Synthesis of some glycodipeptides containing hydroxyamino acids, and their stabilities to acids and bases

A silyl ether-protected building block for O-GlcNAcylated peptide synthesis to enable one-pot acidic deprotection

In this report, we introduce a novel building block for Fmoc/tBu solid phase peptide synthesis (SPPS) of β-linked O-GlcNAcylated peptides. This building block carries acid labile silyl ether protecting groups, which are fully removed under TFA-mediated peptide cleavage conditions from the resin, thus requiring fewer synthetic steps and no intermediate purification as compared to other acid or base labile protecting group strategies.

Synthetic α- and β-Ser-ADP-ribosylated Peptides Reveal α-Ser-ADPr as the Native Epimer

A solid-phase methodology to synthesize oligopeptides, specifically incorporating serine residues linked to ADP-ribose (ADPr), is presented. Through the synthesis of both α- and β-anomers of the phosphoribosylated Fmoc-Ser building block and their usage in our modified solid-phase peptide synthesis protocol, both α- and β-ADPr peptides from a naturally Ser-ADPr containing H2B sequence were obtained. With these, and by digestion studies using the human glycohydrolase, ARH3 (hARH3), compelling evidence is obtained that the α-Ser-ADPr linkage comprises the naturally occurring configuration.

A Synthetic MUC1 Glycopeptide Bearing βGalNAc-Thr as a Tn Antigen Isomer Induces the Production of Antibodies against Tumor Cells

This study presents the synthesis of the novel protected O-glycosylated amino acid derivatives 1 and 2, containing βGalNAc-SerOBn and βGalNAc-ThrOBn units, respectively, as mimetics of the natural Tn antigen (αGalNAc-Ser/Thr), along with the solid-phase assembly of the glycopeptides NHAcSer-Ala-Pro-Asp-Thr[αGalNAc]-Arg-Pro-Ala-Pro-Gly-BSA (3-BSA) and NHAcSer-Ala-Pro-Asp-Thr[βGalNAc]-Arg-Pro-Ala-Pro-Gly-BSA (4-BSA), bearing αGalNAc-Thr or βGalNAc-Thr units, respectively, as mimetics of MUC1 tumor mucin glycoproteins. According to ELISA tests, immunizations of mice with βGalNAc-glycopeptide 4-BSA induced higher sera titers (1:320 000) than immunizations with αGalNAc-glycopeptide 3-BSA (1:40 000). Likewise, flow cytometry assays showed higher capacity of the obtained anti-glycopeptide 4-BSA antibodies to recognize MCF-7 tumor cells. Cross-recognition between immunopurified anti-βGalNAc antibodies and αGalNAc-glycopeptide and vice versa was also verified. Lastly, molecular dynamics simulations and surface plasmon resonance (SPR) showed that βGalNAc-glycopeptide 4 can interact with a model antitumor monoclonal antibody (SM3). Taken together, these data highlight the improved immunogenicity of the unnatural glycopeptide 4-BSA, bearing βGalNAc-Thr as Tn antigen isomer.

Preparation of glycosylated amino acids suitable for Fmoc solid-phase assembly

Many biological interactions and functions are mediated by glycans, consequently leading to the emerging importance of carbohydrate and glycoconjugate chemistry in the design of novel drug therapeutics. Despite the challenges that carbohydrate moieties bring into the synthesis of glycopeptides and glycoproteins, considerable progress has been made during recent decades. Glycopeptides carrying many simple glycans have been chemically synthesized, enzymatic approaches have been utilized to introduce more complex glycans, and most recently native chemical ligation has enabled synthesis of glycoproteins from well-designed peptide and glycopeptide building blocks. Currently, general synthetic methodology for glycopeptides relies on preformed glycosylated amino acids for the stepwise solid-phase peptide synthesis. The formation of glycosidic bonds is of fundamental importance in the assembly of glycopeptides. As such, every glycosylation has to be regarded as a unique problem, demanding considerable systematic research. In this chapter we will summarize the most common chemical methods for the stereoselective synthesis of N- and O-glycosylated amino acids. The particular emphasis will be given to the preparation of building blocks for use in solid-phase glycopeptide synthesis based on the 9-fluorenylmethoxycarbonyl (Fmoc) protective group strategy.

