Molecular modeling of saccharides, 6. Small-ring cyclodextrins: Their geometries and hydrophobic topographies

Cyclic Disaccharide Formation Enforced by a Ring Contraction: 2,3-Dideoxy Pyranoside Glycoside Donor to a Furanoside Macrocycle

The synthesis of a disaccharide macrocycle through 2,3-dideoxy glucopyranosyl monosaccharide is reported. 2,3-Dideoxy-erythro-hexopyranosyl thioglycoside possessing a free hydroxy functionality at the C-4 carbon is prepared, and cycloglycosylation is conducted. In the event, the cycloglycosylation occurs with a ring contraction of the monosaccharide moiety and affords the cyclic furanoside disaccharide. Solution-phase and single-crystal X-ray diffraction structural characterizations permit the features of the macrocycle to be uncovered. The solubilization and encapsulation properties of the macrocycle are studied in aqueous solutions with 1-aminoadamantane.

Unravelling the structures of sodiated β-cyclodextrin and its fragments

We present cryogenic infrared spectra of sodiated β-cyclodextrin [β-CD + Na]+, a common cyclic oligosaccharide, and its main dissociation products upon collision-induced dissociation (CID). We characterize the parent ions using high-resolution ion mobility spectrometry and cryogenic infrared action spectroscopy, while the fragments are characterized by their mass and cryogenic infrared spectra. We observe sodium-cationized fragments that differ in mass by 162 u, corresponding to Bn/Zm ions. For the m/z 347 product ion, electronic structure calculations are consistent with formation of the lowest energy 2-ketone B2 ion structure. For the m/z 509 product ion, both the calculated 2-ketone B3 and the Z3 structures show similarities with the experimental spectrum. The theoretical structure most consistent with the spectrum of the m/z 671 ions is a slightly higher energy 2-ketone B4 structure. Overall, the data suggest a consistent formation mechanism for all the observed fragments.

The scientific legacy of Frieder W. Lichtenthaler

This article presents a selection of topics from Professor Frieder W. Lichtenthaler's scientific lifework. It describes his contributions to, and further development of, the nitromethane cyclization of dialdehydes leading to amino sugars and amino nucleosides, as well as a new coupling methodology for purine nucleosides. A number of chiral building blocks derived from sugars like the "sugar enolones," enollactones, hydroxyhexenals, and their synthetic applications in natural product syntheses are covered. The article further describes the chemistry of "ulosyl bromides" and their glycosidation reactions, including those with bifunctional acceptors, which led to the synthesis of spectinomycin and gomphoside. Lichtenthaler's work on the preparation of synthetically useful building blocks from disaccharides that are readily available in bulk quantities, and his studies on the reactivity, as well as the selective O-functionalization of sucrose, higher oligosaccharides, and cyclodextrins based on computer simulations, are highlighted. The article also presents his research on the syntheses of chiral building blocks from readily available ketoses and their synthetic applications. Finally the chapter concludes with his significant contributions in the field of the history of carbohydrate chemistry.

Conformationally supple glucose monomers enable synthesis of the smallest cyclodextrins

Cyclodextrins (CDs) are cyclic oligomers of α-1,4-d-glucopyranoside and are known mainly as hexamers to octamers. The central cavities of CDs can retain small molecules, enabling diverse applications. The smallest members, CD3 and CD4, have ring sizes too small to permit the most stable conformations of glucopyranose and have not been accessible synthetically. In this study, we present methods to chemically synthesize both CD3 and CD4. The main factor in the successful synthesis is the creation of a glucopyranose ring conformationally counterbalanced between equatorial- and axial-rich forms. This suppleness is imparted by a bridge between O-3 and O-6 of glucose, which enables the generation of desirable, albeit deformed, conformers when synthesizing the cyclic trimer and tetramer.

Validation and Comparison of Force Fields for Native Cyclodextrins in Aqueous Solution

Molecular dynamics simulations of native α-, β-, and γ-cyclodextrin in aqueous solution have been conducted with the goal to investigate the performance of the CHARMM36 force field, the AMBER-compatible q4md-CD force field, and five variants of the GROMOS force field. The properties analyzed are structural parameters derived from X-ray diffraction and NMR experiments as well as hydrogen bonds and hydration patterns, including hydration free enthalpies. Recent revisions of the torsional-angle parameters for carbohydrate systems within the GROMOS family of force fields lead to a significant improvement of the agreement between simulated and experimental NMR data. Therefore, we recommend using the variant 53A6_GLYC instead of 53A6 and 56A6_{CARBO_R} or 2016H66 instead of 56A6_CARBO to simulate cyclodextrins in solution. The CHARMM36 and q4md-CD force fields show a similar performance as the three recommended GROMOS parameter sets. A significant difference is the more flexible nature of the cyclodextrins modeled with the CHARMM36 and q4md-CD force fields compared to the three recommended GROMOS parameter sets.

