A convenient synthesis of 6′-C-substituted β-maltose heptaacetates and of panose

Synthesis of [¹⁸F]-labelled maltose derivatives as PET tracers for imaging bacterial infection

PURPOSE

To develop novel positron emission tomography (PET) agents for visualization and therapy monitoring of bacterial infections.

PROCEDURES

It is known that maltose and maltodextrins are energy sources for bacteria. Hence, (18)F-labelled maltose derivatives could be a valuable tool for imaging bacterial infections. We have developed methods to synthesize 4-O-(α-D-glucopyranosyl)-6-deoxy-6-[(18)F]fluoro-D-glucopyranoside (6-[(18)F]fluoromaltose) and 4-O-(α-D-glucopyranosyl)-1-deoxy-1-[(18)F]fluoro-D-glucopyranoside (1-[(18)F]fluoromaltose) as bacterial infection PET imaging agents. 6-[(18)F]fluoromaltose was prepared from precursor 1,2,3-tri-O-acetyl-4-O-(2',3',-di-O-acetyl-4',6'-benzylidene-α-D-glucopyranosyl)-6-deoxy-6-nosyl-D-glucopranoside (5). The synthesis involved the radio-fluorination of 5 followed by acidic and basic hydrolysis to give 6-[(18)F]fluoromaltose. In an analogous procedure, 1-[(18)F]fluoromaltose was synthesized from 2,3, 6-tri-O-acetyl-4-O-(2',3',4',6-tetra-O-acetyl-α-D-glucopyranosyl)-1-deoxy-1-O-triflyl-D-glucopranoside (9). Stability of 6-[(18)F]fluoromaltose in phosphate-buffered saline (PBS) and human and mouse serum at 37 °C was determined. Escherichia coli uptake of 6-[(18)F]fluoromaltose was examined.

RESULTS

A reliable synthesis of 1- and 6-[(18)F]fluoromaltose has been accomplished with 4-6 and 5-8% radiochemical yields, respectively (decay-corrected with 95 % radiochemical purity). 6-[(18)F]fluoromaltose was sufficiently stable over the time span needed for PET studies (∼96% intact compound after 1-h and ∼65% after 2-h incubation in serum). Bacterial uptake experiments indicated that E. coli transports 6-[(18)F]fluoromaltose. Competition assays showed that the uptake of 6-[(18)F]fluoromaltose was completely blocked by co-incubation with 1 mM of the natural substrate maltose.

CONCLUSION

We have successfully synthesized 1- and 6-[(18)F]fluoromaltose via direct fluorination of appropriate protected maltose precursors. Bacterial uptake experiments in E. coli and stability studies suggest a possible application of 6-[(18)F]fluoromaltose as a new PET imaging agent for visualization and monitoring of bacterial infections.

Synthesis of fluorinated maltose derivatives for monitoring protein interaction by (19)F NMR

A novel reporter system, which is applicable to the ¹⁹F NMR investigation of protein interactions, is presented. This approach uses 2-F-labeled maltose as a spy ligand to indirectly probe protein–ligand or protein–protein interactions of proteins fused or tagged to the maltose-binding protein (MBP). The key feature is the simultaneous NMR observation of both ¹⁹F NMR signals of gluco/manno-type-2-F-maltose-isomers; one isomer (α-gluco-type) binds to MBP and senses the protein interaction, and the nonbinding isomers (β-gluco- and/or α/β-manno-type) are utilized as internal references. Moreover, this reporter system was used for relative affinity studies of fluorinated and nonfluorinated carbohydrates to the maltose-binding protein, which were found to be in perfect agreement with published X-ray data. The results of the NMR competition experiments together with the established correlation between ¹⁹F chemical shift data and molecular interaction patterns, suggest valuable applications for studies of protein–ligand interaction interfaces.

Tuning the cavity of cyclodextrins: altered sugar adaptors in protein pores

Cyclodextrins (CDs) have been widely used in host-guest molecular recognition because of their chiral and hydrophobic cavities. For example, β-cyclodextrin (βCD) lodged as a molecular adaptor in protein pores such as α-hemolysin (αHL) is used for stochastic sensing. Here, we have tuned the cavity and overall size of βCD by replacing a single oxygen atom in its ring skeleton by a disulfide unit in two different configurations to both expand our ability to detect analytes and understand the interactions of βCD with protein pores. The three-dimensional structures of the two stereoisomeric CDs have been determined by the combined application of NMR spectroscopy and molecular simulation and show distorted conformations as compared to natural βCD. The interactions of these synthetic βCD analogues with mutant αHL protein pores and guest molecules were studied by single-channel electrical recording. The dissociation rate constants for both disulfide CDs from the mutant pores show ∼1000-fold increase as compared to those of unaltered βCD, but are ∼10-fold lower than the dissociation rate constants for βCD from wild-type αHL. Both of the skeleton-modified CDs show altered selectivity toward guest molecules. Our approach expands the breadth in sensitivity and diversity of sensing with protein pores and suggests structural parameters useful for CD design, particularly in the creation of asymmetric cavities.

