The active ester N-fmoc-3-O-[Ac4-α-D-Manp-(1→2)-Ac3-α-D-Manp-1-]=thre-O-Pfp as a building block in solid-phase synthesis of an o-linked dimannosyl glycopeptide

Parallel solid-phase synthesis of mucin-like glycopeptides

The glycopeptide, Ac-Pro-Thr(alpha-D-GalNAc)-Thr(alpha-D-GalNAc)-Thr(alpha-d-GalNAc)-Pro-Leu-Lys-NH(2) (1), which features three consecutive O-glycosylated Thr residues and mimics a portion of mucin 2, has been prepared by solid-phase synthesis. Seven related, partially glycosylated peptides (2-8) were synthesized as well. This suite of molecules allowed a systematic analysis of synthetic protocols. N(alpha)-(9-Fluorenylmethoxycarbonyl)-O-(3,4,6-tri-O-acetyl-2-azido-2-deoxy-alpha-D-galactopyranosyl)-L-threonine pentafluorophenyl ester [Fmoc-L-Thr(Ac(3)-alpha-D-GalN(3))-OPfp] was used as a building block that coupled efficiently when used in a relatively low molar excess, that is, approximately 1.5 equiv, with N,N-dimethylformamide (DMF) as the solvent. For conversion of the azido group to the N-acetyl function, direct treatment with thioacetic acid was preferred over a two-step procedure involving reduction with dithiothreitol (DTT) followed by N-acetylation. Effective O-deacetylation of 1-8 in solution was achieved by treatment with sodium methoxide (10-15 mM; approximately 5 equiv) in methanol. On-resin deacetylation techniques were also examined, using sodium methoxide (6-10 mM) in DMF-methanol (17:3) (for 4 and 11) or hydrazine (70 mM) in methanol (for 8). The more convenient on-resin technique in DMF-methanol gave yields similar to solution conditions, and promises to be widely useful for solid-phase glycopeptide synthesis. HPLC profiles showed that free glycopeptides elute earlier than the corresponding O-acetylated derivatives, and that retention times vary systematically with the number of sugar moieties. (1)H NMR studies carried out in water showed an increase in conformational organization of glycopeptides with increased density of glycosylation.

Solid-phase synthesis of O-linked glycopeptide analogues of enkephalin

The synthesis of 18 N-alpha-FMOC-amino acid glycosides for solid-phase glycopeptide assembly is reported. The glycosides were synthesized either from the corresponding O'Donnell Schiff bases or from N-alpha-FMOC-amino protected serine or threonine and the appropriate glycosyl bromide using Hanessian's modification of the Koenigs-Knorr reaction. Reaction rates of D-glycosyl bromides (e.g., acetobromoglucose) with the L- and D-forms of serine and threonine are distinctly different and can be rationalized in terms of the steric interactions within the two types of diastereomeric transition states for the D/L and D/D reactant pairs. The N-alpha-FMOC-protected glycosides [monosaccharides Xyl, Glc, Gal, Man, GlcNAc, and GalNAc; disaccharides Gal-beta(1-4)-Glc (lactose), Glc-beta(1-4)-Glc (cellobiose), and Gal-alpha(1-6)-Glc (melibiose)] were incorporated into 22 enkephalin glycopeptide analogues. These peptide opiates bearing the pharmacophore H-Tyr-c[DCys-Gly-Phe-DCys]- were designed to probe the significance of the glycoside moiety and the carbohydrate-peptide linkage region in blood-brain barrier (BBB) transport, opiate receptor binding, and analgesia.

Synthesis of tumor associated sialyl-T-glycopeptides and their immunogenicity

Sialyl-T-glycopeptides were synthesized by solid-phase techniques, using a PEGA resin as the solid support. An appropriately protected building block containing alpha-Neu5Ac-(2 --> 3)-beta-Gal-(1 --> 3)-alpha-GalN3-(1-->) attached to Fmoc-Thr/Ser-OPfp was employed in a solid phase glycopeptide assembly of a 10-mer glycopeptide, using a general Fmoc/OPfp-ester strategy. Reduction of the azido group of the GalN3 residue was effected on solid-phase, using DTT and DBU. After acidolytic cleavage from the resin, the methyl ester of the sialic acid residue and acetyl groups were removed with 30% NaOMe/MeOH in MeOH and water pH 14, at -30 degrees C for 2 h. At this low temperature, the highly basic conditions did not result in any detectable beta-elimination. However, one O-acetyl group, located at the 2-position of the Gal was resistant to hydrolysis. To remove this remaining acetyl group, reaction with hydrazine hydrate in CHCl3 and MeOH at room temperature for 2.5 h was successful. The two target sequences of sialyl-T-glycopeptides were obtained in good yield. In contrast to the the analogs carrying the T-antigen, the Sial-T-glycopeptides were nonimmunogenic, supporting the idea that the sialylation is a method of circumventing the recognition by the immune system.

