Chemical synthesis and NMR spectra of a protected branched-tetrasaccharide thioglycoside, a useful intermediate for the synthesis of branched oligosaccharides

Stereoselective synthesis of a 4-⍺-glucoside of valienamine and its X-ray structure in complex with Streptomyces coelicolor GlgE1-V279S

Glycoside hydrolases (GH) are a large family of hydrolytic enzymes found in all domains of life. As such, they control a plethora of normal and pathogenic biological functions. Thus, understanding selective inhibition of GH enzymes at the atomic level can lead to the identification of new classes of therapeutics. In these studies, we identified a 4-⍺-glucoside of valienamine (8) as an inhibitor of Streptomyces coelicolor (Sco) GlgE1-V279S which belongs to the GH13 Carbohydrate Active EnZyme family. The results obtained from the dose-response experiments show that 8 at a concentration of 1000 µM reduced the enzyme activity of Sco GlgE1-V279S by 65%. The synthetic route to 8 and a closely related 4-⍺-glucoside of validamine (7) was achieved starting from readily available D-maltose. A key step in the synthesis was a chelation-controlled addition of vinylmagnesium bromide to a maltose-derived enone intermediate. X-ray structures of both 7 and 8 in complex with Sco GlgE1-V279S were solved to resolutions of 1.75 and 1.83 Å, respectively. Structural analysis revealed the valienamine derivative 8 binds the enzyme in an E₂ conformation for the cyclohexene fragment. Also, the cyclohexene fragment shows a new hydrogen-bonding contact from the pseudo-diaxial C(3)-OH to the catalytic nucleophile Asp 394 at the enzyme active site. Asp 394, in fact, forms a bidentate interaction with both the C(3)-OH and C(7)-OH of the inhibitor. In contrast, compound 7 disrupts the catalytic sidechain interaction network of Sco GlgE1-V279S via steric interactions resulting in a conformation change in Asp 394. These findings will have implications for the design other aminocarbasugar-based GH13-inhibitors and will be useful for identifying more potent and selective inhibitors.

Vanillin formation from ferulic acid in Vanilla planifolia is catalysed by a single enzyme

Vanillin is a popular and valuable flavour compound. It is the key constituent of the natural vanilla flavour obtained from cured vanilla pods. Here we show that a single hydratase/lyase type enzyme designated vanillin synthase (VpVAN) catalyses direct conversion of ferulic acid and its glucoside into vanillin and its glucoside, respectively. The enzyme shows high sequence similarity to cysteine proteinases and is specific to the substitution pattern at the aromatic ring and does not metabolize caffeic acid and p-coumaric acid as demonstrated by coupled transcription/translation assays. VpVAN localizes to the inner part of the vanilla pod and high transcript levels are found in single cells located a few cell layers from the inner epidermis. Transient expression of VpVAN in tobacco and stable expression in barley in combination with the action of endogenous alcohol dehydrogenases and UDP-glucosyltransferases result in vanillyl alcohol glucoside formation from endogenous ferulic acid. A gene encoding an enzyme showing 71% sequence identity to VpVAN was identified in another vanillin-producing plant species Glechoma hederacea and was also shown to be a vanillin synthase as demonstrated by transient expression in tobacco.

Vanilla is derived from vanillin isolated from a vanillin-producing orchid, but the process is laborious, costly and results in a small yield. Here, the authors identified an enzyme from the orchid, Vanilla planifolia, that is able to catalyse the formation of vanillin and vanillin glucoside from ferulic acid and its glucoside in vitro, respectively.

Synthesis of zearalenone-16-β,D-glucoside and zearalenone-16-sulfate: A tale of protecting resorcylic acid lactones for regiocontrolled conjugation

The development of a reliable procedure for the synthesis of the 16-glucoside and 16-sulfate of the resorcylic acid lactone (RAL) type compound zearalenone is presented. Different protective group strategies were considered and applied to enable the preparation of glucosides and sulfates that are difficult to access up to now. Acetyl and p-methoxybenzyl protection led to undesired results and were shown to be inappropriate. Finally, triisopropylsilyl-protected zearalenone was successfully used as intermediate for the first synthesis of the corresponding mycotoxin glucoside and sulfate that are highly valuable as reference materials for further studies in the emerging field of masked mycotoxins. Furthermore, high stability was observed for aryl sulfates prepared as tetrabutylammonium salts. Overall, these findings should be applicable for the synthesis of similar RAL type and natural product conjugates.