Influence of serine O-glycosylation or O-phosphorylation close to the vJun nuclear localisation sequence on nuclear import

Nuclear import triggered by the nuclear-localisation sequence (NLS) of the viral Jun (vJun) protein is mediated by phosphorylation of a serine close to the NLS. Since phosphorylation and glycosylation of serine residues are often in a reciprocal "yin-yang" relationship, we investigated whether glycosylation of this serine with O-linked N-acetylglucosamine (O-GlcNAc) would also regulate nuclear import via the vJun NLS. Peptides containing the vJun NLS with an adjacent O-phosphorylated, O-GlcNAc-functionalised or unmodified serine, and equipped with an N-terminal biotin or a 7-nitrobenz-2-oxa-1,3-diazolyl (NBD) fluorescent label, were synthesised on the solid phase by means of an Fmoc/Boc strategy and a Pd0-sensitive HYCRON linker. Fluorescence-polarisation measurements on the NBD-labelled peptides indicated that modification with phosphate or O-GlcNAc leads to a decrease in affinity to the import-mediating adapter protein, importin alpha, of about one order of magnitude compared to the unmodified NLS. Microinjection of biotinylated NLS peptide conjugated with fluorescently labelled avidin into NIH/3T3 and MDCK cells, revealed that avidin-unmodified-NLS peptide was rapidly imported into the nucleus. However, either phosphate or O-GlcNAc next to the NLS caused almost complete exclusion of the protein conjugate from nuclear import. These findings indicate that nuclear import by the vJun NLS might not be regulated by a "yin-yang" modification of an adjacent serine with phosphate or O-GlcNAc. Rather, negative regulation of binding between the polybasic NLS and importin by a negatively charged or a bulky, uncharged residue appears likely.

The First Total Synthesis of α-d-Glucopyranosyl-(1→2)-β-d-galactopyranosyl-O-pyridinoline, an Important Hallmark of Human Joint Diseases

The first total synthesis of alpha-D-glucopyranosyl-(1-->2)-beta-D-galactopyranosyl-O-pyridinoline using a short "one-pot" assembly of its 1,4,5-trisubstituted 3-hydroxypyridinium ring is reported.

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.

A concise methodology for the stereoselective synthesis of O-glycosylated amino acid building blocks: complete 1H NMR assignments and their application in solid-phase glycopeptide synthesis

