAc alpha 1-->O-Ser), was synthesized with a spacer arm, and assembled to dimeric and trimeric structures using N-tert-butyloxycarbonyl-O-(2-acetamido-3,4,6-tri-O-acetyl-2-deoxy-alpha- D-galactopyranosyl)-L-serine as a key building block. The synthetic antigens were conjugated with OSA and their immunogenicity examined in mice. Mice immunized with dimeric or trimeric Tn antigen showed a stronger antibody (IgM) response to a Tn-glycoprotein (asialo-ovine submaxillary mucin) than mice immunized with monomeric Tn antigen. The dimeric and trimeric Tn antigens also induced measurable IgG responses. The dimeric Tn antigen was further coupled to a Starburst dendrimer (5th generation) and to tripalmitoyl-S-glycerylcysteinyl-serine, a synthetic lipopeptide of the active moiety of a major lipoprotein of Escherichia coli. Unexpectedly, the Starburst dendrimer conjugate did not stimulate any immune response specific to Tn antigen. On the other hand, immunization of mice with the lipopeptide conjugate produced not only a high IgM response but also significant IgG anti-Tn response without any carrier molecules or additional adjuvants. The production of IgG antibody is quite significant since carbohydrate antigens are in general known to produce only IgM antibody response. Being a totally synthetic, low-molecular weight, and carrier-free immunogen, the lipopeptide conjugate could be a prototype of synthetic carbohydrate vaccines.

Glycosylation of synthetic T helper cell epitopic peptides influences their antigenic potency and conformation in a sugar location-specific manner

The immunodominant T helper cell epitopes 31D and VF13N of rabies virus nucleoprotein and glycoprotein, respectively, correspond to peptide sequences AVYTRIMMNGGRLKR and VVEDEGCTNLSGF, and are expressed between amino acids 404-418 and 29-41, of the appropriate proteins. We investigated how internal or external glycosylation affects the biological activity and conformation of the peptides 31D and VF13N. Mid-chain incorporation of maltobiose or N-acetylglucosamine moieties into the asparagine residues greatly diminished the T-cell stimulatory activity in vitro (due to the diminished ability of the glycopeptides to bind to major histocompatibility complex determinants) and reduced the characteristic alpha-helicity of the peptides in aqueous trifluoroethanol solutions. In contrast, addition of maltobiose- or N-acetylglucosamine-coupled asparagines to the N-termini of peptides 31D and VF13N resulted in unchanged T-cell activity. Furthermore, N-terminal glycosylation of peptide 31D, as indicated by the functional assay, decreased the sensitivity of the peptide to degradation in human serum and did not affect the alpha-helical conformation. These data indicate that glycosylation of T-cell epitopes is not a preferable method for the preparation of antagonists, but incorporation of the sugars to appropriate positions may be advantageous in the design of T-cell agonists and peptide-based vaccines.

Susceptibility of glycans to beta-elimination in Fmoc-based O-glycopeptide synthesis

In order to investigate the possible extent of beta-elimination occurring in Fmoc-based continuous-flow solid-phase glycopeptide synthesis, the influence of the pKb of the base used for N alpha-deprotection has been studied. A glycosylated pentapeptide was synthesized using 50% morpholine, 10% piperidine or 2% DBU, respectively, in DMF for deprotection. The dehydropentapeptide N alpha-Ac-Thr-Thr-delta Aba-Val-Thr-NH2, which would be formed in the case of beta-elimination, was prepared independently and used as a control in HPLC analysis; however, this product was not formed under any of the deprotection conditions applied. Furthermore, a 23 amino acid long glycopeptide from human intestinal mucin was prepared using 2% DBU as a base for Fmoc cleavage, and similarly no beta-elimination was observed. The glycopeptide products were subjected to a prolonged treatment with sodium hydroxide in methanol/water without significant formation of byproducts, and the pure glycopeptides were isolated and characterized by 1H-NMR spectroscopy.

