Structure of the γ-cyclodextrin–1-propanol–17H2O inclusion complex

Never-Ending Story: New Cyclodextrin-Based Metal-Organic Framework Crystal Structures Obtained Using Different Crystallization Methods

Six novel cyclodextrin (CD)-based metal-organic frameworks (MOFs) were synthesized using distinct crystallization methodologies. A modified vapor diffusion method is introduced for the first time, termed fast crystallization, which enables the rapid solid-state formation of MOF compounds. This innovative method yielded four of the newly synthesized MOFs. The crystal structures of five obtained frameworks were structurally characterized through single-crystal X-ray diffraction, while one, compound 5 (γ-CD-K-5), was additionally characterized as a bulk powder. Structural analysis revealed that two of the newly obtained MOFs, namely, compound 2 (α-CD-K-2) and compound 3 (α-CD-Rb-3), exhibited isostructural characteristics, forming a three-dimensional (3D) framework. Compound 1 (α-CD-K-1) shared the same space group as EVEGET (α-CD-K) and displayed the same framework type. Furthermore, the crystal packing of compound 4 (β-CD-K-4) closely resembled that of compound 1 and EVEGET, with the only distinction lying in the type of CD employed. Notably, compound 6 (γ-CD-K-6) incorporated an iodine ion with an occupancy of 0.2. To discern the intermolecular interactions within the obtained MOFs, the Hirshfeld surface was calculated using Crystal Explorer software.

Caffeine-Cyclodextrin Complexes as Solids: Synthesis, Biological and Physicochemical Characterization

Mechanochemical and in-solution synthesis of caffeine complexes with α-, β-, and γ-cyclodextrins was optimized. It was found that short-duration, low-energy cogrinding, and evaporation (instead of freeze-drying) are effective methods for the formation and isolation of these complexes. The products obtained, their pure components, and their mixtures were examined by powder X-ray diffraction (PXRD), differential scanning calorimetry (DSC), FT-IR and Raman spectroscopy. Moreover, molecular modeling provided an improved understanding of the association process between the guest and host molecules in these complexes. The complexes were found to exhibit high toxicity in zebrafish (Danio rerio) embryos, in contrast to pure caffeine and cyclodextrins at the same molar concentrations. HPLC measurements of the caffeine levels in zebrafish embryos showed that the observed cytotoxicity is not caused by an increased caffeine concentration in the body of the organism, as the concentrations are similar regardless of the administered caffeine form. Therefore, the observed high toxicity could be the result of the synergistic effect of caffeine and cyclodextrins.

Host-guest complexes of anionic porphyrin sensitizers with cyclodextrins

The photodynamic sensitizers zinc(II)- and palladium(II)-5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrins and 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin form 1:1 and/or 1:2 supramolecular complexes with native cyclodextrins (CD) and 2-hydroxypropyl cyclodextrins (hpCD) in aqueous neutral solutions. The formation of these assemblies causes a bathochromic shift of the porphyrin Soret band in the UV-vis spectra and a red shift of the fluorescence emission bands. The binding constants span over three orders of magnitude, from 8.1 × 102 M −1 to 5.4 × 105 M −1 (or 1.1 × 106 M −2) depending on the size of the CD cavity and on the functionalization by adding 2-hydroxypropyl groups. The highest binding constants were obtained for hpβCD and hpγCD. The Nuclear Overhauser spectroscopy signals (ROESY) revealed three binding modes: i) inclusion of the porphyrin 4-sulfonatophenyl or 4-carboxyphenyl groups via the secondary face of βCD and hpβCD with sulfonic or carboxylic groups oriented towards the primary hydroxyl groups. ii) inclusion of the porphyrin groups via the primary face of γCD and hpγCD. iii) non-specific binding of the porphyrin monomers or aggregates on the cyclodextrin exterior. The inclusion host-guest complexation via i) or ii) does not influence the inherent photophysical properties of the monomeric porphyrins such as the quantum yields of fluorescence, the triplet states, and the singlet oxygen formation. Due to the deaggregation effect of cyclodextrins, the inclusion complexes remain efficient supramolecular sensitizers of singlet oxygen even under conditions of extensive aggregation in aqueous solutions.