Carbopeptides: carbohydrates as potential templates for de novo design of protein models

De novo design of proteins has evolved into a powerful approach for studying the factors governing protein folding and stability. Among the families of structures frequently studied is the 'four-helix bundle' in which four alpha-helical peptide strands, linked by loops, form a hydrophobic core. Assembly of protein models on a template has been suggested as a way to reduce the entropy of folding. Here we describe the potential use of a carbohydrate as such a template. The monosaccharide D-galactose was per-O-acylated with (Nbeta-Fmoc-betaAla)2O to give a penta-substituted derivative, which was converted to the corresponding glycosyl bromide and used for the glycosylation of 4-hydroxymethylbenzoic acid pentafluorophenyl ester (HMBA-OPfp). The beta-glycosidic carbohydrate template (Nbeta-Fmoc-3Ala)4-beta-D-Galp-(1-O)-MBA-OPfp thus obtained was coupled to a PAL-PEG-PS resin and simultaneously extended at the four arms to yield, after cleavage from the solid support, a carbopeptide with four identical peptide strands. Extension of this concept to, for example, synthesis of novel multiple antigenic peptides (MAPs) and synthesis of carbohydrate clusters can be easily envisioned. The ability to efficiently synthesize such structures sets the stage for further studies to test whether the carbohydrate templates do indeed nucleate folding.

Chemical synthesis and receptor binding of catfish somatostatin: a disulfide-bridged beta-D-Galp-(1-->3)-alpha-D-GalpNAc O-glycopeptide

The glycopeptide hormone catfish somatostatin (somatostatin-22) has the amino acid sequence H-Asp-Asn-Thr-Val-Thr-Ser-Lys-Pro-Leu-Asn-Cys-Met-Asn-Tyr-Phe-Trp-Lys-Se r-Arg-Thr-Ala-Cys-OH; it includes a cyclic disulfide connecting the two Cys residues, and the major naturally occurring glycoform contains D-GalNAc and D-Gal O-glycosidically linked to Thr5. The linear sequence was assembled smoothly starting with an Fmoc-Cys(Trt)-PAC-PEG-PS support, using stepwise Fmoc solid-phase chemistry. In addition to the nonglycosylated peptide, two glycosylated forms of somatostatin-22 were accessed by incorporating as building blocks, respectively, Nalpha-Fmoc-Thr(Ac3-alpha-D-GalNAc)-OH and Nalpha-Fmoc-Thr(Ac4-beta-D-Gal-(1-->3)-Ac2-alpha-D-GalNAc)-O H. Acidolytic deprotection/cleavage of these peptidyl-resins with trifluoroacetic acid/scavenger cocktails gave the corresponding acetyl-protected glycopeptides with free sulfhydryl functions. Deacetylation, by methanolysis in the presence of catalytic sodium methoxide, was followed by mild oxidation at pH 7, mediated by Nalpha-dithiasuccinoyl (Dts)-glycine, to provide the desired monomeric cyclic disulfides. The purified peptides were tested for binding affinities to a panel of cloned human somatostatin receptor subtypes; in several cases, presence of the disaccharide moiety resulted in 2-fold tighter binding.

Solid-phase synthesis of an O-linked glycopeptide based on a benzyl-protected glycan approach

The solid-phase synthesis of asialo-[Ala18]-B-chain (2) of human alpha 2HS glycoprotein is described. Disaccharide-linked serine unit 12, carrying a benzyl protecting group, was synthesized via stereoselective glycosylation of 8 with 6. Peptide synthesis was carried out by the Fmoc method utilizing an automated peptide synthesizer. A modified procedure using a mechanical shaker at the coupling step with 12 made easy the recovery of unreacted 12. The benzylated glycopeptide thus synthesized was cleaved from the resin and hydrogenated to give 2.