Starch phosphorylation--maltosidic restrains upon 3'- and 6'-phosphorylation investigated by chemical synthesis, molecular dynamics and NMR spectroscopy

Phosphorylation is the only known in vivo substitution of starch, yet no structural evidence has been provided to explain its implications of the amylosidic backbone and its stimulating effects on starch degradation in plants. In this study, we provide evidence for a major influence on the glucosidic bond in starch specifically induced by the 3-O-phosphate. Two phosphorylated maltose model compounds were synthesized and subjected to combined molecular dynamics (MD) studies and 950 MHz NMR studies. The two phosphorylated disaccharides represent the two possible phosphorylation sites observed in natural starches, namely maltose phosphorylated at the 3'- and 6'-position (maltose-3'-O-phosphate and maltose-6'-O-phosphate). When compared with maltose, both of the maltose-phosphates exhibit a restricted conformational space of the alpha(1-->4) glycosidic linkage. When maltose is phosphorylated in the 3'-position, MD and NMR show that the glucosidic space is seriously restricted to one narrow potential energy well which is strongly offset from the global potential energy well of maltose and almost 50 degrees degrees from the Phi angle of the alpha-maltose crystal structure. The driving force is primarily steric, but the configuration of the structural waters is also significantly altered. Both the favored conformation of the maltose-3'-phosphate and the maltose-6'-phosphate align well into the 6-fold double helical structure of amylopectin when the effects on the glucosidic bond are not taken into account. However, the restrained geometry of the glucosidic linkage of maltose-3'-phosphate cannot be accommodated in the helical structure, suggesting a major local disturbing effect, if present in the starch granule semi-crystalline lattice.

Chemical synthesis of a dual branched malto-decaose: a potential substrate for alpha-amylases

A convergent block strategy for general use in efficient synthesis of complex alpha-(1-->4)- and alpha-(1-->6)-malto-oligosaccharides is demonstrated with the first chemical synthesis of a malto-oligosaccharide, the decasaccharide 6,6''''-bis(alpha-maltosyl)-maltohexaose, with two branch points. Using this chemically defined branched oligosaccharide as a substrate, the cleavage pattern of seven different alpha-amylases were investigated. Alpha-amylases from human saliva, porcine pancreas, barley alpha-amylase 2 and recombinant barley alpha-amylase 1 all hydrolysed the decasaccharide selectively. This resulted in a branched hexasaccharide and a branched tetrasaccharide. Alpha-amylases from Asperagillus oryzae, Bacillus licheniformis and Bacillus sp. cleaved the decasaccharide at two distinct sites, either producing two branched pentasaccharides, or a branched hexasaccharide and a branched tetrasaccharide. In addition, the enzymes were tested on the single-branched octasaccharide 6-alpha-maltosyl-maltohexaose, which was prepared from 6,6''''-bis(alpha-maltosyl)-maltohexaose by treatment with malt limit dextrinase. A similar cleavage pattern to that found for the corresponding linear malto-oligosaccharide substrate was observed.

Chemical synthesis of methyl 6'-alpha-maltosyl-alpha-maltotrioside and its use for investigation of the action of starch synthase II

The branched pentasaccharide methyl 6'-alpha-maltosyl-alpha-maltotrioside was chemically synthesised and investigated as a primer for particulate starch synthase II (SSII) using starch granules prepared from the low-amylose pea mutant lam as the enzyme source. For chemical synthesis, the trichloroacetimidate activation method was used to synthesise methyl O-(2,3,4,6-tetra-O-benzyl-alpha-D-glucopyranosyl)-(1-->4)-O-(2,3,6-tri-O-benzyl-alpha-D-glucopyranosyl)-(1-->6)-O-[(2,3,4,6-tetra-O-benzyl-alpha-D-glucopyranosyl-(1-->4)]-O-(2,3-di-O-benzyl-alpha-D-glucopyranosyl)-(1-->4)-2,3,6-tri-O-benzyl-alpha-D-glucopyranoside, which was then debenzylated to provide the desired branched pentasaccharide methyl 6'-alpha-maltosyl-alpha-maltotrioside as documented by 1H and 13C NMR spectroscopy. Using a large excess of the maltoside, the pentasaccharide was tested as a substrate for starch synthase II (SSII). Both of the non-reducing ends of methyl 6'-alpha-maltosyl-alpha-maltotrioside were extended equally resulting in two hexasaccharide products in nearly equal amounts. Thus, SSII catalyses an equimolar and non-processive elongation reaction of this substrate. Accordingly, the presence of the alpha-1,6 linkages does not dictate a specific structure of the pentasaccharide in which only one of the two non-reducing ends are available for extension.