Study on the binding of Thioflavin T to β-sheet-rich and non-β-sheet cavities

Amyloid fibril formation plays a role in more than 20 diseases including Alzheimer's disease. In vitro detection of these fibrils is often performed using Thioflavin T (ThT), though the ThT binding mode is largely unknown. In the present study, spectral properties of ThT in binding environments representing beta-sheet-rich and non-beta-sheet cavities were examined. Acetylcholinesterase and gamma-cyclodextrin induced a characteristic ThT fluorescence similar to that with amyloid fibrils, whereas beta-cyclodextrin and the beta-sheet-rich transthyretin did not. The cavities of acetylcholinesterase and gamma-cyclodextrin were of similar diameter and only these cavities could accommodate two ThT ions according to molecular modelling. Binding stoichiometry studies also showed a possible binding of two ThT ions. Thus, the characteristic ThT fluorescence is induced in cavities with a diameter of 8-9A and a length able to accommodate the entire length of the ThT ion. The importance of a cavity diameter capable of binding two ThT ions, among others, indicates that an excimer formation is a plausible mechanism for the characteristic fluorescence. We propose a similar ThT binding mode in amyloid fibrils, where cavities of an appropriate size running parallel to the fibril axis have previously been proposed in several amyloid fibril models.

Thickness Recognition of Bolaamphiphiles by α-Cyclodextrin

The minimum internal diameters of alpha-, beta-, and gamma-cyclodextrin were calculated by a space filling algorithm, MolShape, from the electron density maps created by semiempirical AM1 and PM3 calculations using Gaussian03. In addition, the minimum diameters of a series of dicationic bolaamphiphiles were calculated by MolShape as well. The calculated diameters of these hosts and guests allowed prognosis about the stabilities of the corresponding inclusion compounds. The experimental binding data, obtained by isothermal titration calorimetry (ITC), revealed indeed a very pronounced thickness recognition and correlate well with the calculated diameters.

Linkage and pyranosyl ring twisting in cyclodextrins

Acylated beta-cyclodextrins (beta-CDs) were studied to gain perspective on maltose octapropanoate, the crystal structure of which was reported in the preceding paper in this issue. Acylated beta-CDs are distorted so we looked at other CDs and gained increased understanding of distortion in CDs and possibly, shapes in starch. Classic CDs have six to eight glucose residues in a doughnut shape that is stabilized by a ring of inter-residue O3,,,O2' hydrogen bonds. On a phi,psi energy map for a maltose analog that does not form hydrogen bonds, classic CD linkages have higher energies than structures that are stabilized by the exo-anomeric effect. In distorted beta-CDs, which lack hydrogen bonding, some linkages attain low-energies from the exo-anomeric effect and acyl stacking. Those linkages result in left-handed helical geometry so other linkages are forced by the CD macrocycle to have counter-balancing right-handed character. Permethylated gamma-CDs have two 'flipping' linkages as do some larger native CDs. Flipping linkages allow two left-handed segments to join into a macrocycle, thus avoiding the higher-energy, right-handed forms. Some glucose rings in derivatized beta-CDs have substantial positive twists of the pseudo torsion angle O1-C1...C4-O4, adding right-handed character to balance the left-handed linkages. In substituted gamma-CD, all residues have negative twists, giving extra left-handed character to the short, pseudo-helical segments. In non-macrocyclic molecules the twists ranged from -14 degrees to +2 degrees , averaging -6.1 degrees. In these beta- and gamma-CDs, the twists ranged from -22 degrees to +16 degrees for (4)C(1) rings, and the (O)S(2) ring in acetylated beta-CD has a twist of +34 degrees . Glucose residues in other CDs were less twisted.

Stereoselective Photodimerization of (E)-Stilbenes in Crystalline gamma-Cyclodextrin Inclusion Complexes

Solid-state irradiation of the crystalline inclusion complex of (E)-stilbene in gamma-cyclodextrin (gamma-CD) yields a single isomer of syn-tetraphenylcyclobutane stereoselectively in high yield. In contrast, the photodimerization of stilbene in solution is very inefficient and unselective, and no photodimer is observed even upon prolonged irradiation of pure crystals. The monosubstituted stilbenes form a pair of photodimers stereoselectively, viz. the syn head-to-head and syn head-to-tail isomers, in comparable yields. The photodimer yields of about 70% and the biphasic decay kinetics of the excited stilbene (as established by picosecond time-resolved diffuse-reflectance spectroscopy) indicate that the stilbene guests are located in at least two distinct sites in the gamma-CD crystal lattice, i.e., a dimerization site where excited stilbene is in close reach of another stilbene guest molecule and an isomerization site where excited stilbene does not find a close neighbor for dimerization and thus undergoes trans --> cis isomerization only.