A facile strategy for the stereoselective synthesis of suitably protected O-glycosylated amino acid building blocks, namely, Nalpha-Fmoc-Ser-[Ac4-beta-D-Gal-(1-3)-Ac2-alpha or beta-D-GalN3]-OPfp and Nalpha-Fmoc-Thr-[Ac4-beta-D-Gal-(1-3)-Ac2-alpha or beta-D-GalN3]-OPfp is described. What is new and novel in this report is that Koenigs-Knorr type glycosylation of an aglycon serine/threonine derivative (i.e. Nalpha-Fmoc-Ser-OPfp or Nalpha-Fmoc-Thr-OPfp) with protected beta-D-Gal(1-3)-D-GalN3 synthon mediated by silver salts resulted in only alpha- and/or beta-isomers in excellent yields under two different reaction conditions. The subtle differences in stereoselectivity were demonstrated clearly when glycosylation was carried out using only AgClO4 at -40 degrees C which afforded a-isomer in a quantitative yield (alpha:beta = 5:1). On the other hand, the beta-isomer was formed exclusively when the reaction was performed in the presence of Ag2CO3/AgClO4 at room temperature. A complete assignment of 1H resonances to individual sugar ring protons and the characteristic anomeric alpha-1 H and beta-1 H in Ac4Galbeta(1-3)Ac2GalN3 alpha and/or beta linked to Ser/Thr building blocks was accomplished unequivocally by two-dimensional double-quantum filtered correlated spectroscopy and nuclear Overhauser enhancement and exchange spectroscopy NMR experiments. An unambiguous structural characterization and documentation of chemical shifts, including the coupling constants for all the protons of the aforementioned alpha- and beta-isomers of the O-glycosylated amino acid building blocks carrying protected beta-D-Gal(1-3)-D-GalN3, could serve as a template in elucidating the three-dimensional structure of glycoproteins. The synthetic utility of the building blocks and versatility of the strategy was exemplified in the construction of human salivary mucin (MUC7)-derived, O-linked glycopeptides with varied degrees of glycosylation by solid-phase Fmoc chemistry. Fmoc/tert-butyl-based protecting groups were used for the peptidic moieties in conjunction with acetyl sugar protection. The transformation of the 2-azido group into the acetamido derivative was carried out with thioacetic acid on the polymer-bound glycopeptides before the cleavage step. After cleaving the glycopeptide from the resin, the acetyl groups used for sugar OH-protection were removed with sodium methoxide in methanol. Finally, the glycopeptides were purified by reversed-phase high-performance liquid chromatography and their integrity was confirmed by proton NMR as well as by mass spectral analysis. Secondary structure analysis by circular dichroism of both the glycosylated and nonglycosylated peptides revealed that carbohydrates did not exert any profound structural effect on the peptide backbone conformation.

Neoglycopeptide synthesis and purification in multi-gram scale: preparation of O-(2,3,4,6-tetra-O-acetyl-beta-D-galactopyranosyl)-N alpha-fluoren-9-yl-methoxycarbonyl-hydroxyproline and its use in the pilot-scale synthesis of the potent analgesic glycopeptide O1.5-beta-D-galactopyranosyl [DMet2, Hyp5]enkephalinamide

The preparation of a beta-galactosylated hydroxyproline derivative and its use in the multi-gram solid-phase synthesis of the potent analgesic neoglycopeptide O1.5-beta-D-galactopyranosyl [D-Met2, Hyp5]enkephalinamide is described in this paper. The most closely related impurities have been identified, isolated and characterized. Significant aspects of the synthesis and purification affecting yields and purity of both the building block and the target neoglycopeptide are discussed.

Chemoselective Removal of Protecting Groups from O-Glycosyl Amino Acid and Peptide (Methoxyethoxy)ethyl Esters Using Lipases and Papain

The selective C-terminal deprotection of O-glycopeptide (methoxyethoxy)ethyl esters is achieved under mild conditions (pH 6.6, 37 degrees C) by enzymatic hydrolysis using papain or lipase M from Mucor javanicus to give building blocks useful for chain-extending glycopeptide synthesis. On the other hand, the selective removal of acetyl protecting groups from the saccharide portion of glycopeptides is accomplished by alternative enzymatic hydrolysis with lipase WG from wheat germ to furnish model substrates for enzymatic glycosyl transfer reactions in order to extend the carbohydrate side chain of these conjugates.

Removal of Acyl Protective Groups from Glycopeptides: Base Does Not Epimerize Peptide Stereocenters, and beta-Elimination Is Slow

Epimerization of glycopeptide stereocenters and beta-elimination have been considered as important potential side reactions on deacylation of glycopeptides which have the carbohydrate moieties protected with O-acyl groups. Since no systematic investigation of these side reactions has been reported, a model acetylated, O-linked glycotripeptide and its three epimers at the alpha-carbon stereocenters were prepared. The model glycopeptide did not undergo any epimerization (<1%) or beta-elimination, as determined by (1)H NMR spectroscopy, under various conditions which are in common use for deacetylation of glycopeptides. Under more severe conditions, which are required for removal of O-benzoyl groups, beta-elimination occurred slowly and was accompanied by slight (<5%) epimerization. The surprisingly low tendency of glycopeptides to undergo base catalyzed epimerization and beta-elimination is most likely due to protection of the alpha-carbon stereocenters by deprotonation of the adjacent amide groups.