A facile protocol for direct conversion of unprotected sugars into phenyl 4,6-O-benzylidene-per-O-acetylated-1,2-trans-thioglycosides

A short and practical methodology for conversion of unprotected D-glucose, maltose, cellobiose and lactose into the corresponding phenyl 4,6-O-benzylidine-per-O-acetylated-1,2-trans-thioglycosides is described. The protocol is based on the execution of five reaction steps (bromoacetylation, thiophenolysis under phase transfer catalysis conditions, deacetylation, benzylidenation and acetylation) in one continuous procedure and provides a fast access to the title compounds as pure crystalline products without chromatographic purification.

NMR spectra of fluorinated carbohydrates

Recent advances in structural and conformational analysis of fluorinated carbohydrates by NMR spectroscopy are reviewed. Characteristic 1H, 13C, and 19F NMR chemical shifts and coupling constants for selected examples are given and the spectral data of a series of fluorinated carbohydrates were collected in continuation of the review of Csuk and Glänzer [Adv. Carbohydr. Chem. Biochem., 46 (1988) 73-177].

Chemical synthesis of 6"'-alpha-maltotriosyl-maltohexaose as substrate for enzymes in starch biosynthesis and degradation

A branched nonasaccharide 6"'-alpha-maltotriosyl-maltohexaose was synthesised in 40 steps from D-glucose and maltose. Phenyl O-(2,3,4,6-tetra-O-benzyl-alpha-D-glucopyranosyl)-(1-->4)-O- (2,3,6-tri-O-benzyl-alpha-D-glucopyranosyl)-(1-->4)-2,3-di-O-benzyl-1-th io- beta-D-glucopyranoside and O-(2,3,4,6-tetra-O-benzyl-alpha-D-glucopyranosyl)-(1-->4)-O-(2,3,6-tri- O-benzyl-alpha-D-glucopyranosyl)-(1-->4)-2,3,6-tri-O-benzyl-alpha, beta-D-glucopyranosyl trichloroacetimidate were coupled by a general condensation reaction to form the per-O-benzylated branched hexasaccharide phenyl thioglycoside. The phenylthio group of this compound was converted into a trichloroacetimidate, which was coupled with phenyl O-(2,3,6-tri-O-benzyl-alpha-D-glucopyranosyl)-(1-->4)-O-(2,3,6-tri-O- benzyl-alpha-D-glucopyranosyl)-(1-->4)-2,3,6-tri-O-benzyl-1-thio-beta-D- glucopyranoside to afford the per-O-benzylated branched nonasaccharide phenyl thioglycoside. Replacement of the phenylthio group with a free OH-group followed by hydrogenolysis gave the desired product. The synthons reported for this synthesis constitute a versatile tool for the chemical synthesis of other complex carbohydrates.

Proton NMR spectroscopy assignment of D-glucose residues in highly acetylated starch

1H NMR spectroscopy assignments have been obtained for starch acetates using COSY and HOHAHA experiments by comparison with the spectra of amylose triacetate and of peracetylated malto-oligosaccharides (maltotriose, maltotetraose, maltoheptaose). These assignments are valuable for the location and evaluation of the substitution pattern in modified starches. The bulk of the 1H NMR spectra of highly acetylated starch strongly resembles the spectrum of amylose triacetate in which all protons are identified and display distinct chemical shifts. The resolving power of the HOHAHA experiment allowed the distinction of minor spin systems. Beside these strong signals pertaining to an average 2.3.6-tri-O-acetyl-alpha-(1-->4) linked D-glucopyranose unit in an infinite chain, the combination of COSY and HOHAHA experiments allowed the identification of these systems to the terminal, n-1, n-2, and to partially acetylated glucopyranosyl units. As an example, two different preparations of starch acetates with degrees of substitution 2.74 and 2.63 were examined. In one case, NMR demonstrates that the defects of acetylation are random on the polymeric chain (with corresponding signals for unacylated secondary hydroxyl positions at delta 3.61 and 3.40) while in another case, these signals are not detectable, probably due to the presence of clusters of non-acetylated residue forming solid-like zones.