Solid-phase synthesis of a glycosylated hexapeptide of human sialophorin, using the trichloroacetimidate method

A hexapeptide containing a beta-D-Gal p-(1-->3)-alpha-D-Gal pNAc-(1-->O)-L-threonine unit was synthesized using glycosylated pentafluorophenyl esters in an Fmoc-based strategy. In all of the glycosylation reactions, trichloroacetimidates were successfully employed. The disaccharide moiety was prepared from tetra-O-acetyl-alpha-D-galactopyranosyl trichloroacetimidate and tert-butyldimethylsilyl 2-azido-6-O-benzoyl-2-deoxy-beta-E-galactopyranoside with boron trifluoride etherate as a catalyst. The glycosylated active esters were obtained in the reaction of alpha and beta 2,3,4,6-tetra-O-acetyl-beta-D-galactopyranosyl-(1-->3)-4-O-acetyl-2-azid o-6- O-benzoyl-2-deoxy-D-galactopyranosyl trichloroacetimidates with Fmoc-protected pentafluorophenyl esters of L-serine and L-threonine in the presence of trimethylsilyl trifluoromethanesulfonate as Lewis acid. The glycosylated pentafluorophenyl ester of L-threonine was transformed into glycopeptides via a solid-phase synthesis. Azide reduction and N-acetylation were performed on the solid phase with a thioacetic acid-pyridine mixture. The glycopeptide was then cleaved from the resin with strong acid, also removing the acid-labile protecting groups of the peptide chain. Finally, the acyl groups used for sugar protection were cleaved with sodium methoxide, affording the completely deprotected N-acetyl-L-leucyl-L-glutamyl-O-[beta-D-galactopyranosyl-(1-->3)-2- acetamido-2-deoxy-alpha-D-galactopyranosyl]-L-threonyl-L-seryl-L-threony l- glycinamide (1) in high purity.

Chemical and enzymatic synthesis of multivalent sialoglycopeptides

Linear and branched glycopeptides containing multiple sialyl-N-acetyllactosamine side chains have been synthesized using a combined chemical and enzymatic approach. Peptide backbones in which beta-GlcNAc-Asn residues were incorporated were obtained in good yields by optimized solid-phase synthesis following the Boc strategy. The resulting multivalent glycopeptides were galactosylated in near-quantitative yields using bovine galactosyltransferase, UDP-galactose, and calf alkaline phosphatase that destroys the inhibiting side product UDP. Subsequent enzymatic sialylation yielded the desired glycopeptides containing asparagine-linked sialyl-N-acetyllactosamine side chains. The compounds were characterized by 1H NMR and FABMS. Recombinant sialyltransferase and CMP-sialate synthetase were used for the enzymatic synthesis of sialosides on a preparative scale. The synthetic glycopeptides were tested as inhibitors of influenza virus to cells, revealing that most of the multivalent sialoglycopeptides exhibit increased binding that depends on the spacing when compared to monovalent compounds. A possible mechanism for increased binding is proposed.

Solid-phase synthesis of O-mannosylated peptides: two strategies compared

A comparison of the O-glycosylation of resin-bound assembled peptides with the incorporation of glycosylated amino acids using established chemistry is presented. Fmoc/tert-butyl-based protecting groups were used for the peptidic moieties in conjunction with acetyl sugar protection. Koenigs-Knorr glycosylations were carried out using protected bromomannose derivatives, the acceptor being threonine or serine, either in solution or within a resin-bound peptide. The characterisation of microgram quantities of glycopeptides by the use of glycosidases in combination with mass spectrometry is also described.

Synthesis and conformational and NMR studies of alpha-D-mannopyranosyl and alpha-D-mannopyranosyl-(1----2)-alpha-D-mannopyranosyl linked to L-serine and L-threonine

alpha-D-Mannopyranosyl and alpha-D-mannopyranosyl-(1----2)-alpha-D- mannopyranosyl linked to L-serine and L-threonine have been synthesised as model substances for the linkage region in certain O-linked glycoproteins. Metropolis Monte Carlo simulations were performed with a modified version of the GESA program, to yield theoretical NOEs and interatomic distances as ensemble-average values, and these were compared with results from steady-state NOE experiments. The NOEs were determined as ensemble-average and as global minimum values. NMR chemical shift differences, obtained for signals of the glycopeptides relative to those of the respective monomers, were interpreted in terms of short inter-residue atomic distances as found within the global minima, and on the basis of averaged distances derived from Monte Carlo simulations.

Solid phase synthesis of the fibronectin glycopeptide V(Gal beta 3GalNAc alpha)THPGY, its beta analogue, and the corresponding unglycosylated peptide

The fibronectin fragment VTHPGY and the corresponding glycopeptides V(Gal beta 3GalNAc alpha)THPGY and V(Gal beta 3GalNAc beta)THPGY were synthesized by the FMOC/solid phase approach. FMOC derivatives of threonine, carrying O-linked, peracetylated Gal beta 3GalNAc chains were used for introduction (HOBt-mediated coupling) of the disaccharide moieties.