Stereoselective synthesis of O-serinyl/threoninyl-2-acetamido-2-deoxy-alpha- or beta-glycosides

General glycosidation methodology has been developed which can selectively provide 2-acetamido-2-deoxy-alpha- or beta-glycosides of beta-hydroxy-alpha-amino acid derivatives [glucopyranoside-(8, 43), galactopyranoside- (9, 13), mannopyranoside- (10), lactoside analogs (11, 38) and 3-O-beta-galactopyranosyl-mannopyranoside (12)] stereoselectively in excellent yield from the highly nucleophilic alpha-imino esters (Schiff bases) of L-serine and L-threonine. Various glycosides were converted via their amino and acetamido derivatives to Fmoc-protected serinyl- or threoninyl-glycosides (24-28, 37, 41, 46) which are all suitable building blocks for the solid-phase synthesis of O-glycopeptides. Complete 1H- and 13C-NMR data are provided for all compounds.

Structure of a novel oligosaccharide-mycosporine-amino acid ultraviolet A/B sunscreen pigment from the terrestrial cyanobacterium Nostoc commune

Water-soluble UV-A/B-absorbing pigments are secreted by cells of the cosmopolitan terrestrial cyanobacterium Nostoc commune. The pigments constitute a complex mixture of monomers with molecular masses of up to 1801 Da. Two different chromophores with absorption maxima at 312 and 335 nm are linked to different amino acids and to oligosaccharides consisting of galactose, glucose, xylose, glucuronic acid, and glucosamine. The 335 nm chromophore is a 1,3-diaminocyclohexen derivative, while the chromophore with an absorption maximum at 312 nm is most likely a 3-aminocyclohexen-1-one derivative. These UV-inducible substances are the first mycosporines to be described covalently linked to oligosaccharides. The pigments are located in the extracellular glycan sheath of Nostoc colonies, where they form complexes of extremely high molecular mass that are attached noncovalently to the glycan sheath. Pigments occur in concentrations that permit the cells to attenuate a significant part of incident UV-B radiation.

Studies on the formation of lysinomethylalanine and histidinomethylalanine in milk products

From reaction mixtures consisting of N-acetyldehydroaminobutyric acid methyl ester and N alpha-acetyl-L-lysine or N alpha-acetyl-L-histidine, respectively, distinct amounts of the cross-link amino acids N epsilon-(2-amino-2-carboxy-1-methyl-ethyl)-L-lysine (lysinomethylalanine, LMeAL) and N tau-(2'-amino-2'-carboxy-1'-methyl-ethyl)-L-histidine (histidinomethylalanine, HMeAL) were isolated via preparative ion-exchange chromatography and identified by 1H- and 13C-nuclear magnetic resonance. In the amino acid chromatogram, both compounds eluted clearly separated from other basic amino acids. However, neither LMeAL nor HMeAL could be detected in numerous acid hydrolysates of a range of milk products. In model studies, threonine showed a significantly lower tendency for an alkali-induced beta-elimination reaction compared to serine. The reactivity of the resulting dehydroaminobutyric acid towards nucleophiles was more than tenfold lower as compared to dehydroalanine. Thus, the formation of LMeAL as well as of HMeAL during food processing is negligible.

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.

Solid-phase synthesis and conformational studies of glycosylated derivatives of helper-T-cell immunogenic peptides from hen-egg lysozyme

3-Mercaptopropionic acid and N alpha-Fmoc serine, both with unprotected carboxyl groups, were stereospecifically glycosylated in 62-82% yields, using saccharide 1,2-trans peracetates and Lewis acid catalysis. The resulting glycosylated building blocks were used in the synthesis of derivatives of helper-T-cell stimulating peptides, with the carbohydrate moiety located at the amino terminus, or internally in the peptide chain. 1H NMR spectroscopy in Me2SO-d6 showed that the glycopeptides assumed random conformations, which were not influenced by the glycosylation or by single substitutions of amino acids in the peptide moiety.

