Synthesis and Characterization of Sulfated Gal‐β‐1,3/4‐GlcNAc Disaccharides through Consecutive Protection/Glycosylation Steps

We have developed an expeditious procedure to yield large amounts of orthogonally protected Gal‐β1,3/4‐GlcNAc, which allowed for the systematic introduction of a sulfate group onto the C3/C6 positions of Gal and/or the C6 position of GlcNAc. In particular, the disaccharide precursors were prepared in five or six steps and high overall yield from para‐tolyl‐6‐O‐tert‐butyldiphenylsilyl‐1‐thio‐β‐D ‐galactopyranoside. After deprotection and sulfation steps, the final products were characterized by using several NMR methods to unambiguously confirm the location of each introduced sulfate group and they were examined for their binding specificity of human galectin‐1 and galectin‐8.     

Photochemistry of α‐Diketones in Carbohydrates: Anomalous Norrish Type II Photoelimination and Norrish–Yang Photocyclization Promoted by the Internal Carbonyl Group

A series of four α‐diketones placed as 1α‐pyruvoyl tethers on D ‐glucopyranose and D ‐glucopyranosiduronic acid skeletons was prepared in order to determine the influence of captodative and stereoelectronic effects on the regioselectivity of the hydrogen atom transfer (HAT) in Norrish type II photochemical processes. We observed that the 1,5‐HAT regioselectivity can be switched between the two potentially abstractable syn‐1,3‐diaxial hydrogens at H6 and H8. Highly unusual photoproducts from Norrish type II photoelimination and Norrish–Yang photocyclization initiated by the excited internal carbonyl group were obtained, in some cases in excellent synthetic yield. The 1,5‐HAT transition state in the Norrish type II photoelimination was investigated by photochemical experiments in the crystalline state.     

Peptide‐Based Carbohydrate Receptors

A broad spectrum of physiological processes is mediated by highly specific noncovalent interactions of carbohydrates and proteins. In a recent communication we identified several cyclic hexapeptides in a dynamic combinatorial library that interact selectively with carbohydrates with high binding constants in water. Herein, we report a detailed investigation of the noncovalent interaction of two cyclic hexapeptides (Cys‐His‐Cys (which we call HisHis) and Cys‐Tyr‐Cys (which we call TyrTyr)) with a selection of monosaccharides and disaccharides in aqueous solution. The parallel and antiparallel isomers of HisHis or TyrTyr were synthesized separately, and their interaction with monosaccharides and disaccharides in aqueous solution was studied by isothermal titration calorimetry, NMR spectroscopic titrations, and circular dichroism spectroscopy. From these measurements, we identified particularly stable complexes (K_a>1000 M ^?1) of the parallel isomer of HisHis with N‐acetylneuraminic acid and with methyl‐α‐D ‐galactopyranoside as well as of both isomers of TyrTyr with trehalose. To gain further insight into the structure of the peptide–carbohydrate complexes, structure prediction was performed using quantum chemical methods. The calculations confirm the selectivity observed in the experiments and indicate the formation of multiple intermolecular hydrogen bonds in the most stable complexes.     

Cooperative Hydrogen Bonding in Glyco–Oligoamides: DNA Minor Groove Binders in Aqueous Media

A strategy to create cooperative hydrogen‐bonding centers by using strong and directional intramolecular hydrogen‐bonding motifs that can survive in aqueous media is presented. In particular, glyco–oligoamides, a family of DNA minor groove binders, with cooperative and non‐cooperative hydrogen‐bonding donor centers in the carbohydrate residues have been designed, synthesized, and studied by means of NMR spectroscopy and molecular modeling methods. Indeed, two different sugar moieties, namely, β‐D ‐Man‐Py‐γ‐Py‐Ind ( 1 ; Ind=indole, Man=mannose, Py=pyrrole) and β‐D ‐Tal‐Py‐γ‐Py‐Ind ( 2 ; Tal=talose), were chosen according to our design. These sugar molecules should present one‐ or two‐directional intramolecular hydrogen bonds. The challenge has been to study the conformation of the glyco–oligoamides at low temperature in physiological media by detecting the exchangeable protons (amide NH and OH resonances) by means of NMR spectroscopic analysis. In addition, two more glyco–oligoamides with non‐cooperative hydrogen‐bonding centers, that is, β‐D ‐Glc‐Py‐γ‐Py‐Ind ( 3 ; Glc=glucose), β‐D ‐Gal‐Py‐γ‐Py‐Ind ( 4 ; Gal=galactose), and the model compounds β‐D ‐Man‐Py‐NHAc ( 5 ) and β‐D ‐Tal‐Py‐NHAc ( 6 ) were synthesized and studied for comparison. We have demonstrated the existence of directional intramolecular hydrogen bonds in 1 and 2 in aqueous media. The unexpected differences in terms of stabilization of the intramolecular hydrogen bonds in 1 and 2 relative to 5 and 6 promoted us to evaluate the influence of CH—π interactions on the establishment of intramolecular hydrogen bonds by using computational methods. Initial binding studies of 1 and 2 with calf‐thymus DNA and poly(dA‐dT)₂ by NMR spectroscopic analysis and molecular dynamics simulations were also carried out. Both new sugar–oligoamides are bound in the minor groove of DNA, thus keeping a stable hairpin structure, as in the free state, in which both intramolecular hydrogen‐bonding and CH—π interactions are present.     