Stereoselective total synthesis of glycopeptides bearing the dimeric and trimeric sialosyl-Tn epitope

The dimeric and trimeric sialosyl-Tn epitopes, [alpha-D-Neup5Ac-(2----6)-alpha-D-GalpNAc-(1----3)-L-Ser]n-L-Val (n = 2 and 3), which represent part of a clustered carbohydrate region of glycophorin A, a human erythrocyte glycoprotein, have been synthesised stereoselectively. 2-Azido-3-O-benzyl-4,6-O-benzylidene-2-deoxy-D-galactopyranosyl fluoride (GalpNAc unit), Fmoc-L-serine phenacyl ester (Ser unit), and benzyl 5-acetamido-4,7,8,9-tetra-O-benzyl-5-deoxy-3-S-phenyl-3-thio-D-erythro-L - gluco-2-nonulopyranosylonate bromide (Neup5Ac unit) were the key intermediates for stereoselective glycosylation. 2-Ethoxy-1-ethoxycarbonyl-1,2-dihydroquinoline-promoted elongation of the peptide chain and then hydrogenolysis afforded the title compounds.

Solid phase peptide synthesis utilizing 9-fluorenylmethoxycarbonyl amino acids

9-Fluorenylmethoxycarbonyl (Fmoc) amino acids were first used for solid phase peptide synthesis a little more than a decade ago. Since that time, Fmoc solid phase peptide synthesis methodology has been greatly enhanced by the introduction of a variety of solid supports, linkages, and side chain protecting groups, as well as by increased understanding of solvation conditions. These advances have led to many impressive syntheses, such as those of biologically active and isotopically labeled peptides and small proteins. The great variety of conditions under which Fmoc solid phase peptide synthesis may be carried out represents a truly "orthogonal" scheme, and thus offers many unique opportunities for bioorganic chemistry.

Solid phase synthesis and conformation of sequential glycosylated polytripeptide sequences related to antifreeze glycoproteins

Sequential glycopeptides [Thr(beta-D-galactose)-Ala-Ala]n, with n ranging from 2 to 7, as models of natural antifreeze glycoproteins were synthesized by the continuous flow, solid phase procedure. The conformational properties of these materials in solution were investigated by c.d. and 1H-n.m.r. spectroscopy. In aqueous solution the c.d. pattern is practically independent of chain length and is very similar to that of natural antifreeze glycoproteins. The results are interpreted in terms of random coil structure. The absence of ordered structures is further confirmed by n.m.r. data. A small amount of ordered conformation can be induced either by increasing the temperature of the aqueous solution or by addition of TFE. The c.d. pattern of all glycopeptides in water at temperatures higher than 50 degrees C are compatible with the presence of a small amount of alpha-helix or 3(10) helix. Since the glyco-hexapeptide is too short to form an alpha-helix, the hypothesis is made that in the glycopeptides in water at high temperature a small amount of 3(10) helix is formed. The same is observed for the 21-residue glycopeptide in presence of 85% (v/v) TFE. In this medium, the c.d. data on the glyco-hexapeptide are more compatible with the presence of a small amount of beta-structure.

Blood group antigens: synthesis of Ss antigenic peptides related to human glycophorin B

The human Ss blood group antigens are located on glycophorin B, a minor human erythrocyte membrane glycoprotein. The structural difference in Ss antigens is determined by a Met/Thr polymorphism at position 29. This report describes the first synthesis of the two peptides carrying the Ss specificities, SS: Asn-Gly-Glu-Met-Gly-Gln-Leu-Val and ss: Asn-Gly-Glu-Thr-Gly-Gln-Leu-Val.

Possible role of the carbohydrate residues on the structure of the N-terminus of glycophorin AM

Natural-abundance 13C nuclear magnetic resonance (13C-n.m.r.) was used to study the effect of monoglycosylation on the structure and dynamics of a pentapeptide related to the N-terminus of glycophorin AM. The results of this study indicate that a single point of glycosylation, on the pentapeptide, can significantly affect its structure. Moreover, glycosylation of this pentapeptide also affects its dynamic motion in solution. This study further defines the role that the carbohydrate residue plays in determining the structure about the N-terminus of glycophorin AM.

Structural, dynamic, and metal-ion binding studies of the core glycopeptides beta-D-Gal-(1----3)-alpha-D-GalNAc----Ser, Thr by 13C-N.m.r. spectroscopy

13C-N.m.r. spectral data as well as spin-lattice relaxation times (T1 values) are presented for the core glycopeptides beta-D-Gal-(1----3)-alpha-D-GalNAc----Ser, Thr. The binding of Gd3+ to these model compounds containing N-terminal blocking groups and esterified carboxyl groups indicates that the disaccharide contains a rather weak, but unique, binding-site in the vicinity of C-2 of alpha-D-GalNAc (possibly involving N-2', the acetamido carbonyl group, O-3' and/or possibly the glycosidic oxygen atom (O-3)).