Trimodal Control of Ion-Transport Activity on Cyclo-oligo-(1→6)-β-D-glucosamine-Based Artificial Ion-Transport Systems

Cyclo-oligo-(1→6)-β-D-glucosamines functionalized with hydrophobic tails are reported as a new class of transmembrane ion-transport system. These macrocycles with hydrophilic cavities were introduced as an alternative to cyclodextrins, which are supramolecular systems with hydrophobic cavities. The transport activities of these glycoconjugates were manipulated by altering the oligomericity of the macrocycles, as well as the length and number of attached tails. Hydrophobic tails of 3 different sizes were synthesized and coupled with each glucosamine scaffold through the amide linkage to obtain 18 derivatives. The ion-transport activity increased from di- to tetrameric glucosamine macrocycles, but decreased further when flexible pentameric glucosamine was introduced. The ion-transport activity also increased with increasing length of attached linkers. For a fixed length of linkers, the transport activity decreased when the number of such tails was reduced. All glycoconjugates displayed a uniform anion-selectivity sequence: Cl(-) >Br(-) >I(-) . From theoretical studies, hydrogen bonding between the macrocycle backbone and the anion bridged through water molecules was observed.

Versatility of cyclodextrins in self-assembly systems of amphiphiles

Recently, cyclodextrins (CDs) were found to play important yet complicated (or even apparently opposite sometimes) roles in self-assembly systems of amphiphiles or surfactants. Herein, we try to review and clarify the versatility of CDs in surfactant assembly systems by 1) classifying the roles played by CDs into two groups (modulator and building unit) and four subgroups (destructive and constructive modulators, amphiphilic and unamphiphilic building units), 2) comparing these subgroups, and 3) analyzing mechanisms. As a modulator, although CDs by themselves do not participate into the final surfactant aggregates, they can greatly affect the aggregates in two ways. In most cases CDs will destroy the aggregates by depleting surfactant molecules from the aggregates (destructive), or in certain cases CDs can promote the aggregates to grow by selectively removing the less-aggregatable surfactant molecules from the aggregates (constructive). As an amphiphilic building unit, CDs can be chemically (by chemical bonds) or physically (by host-guest interaction) attached to a hydrophobic moiety, and the resultant compounds act as classic amphiphiles. As an unamphiphilic building unit, CD/surfactant complexes or even CDs on their own can assemble into aggregates in an unconventional, unamphiphilic manner driven by CD-CD H-bonds. Moreover, special emphasis is put on two recently appeared aspects: the constructive modulator and unamphiphilic building unit.

An enzymatically produced novel cyclomaltopentaose cyclized from amylose by an α-(1→6)-linkage, cyclo-{→6)-α-d-Glcp-(1→4)-α-d-Glcp-(1→4)-α-d-Glcp-(1→4)-α-d-Glcp-(1→4)-α-d-Glcp-(1→}

A bacterial strain AM7, isolated from soil and identified as Bacillus circulans, produced two kinds of novel cyclic oligosaccharides. The cyclic oligosaccharides were produced from amylose using a culture supernatant of the strain as the enzyme preparation. The major product was a cyclomaltopentaose cyclized by an alpha-(1-->6)-linkage, cyclo-{-->6)-alpha-D-Glcp-(1-->4)-alpha-D-Glcp-(1-->4)-alpha-D-Glcp-(1-->4)-alpha-D-Glcp-(1-->4)-alpha-D-Glcp-(1-->}. The other minor product was cyclomaltohexaose cyclized by an alpha-(1-->6)-linkage, cyclo-{-->6)-alpha-D-Glcp-(1-->4)-alpha-D-Glcp-(1-->4)-alpha-D-Glcp-(1-->4)-alpha-D-Glcp-(1-->4)-alpha-D-Glcp-(1-->4)-alpha-D-Glcp-(1-->}. We propose the names isocyclomaltopentaose (ICG5) and isocyclomaltohexaose (ICG6) for these novel cyclic maltooligosaccharides having one alpha-(1-->6)-linkage. ICG5 was digested by alpha-amylase derived from Aspergillus oryzae, cyclomaltodextrin glucanotransferase (CGTase) from Bacillus stearothermophilus, and maltogenic alpha-amylase. On the other hand, ICG6 was digested by CGTase from B. stearothermophilus and B. circulans, and maltogenic alpha-amylase. This is the first report of enzymatically produced cyclomaltopentaose and cyclomaltohexaose, which have an alpha-(1-->6)-linkage in their molecules.

