Modes of Micromolar Host-Guest Binding of β-Cyclodextrin Complexes Revealed by NMR Spectroscopy in Salt Water

Two Squares in a Barrel: An Axially Disubstituted Conformationally Rigid Aliphatic Binding Motif for Cucurbit[6]uril

Novel binding motifs suitable for the construction of multitopic guest-based molecular devices (e.g., switches, sensors, data storage, and catalysts) are needed in supramolecular chemistry. No rigid, aliphatic binding motif that allows for axial disubstitution has been described for cucurbit[6]uril (CB6) so far. We prepared three model guests combining spiro[3.3]heptane and bicyclo[1.1.1]pentane centerpieces with imidazolium and ammonium termini. We described their binding properties toward CB6/7 and α-/β-CD using NMR, titration calorimetry, mass spectrometry, and single-crystal X-ray diffraction. We found that a bisimidazolio spiro[3.3]heptane guest forms inclusion complexes with CB6, CB7, and β-CD with respective association constants of 4.0 × 104, 1.2 × 1012, and 1.4 × 102. Due to less hindering terminal groups, the diammonio analogue forms more stable complexes with CB6 (K = 1.4 × 106) and CB7 (K = 3.8 × 1012). The bisimidazolio bicyclo[1.1.1]pentane guest forms a highly stable complex only with CB7 with a K value of 1.1 × 1011. The high selectivity of the new binding motifs implies promising potential in the construction of multitopic supramolecular components.

NMR exchange dynamics studies of metal-capped cyclodextrins reveal multiple populations of host-guest complexes in solution

Metal-capped molecular hosts are unique in supramolecular chemistry, benefitting from the inner cavity's hydrophobic nature and the metal center's electrochemical properties. It is shown here that the paramagnetic properties of the metals in lanthanide-capped cyclodextrins (Ln-α-CDs and Ln-β-CDs) are a convenient NMR indicator for different populations of host-guest complexes in a given solution. The paramagnetic guest exchange saturation transfer (paraGEST) method was used to study the exchange dynamics in systems composed of Ln-α-CDs or Ln-β-CDs with fluorinated guests, revealing multiple co-existing populations of host-guest complexes exclusively in solutions containing Ln-β-CDs. The enhanced spectral resolution of paraGEST, achieved by a strong pseudo contact shift induction, revealed that different molecular guests can adopt multiple orientations within Ln-β-CDs' cavities and, in contrast, only a single orientation inside Ln-α-CDs. Thus, paraGEST, which can significantly improve NMR detectability and spectral resolution of host-guest systems that experience fast exchange dynamics, is a convenient tool for studying supramolecular systems of metal-capped molecular hosts.

Assembling a supramolecular 3D network with tuneable mechanical properties using adamantylated cross-linking agents and β-cyclodextrin-modified hyaluronan

Hydrogels based on the supramolecular host-guest concept can be prepared if at least one constituent is a polymer chain modified with supramolecular host or guest (or both) units. Low-molecular-weight multitopic counterparts can also be used, however, guest molecules in the role of cross-linking agents are seldom reported, although such an approach offers wide-ranging possibilities for tuning the system properties via easily achievable structural modifications. In this paper, a series of adamantane-based star-like guest molecules was used for cross-linking of two types of β-cyclodextrin-modified hyaluronan (CD-HA). The prepared 3D supramolecular networks were characterised using nuclear magnetic resonance, titration calorimetry and rheological measurements to confirm the formation of the host-guest complexes between adamantane moieties and β-cyclodextrin units, including their typical properties such as self-healing and dynamic nature. The results indicate that the nature of the cross-linker (amides versus esters) has a greater impact on mechanical properties than the length of the guest's arms. In addition, the results show that the length of the HA polymer chain is more important than the degree of modification with supramolecular units. In conclusion, it was proven that the modular concept employing low-molecular-weight cross-linking guests is valuable for the formulation of supramolecular networks, including hydrogels.

Adamantane-Substituted Purine Nucleosides: Synthesis, Host-Guest Complexes with β-Cyclodextrin and Biological Activity

Purine nucleosides represent an interesting group of nitrogen heterocycles, showing a wide range of biological effects. In this study, we designed and synthesized a series of 6,9-disubstituted and 2,6,9-trisubstituted purine ribonucleosides via consecutive nucleophilic aromatic substitution, glycosylation, and deprotection of the ribofuranose unit. We prepared eight new purine nucleosides bearing unique adamantylated aromatic amines at position 6. Additionally, the ability of the synthesized purine nucleosides to form stable host-guest complexes with β-cyclodextrin (β-CD) was confirmed using nuclear magnetic resonance (NMR) and mass spectrometry (ESI-MS) experiments. The in vitro antiproliferative activity of purine nucleosides and their equimolar mixtures with β-CD was tested against two types of human tumor cell line. Six adamantane-based purine nucleosides showed an antiproliferative activity in the micromolar range. Moreover, their effect was only slightly suppressed by the presence of β-CD, which was probably due to the competitive binding of the corresponding purine nucleoside inside the β-CD cavity.

Application of Molecular Dynamics Simulations in the Analysis of Cyclodextrin Complexes

Cyclodextrins (CDs) are highly respected for their ability to form inclusion complexes via host-guest noncovalent interactions and, thus, ensofance other molecular properties. Various molecular modeling methods have found their applications in the analysis of those complexes. However, as showed in this review, molecular dynamics (MD) simulations could provide the information unobtainable by any other means. It is therefore not surprising that published works on MD simulations used in this field have rapidly increased since the early 2010s. This review provides an overview of the successful applications of MD simulations in the studies on CD complexes. Information that is crucial for MD simulations, such as application of force fields, the length of the simulation, or solvent treatment method, are thoroughly discussed. Therefore, this work can serve as a guide to properly set up such calculations and analyze their results.