Radiology services for remote communities: cost minimisation study of telemedicine

OBJECTIVES

To determine the social costs of providing a rural population with radiology services under three different systems: the existing system (a small x ray unit at the remote site and all other examinations at the nearest radiology department (the host site)); a teleradiology system (most examinations at the remote site and more advanced examinations at the host site); and all examinations at the host site.

DESIGN

Cost minimisation study.

SETTING

Primary health care in a remote community in Norway.

SUBJECTS

A randomly selected sample (n = 597) of all patients (n = 1793) having radiological examinations in 1993.

MAIN OUTCOME MEASURES

Annual direct medical costs, direct non-medical (travel) costs, and indirect costs (lost production) of the three options.

RESULTS

After exclusion of costs common to the three systems the direct medical, direct non-medical, and indirect costs of the three options were, respectively, 9000 pounds, 51,000 pounds, and 31,500 pounds (total 91,500 pounds) for the existing system; 108,000 pounds, 2,000 pounds, and 13,500 pounds (total 123,500 pounds) for the teleradiology option; and 0 pounds, 75,000 pounds, and 42,000 pounds (117,000 pounds in total) for the "all at host" option. Sensitivity analyses indicated that the existing system is the least costly option except when lost leisure is valued as highly as lost production.

CONCLUSION

The teleradiology option did not seem to be cost saving in the study community. Such systems, however, may be justified on the grounds of equity of access and quality of care.

Chemical synthesis of 6'-alpha-maltosyl-maltotriose, a branched oligosaccharide representing the branch point of starch

Chemical synthesis of the branched pentasaccharide 6'-alpha-maltosyl-maltotriose (15) is reported, based on the use of one synthon as a glycosyl acceptor and another synthon as a glycosyl donor. The synthon used as glycosyl acceptor was phenyl 2,3,6-tri-O-benzyl-1-thio-beta-D-glucopyranoside (7) and was synthesized from D-glucose with phenyl 2,3-di-O-acetyl-4,6-O-benzylidene-1-thio-beta-D-glucopyranoside and phenyl 2,3-di-O-benzyl-4,6-O-benzylidene-1-thio-beta-D-glucopyranoside as key intermediates. The synthon used as glycosyl donor was O-(2,3,4,6-tetra-O-benzyl-alpha-D-glucopyranosyl)-(1-->4)-O-(2,3,6-tri-O -benzyl - alpha-D-glucopyranosyl)-(1-->6)-O-[(2,3,4,6-tetra-O-benzyl-alpha-D- glucopyranosyl)-(1-->4)]-2,3-di-O-benzyl-alpha,beta-D-glucopyranosyl trichloroacetimidate (12) and was synthesized from phenyl O-2,3,4,6-tetra-O-benzyl-alpha-D-glucopyranosyl)-(1-->4)-O-(2,3,6-tri-O- benzyl- alpha-D-glucopyranosyl)-(1-->6)-O-[(2,3,4,6-tetra-O-acetyl-alpha-D- glucopyranosyl)-(1-->4)]-2,3-di-O-acetyl-1-thio-beta-D-glucopyranoside with O-(2,3,4,6-tetra-O-benzyl-alpha-D-glucopyranosyl)-(1-->4)-O-(2,3,6-tri-O - benzyl-alpha-D-glucopyranosyl)-(1-->4)]-2,3-di-O-benzyl-D-glucopyranose as an intermediate. Condensation of compounds 7 and 12 followed by removal of the phenylthio group and debenzylation provided the branched pentasaccharide 15. Alternatively, the branched pentasaccharide was produced from amylopectin by consecutive alpha- and beta-amylase treatments and purified by chromatography. The identity of the products obtained by chemical synthesis and enzymatic hydrolysis is documented by 1H and 13C NMR spectra.