Structural analysis of O-glycosidic type of sialyloligosaccharide-alditols derived from urinary glycopeptides of a sialidosis patient

Sialidosis urine was fractionated by gel filtration on Bio-Gel P-6. All pooled fractions containing carbohydrates showed the presence of small amounts of GalNAc in non-reducing position, besides free N-acetyllactosamine type of oligosaccharides as major constituents. The fractions were subjected to reductive alkaline borohydride degradation, after which the major part of GalNAc was recovered as N-acetyl-D-galactosaminitol (GalNAc-ol). The GalNAc-ol-containing material was separated from the N-glycosidic oligosaccharides by a second gel-filtration step on AcA 202. Subsequently, the O-glycosidic sialyloligosaccharide-alditols were subfractionated by anion-exchange chromatography on Mono Q. Structural analysis by 500-MHz 1H-NMR spectroscopy revealed two major components in all fractions, namely: NeuAc alpha 2-3Gal beta 1-3GalNAc-ol and NeuAc alpha 2-3Gal beta 1-3[NeuAc alpha 2-6]GalNAc-ol. Furthermore, NeuAc alpha 2-3Gal beta 1-3[NeuAc alpha 2-3Gal beta 1-4GlcNAc beta 1-6]GalNAc-ol was found as a minor component in some of the fractions. The presence of these carbohydrate chains in Bio-Gel fractions differing in molecular mass suggested that they are derived from glycopeptides which are heterogeneous in their peptide part.

The structure of sialyl-glycopeptides of the O-glycosidic type, isolated from sialidosis (mucolipidosis I) urine

Sialyl-glycopeptides containing an O-glycosidically linked tetrasaccharide chain were obtained from the urine of a patient suffering from mucolipidosis I. Isolation of these compounds was achieved by gel filtration, ion-exchange chromatography and preparative paper chromatography. Their structures were determined by a combination of carbohydrate and amino acid analysis, dansylation, periodate oxidation, methylation studies, enzymatic hydrolysis and 1H-NMR spectroscopy, to be as follows: (formula; see text) wherein R = peptide linked through -Thr-, -Ser-Thr- or -Thr-Ser-. The finding of these glycopeptides in urine shows that mucolipidosis I is characterized by a general "glycoprotein-specific" sialidase deficiency. The possibility of the existence of a human endo-alpha-N-acetylgalactosaminidase is discussed.

Partial deglycosylation of blood-group-specific glycoproteins

The time course for the partial deglycosylation of blood-group-specific glycoproteins from human ovarian-cyst fluids with 0.25 M-H2SO4/acetic acid and 6 M-HCl in methanol was studied. Either reagent readily removed about 80% of the carbohydrate from the glycoproteins to leave non-diffusible glycopeptides that contain N-acetylgalactosamine as the predominant sugar. Some changes in amino acid distribution were observed during the deglycosylation, which were attributed to an accelerated break-up of the nonglycosylated regions of the parent glycoprotein. The N-acetylgalactosaminyl-peptides isolated were judged to be polydisperse by gel filtration, and ion-exchange chromatography divided the glycopeptide population into several fractions with differing amino acid compositions. A Lumbricus terrestris hexosaminidase preparation was successful in removing almost all the remaining sugar from the glycopeptides, but caused further rupture of the peptide. When a per O-acetylated glycoprotein was treated with the H2SO4/acetic acid reagent the glycopeptide contained, in addition to N-acetylgalactosamine, about 50% of the sialic acid present in the parent glycoprotein, indicating that most of this sugar is located near the peptide end of the carbohydrate chains.