Enzymatic synthesis of 4-methylumbelliferyl (1-->3)-beta-D-glucooligosaccharides-new substrates for beta-1,3-1,4-D-glucanase

The transglycosylation reactions catalyzed by beta-1,3-D-glucanases (laminaranases) were used to synthesize a number of 4-methylumbelliferyl (MeUmb) (1-->3)-beta-D-gluco-oligosaccharides having the common structure [beta-D-Glcp-(1-->3)](n)-beta-D-Glcp-MeUmb, where n=1-5. The beta-1,3-D-glucanases used were purified from the culture liquid of Oerskovia sp. and from a homogenate of the marine mollusc Spisula sachalinensis. Laminaran and curdlan were used as (1-->3)-beta-D-glucan donor substrates, while MeUmb-beta-D-glucoside (MeUmbGlcp) was employed as a transglycosylation acceptor. Modification of [beta-D-Glcp-(1-->3)](2)-beta-D-Glcp-MeUmb (MeUmbG(3)) gives 4,6-O-benzylidene-D-glucopyranosyl or 4,6-O-ethylidene-D-glucopyranosyl groups at the non-reducing end of artificial oligosaccharides. The structures of all oligosaccharides obtained were solved by 1H and 13C NMR spectroscopy and electrospray tandem mass spectrometry. The synthetic oligosaccharides were shown to be substrates for a beta-1,3-1,4-D-glucanase from Rhodothermus marinus, which releases MeUmb from beta-di- and beta-triglucosides and from acetal-protected beta-triglucosides. When acting upon substrates with d.p.>3, the enzyme exhibits an endolytic activity, primarily cleaving off MeUmbGlcp and MeUmbG(2).

Synthesis of 4'-O-acetyl-maltose and alpha-D-galactopyranosyl-(1-->4)-D-glucopyranose for biochemical studies of amylose biosynthesis

The chemical synthesis of the title compounds as maltose analogs, in which the non-reducing end is modified by acetylation of the 4'-OH group or by reversing its configuration, is reported. For synthesis of the 4'-O-acetylated analog, beta-maltose was converted into its per-O-benzylated-4',6'-O-benzylidene derivative followed by removal of the benzylidene acetal function and selective silylation at C-6'. Acetylation at C-4' of the obtained silylated compound followed by removal of the benzyl ether protecting groups and subsequent desilylation afforded the desired analog. The other maltose analog was synthesized via the glycosidation reaction between the glycosyl donor, O-(2,3,4,6-tetra-O-benzyl-alpha/beta-D-galactopyranosyl)trichloroacetimidate and the glycosyl acceptor, phenyl 2,3,6-tri-O-benzyl-1-thio-beta-D-glucopyranoside followed by removal of the phenylthio group and debenzylation to provide the desired analog.

Formation of alpha-(1-->6), alpha-(1-->3), and alpha-(1-->2) glycosidic linkages by dextransucrase from Streptococcus sanguis in acceptor-dependent reactions

Dextransucrase from Streptococcus sanguis 10558 was found to synthesize alpha-(1-->6), alpha-(1-->3), and alpha-(1-->2) linkages during an acceptor-dependent glucosyl transfer reaction. Normally, new glucosyl residues are added at C-6 of monosaccharide acceptors. However, sugars blocked at C-6 also can serve as good acceptors. The disaccharide and trisaccharide products formed when methyl 6-bromo-6-deoxy-alpha-D-glucopyranoside was used as acceptor were isolated and characterized. Both were found to contain only alpha-(1-->3) glycosidic bonds. This supports the hypothesis that when C-6 is blocked the acceptor binds to the enzyme in a flipped orientation, resulting in an approximate exchange in space of the C-3 and C-6, thereby putting C-3 adjacent to the active site. The second alpha-(1-->3) links in the trisaccharide are formed by a single-chain mechanism without release of the intermediate disaccharide. With maltose as acceptor, new glucosyl residues are added at C-6'. However, if that position is blocked with a bromine atom, the resulting compound, 6'-bromo-6'-deoxy-maltose, can still serve as an acceptor. The product in this case was isolated and characterized. The new glycosidic link was found to be alpha-(1-->2).

Strong competitive inhibition of porcine pancreatic alpha-amylase by aminodeoxy derivatives of maltose and maltotriose

The syntheses are described of 6-amino-6-deoxymaltose (2), the 6-amino-6-deoxy (4), 6'-amino-6'-deoxy (6), and 6"-amino-6"-deoxy (8) derivatives of maltotriose, and the methyl alpha- (10) and beta-glycoside (12) and the 1-deoxy derivative (16) of 4. The Ki values (microM) of these competitive inhibitors of porcine pancreatic alpha-amylase were: 2, 88; 4, 1.9; 6, 2.0; 8, 175; 10, 360; 12, 9000; 16, 7600 (cf. 1800 for maltotriose and 3000 for methyl alpha-maltotrioside). The low values for 4 and 6 reflect reinforcement of the normal binding by ionic attraction and, possibly, interaction of the reducing end groups with the protein.

Photolabile derivatives of maltose and maltotriose as ligands for the affinity labelling of the maltodextrin-binding site in porcine pancreatic alpha-amylase

The 3-azibutyl group was linked through sulfur to the anomeric position of maltose and maltotriose to yield the photolabile thioglycosides 3-azibutyl 1-thio-alpha-maltoside (11) and 3-azibutyl 1-thio-alpha-maltotrioside (12), and to the 4'- and 6'-position of maltose to give the thioethers 4'-S-(3-azibutyl)-4'-thiomaltose (8) and 6'-S-(3-azibutyl)-6'-thiomaltose (15). All four compounds were good competitive inhibitors of the action of porcine pancreatic alpha-amylase. Compound 12 irreversibly deactivated the enzyme to approximately 100% when irradiated together with the protein. The other compounds were much less effective. It is likely that separate areas of the enzyme binding site are chemically modified by the different ligands.