Tetrafluorination of Sugars as Strategy for Enhancing Protein–Carbohydrate Affinity: Application to UDP‐Galp Mutase Inhibition

Tetrafluorinated analogues of both UDP‐galactopyranose and UDP‐galactofuranose have been synthesized and assayed against UDP‐galactopyranose mutase, a key enzyme for Mycobacterium tuberculosis cell wall biosynthesis. Competition assays and STD‐NMR spectroscopy techniques have evidenced not only the first unambiguous case of affinity enhancement through local sugar polyfluorination, but also showed that tetrafluorination can still have a beneficial effect on binding when monofluorination at the same position does not.     

Direct Umpolung of Glycals and Related 2,3‐Unsaturated N‐Acetylneuraminic Acid Derivatives Using Samarium Diiodide

The umpolung of glycals with samarium diiodide offers a simple route to novel carbohydrate‐derived nucleophilic reagents in a single step using a readily available reductant. The corresponding allyl samarium reagent that arises from the hexose series reacts with ketones at the C3 position with high stereoselectivity; carbon–carbon bond formation takes place only anti to the substituent at the C4 position of the dihydropyran ring. For the sialic acid series, the completely regio‐ and stereoselective coupling process of the samarium reagent occurs at the anomeric carbon atom and provides a new approach to the α‐C‐glycosides of N‐acetyl neuraminic acid.     

Self‐Repairable Polyurethane Networks by Atmospheric Carbon Dioxide and Water

Sugar moieties were incorporated into cross‐linked polyurethane (PUR) networks in an effort to achieve self‐repairing in the presence of atmospheric carbon dioxide (CO₂) and water (H₂O). When methyl‐α‐D ‐glucopyranoside (MGP) molecules are reacted with hexamethylene diisocyanate trimer (HDI) and polyethylene glycol (PEG) to form cross‐linked MGP‐polyurethane (PUR) networks, these materials are capable of self‐repairing in air. This process requires atmospheric amounts of CO₂ and H₂O, thus resembling plant behavior of carbon fixation during the photosynthesis cycle. Molecular processes responsible for this unique self‐repair process involve physical diffusion of cleaved network segments as well as the formation of carbonate and urethane linkages. Unlike plants, MGP‐PUR networks require no photo‐initiated reactions, and they are thus capable of repair in darkness under atmospheric conditions.     

Polarimetry as a Tool for the Study of Solutions of Chiral Solutes

Optical rotation of aqueous solutions of D ‐levoglucosan was studied experimentally in the 0.03–4.0 mol L^?1 concentration range and a nonlinear concentration dependence of specific optical rotation (SR) was revealed. Discontinuities observed in the concentration plot of SR (at 0.1, 0.3, 0.5, 1.0, and 2.0 mol L^?1) are well correlated with those found by static and dynamic light scattering and identify concentration ranges in which different solution domains (supramers) may exist. The average SR experimental value for a D ‐levoglucosan aqueous solution ([α]_D²⁸ ?58.5±8.7 deg dm^?1 cm^?3 g^?1) was found to be in good agreement with values obtained by theoretical calculation (TD‐DFT/GIAO) of SR for 15 different conformers revealed by conformational sampling at the PCM/B3LYP/6‐311++G(2d,2p)//B3LYP/6‐31+G(d,p) level, which were shown to be strongly affected by the solvation microenvironment (0, 1, 2, and 3 explicit solvent molecules considered) due to local geometrical changes induced in the solute molecule. This exceptionally high sensitivity of SR makes polarimetry a unique method capable of sensing changes in the structure of supramers detected in this study.     

Studying the Glycan Moiety of RNase B by Means of Raman and Raman Optical Activity

Raman and Raman optical activity (ROA) spectroscopy are used to study the solution‐phase structure of the glycan moiety of the protein ribonuclease B (RNase B). Spectral data of the intact glycan moiety of RNase B is obtained by subtracting high‐quality spectral data of RNase A, the non‐glycosylated form of the RNase, from the spectra of the glycoprotein. The remaining difference spectra are compared to spectra generated from Raman and ROA data of the constituent disaccharides of the RNase glycan, achieving convincing spectral overlap. The results show that ROA spectroscopy is able to extract detailed spectral data of the glycan moieties of proteins, provided that the non‐glycosylated isoform is available. Furthermore, good comparison between the full glycan spectrum and the regenerated spectra based on the disaccharide data lends great promise to ROA as a tool for the solution‐phase structural analysis of this structurally elusive class of biomolecules.