Saccharides as efficacious solubilisers for highly lipophilic compounds in aqueous media

The bioavailability of lipophilic substrates is critical for biotransformations with isolated enzymes as well as with whole cells. With the example of a series of lipophilic ketones the suitability of saccharides as potent solubilisers for highly lipophilic substrates was demonstrated. Best results were obtained for d-glucose, which increased substrate solubility up to 50 times. In whole-cell biocatalysis the sugar acts both as solubiliser and as carbon source for which reason this procedure does not impair cell physiology and is unique in being environmentally benign. The capability of saccharides to solubilise lipophilic compounds in aqueous media sources from their ability to form hydrophilic and lipophilic domains at hydrophobic interfaces, thus forming cyclodextrin-like structures around the lipophilic substrate.

Diastereospecific synthesis of phosphinidene-capped cyclodextrins leading to “introverted” ligands

alpha-Cyclodextrins (alpha-CDs) containing "PPh" units which cap the primary face of the CD were obtained in high yield by reaction of Li(2)PPh with A,B- or A,C-dimesylated and A,B,D,E-tetramesylated precursors; the resulting phosphines are diastereomerically pure and constitute valuable precursors for the synthesis of metallo-cavitands.

Topography of the 1:1 alpha-cyclodextrin-nitromethane inclusion complex

Dissolution of alpha-cyclodextrin (alpha-CD) in 9:1 water-nitromethane smoothly generates the title compound, which crystallizes as the pentahydrate in the orthorhombic space group P2(1)2(1)2(1) with a = 9.452(4), b = 14.299(3), c = 37.380(10) A, and Z = 4. Its crystal structure analysis revealed the alpha-CD macrocycle in an unstrained conformation stabilized through a ring of O-2...O-3' hydrogen bonds between five of the six adjacent glucose residues. The nitromethane is located in the alpha-CD cavity in an orientation parallel to the plane of the macrocycle, and assumes two sites of equal population with the nitro group in excessive thermal motion; the guest is held by van der Waals contacts and C-H...O-type hydrogen bonds to the pyranose H-3 and H-5 protons. The packing of the macrocycles in the crystal lattice is of cage herringbone-type with an extensive intra- and intermolecular hydrogen bonding network. The ready formation of a nitromethane inclusion complex in aqueous nitromethane, and the subtleties of its molecular structure amply demonstrate the ease with which water is expelled from the alpha-CD cavity by a more hydrophobic co-solvent.

Cyclofructins with six to ten beta-(1-->2)-linked fructofuranose units: geometries, electrostatic profiles, lipophilicity patterns, and potential for inclusion complexation

Cyclofructins composed of six (1, "CF6") to ten (5, "CF10") beta-(1-->2)-linked fructofuranose units were subjected to conformational analysis using Monte Carlo simulations based on the PIMM91 force-field. Breaking the molecular symmetry partially by alternating inclination of the spiro-type anellated fructofuranoses relative to the crown ether ring core, i.e. the 3-OH groups pointing either towards or away from the molecular center, substantially lowers the strain energy of the cyclofructins. The global energy-minimum geometries of CF6, CF8, and CF10 exhibit Cn/2 rotational symmetry, whilst the odd-membered macrocycles in CF7 and CF9 adopt C1 symmetry. Identical conformations of the solid-state geometry of CF6 (1) and its computer-generated form manifest the reliability of the computational analysis. The molecular surfaces calculated for the energy-minimum structures establish a disk-type shape for CF6 (1), CF7 (2), and CF8 (3), whereas further ring enlargement to CF9 (4) and CF10 (5) leads to torus-shaped molecules with through-going cavities. Color-coded projection of the molecular lipophilicity patterns (MLPs) and the electrostatic potential profiles (MEPs) onto these surfaces cogently displays the crown ether-like properties, favoring the complexation of metal cations via strong electrostatic interactions through the 3-OH groups located on the hydrophilic molecular side. The central cavities of CF9 and CF10 are characterized not only by significantly enhanced hydrophobicity, but also by highly negative electrostatic potentials around the narrow aperture of the tori made up by the 3-OH/4-OH groups, and positive potentials on the opposite rim. Accordingly, CF9 and CF10 are capable to form inclusion complexes, the cavity of the latter being approximately as large as the one of alpha-cyclodextrin. Calculation of the inclusion complex geometries of CF9 with beta-alanine and of CF10 with p-aminobenzoic acid revealed the guest to be deeply incorporated into the respective cavities, masking the guest's hydrophobic parts. Analysis of the electrostatic interactions at the interface of the zwitter-ionic guests with the oppositely polarized hosts predicts a high degree of regiospecificity for complex formation.