Exploring pH Dependent Host/Guest Binding Affinities

When the electrostatic environment surrounding binding partners changes between unbound and bound states, the net uptake or release of a proton is possible by either binding partner. This process is pH-dependent in that the free energy required to uptake or release the proton varies with pH. This pH-dependence is typically not considered in conventional free energy methods where the use of fixed protonation states is the norm. In the present paper, we apply a simple two-step approach to calculate the pH-dependent binding free energy of a model cucubit[7]uril host/guest system. By use of λ-dynamics with an enhanced sampling protocol, adaptive landscape flattening, pK_a shifts and reference binding free energies upon complexation were determined. This information enables the construction of pH-dependent binding profiles that accurately capture the pK_a shifts and reproduce binding free energies at the different pH conditions that were observed experimentally. Our calculations illustrate a general framework for computing pH-dependent binding free energies but also point to some issues in modeling the molecular charge distributions within this series of molecules with CGenFF. However, by introducing some minor charge modifications to the CGenFF force field, we saw significant improvement in accuracy of the calculated pK_a shifts.

Unveiling the formation 1 : 2 supramolecular complexes between cucurbit[7]uril and a cationic calix[4]arene derivative

The formation of host–guest complexes between cucurbit[7]uril (CB7) and a tetracationic calix[4]arene derivative in the so-called cone conformation was investigated by ¹H NMR, DOSY NMR, isothermal titration calorimetry and ESI-MS.

Contrasting pK a Shifts in Cucurbit[7]uril Host-Guest Complexes Governed by an Interplay of Hydrophobic Effects and Electrostatic Interactions

Cucurbit[7]uril inclusion complexes with guests bearing dimethylamino groups show the expected upward pK a shifts, whereas their diethylamino counterparts display a decrease in pK a due to the preferential stabilization of the unprotonated form. These results identify the diethylamino group as the substituent of choice to avoid receptor-assisted protonation of guest molecules and present new evidence for the role of the hydrophobic effect as a driving force in cucurbituril complexation.

Host-Guest Complexes of Flavylium Cations and Cucurbit[7]uril: The Influence of Flavylium Substituents on the Structure and Stability of the Complex

The host-guest complexes formed from six differently substituted flavylium cations and cucurbit[7]uril (CB7) have been characterized by UV/Vis absorption, fluorescence emission and ¹ H NMR spectroscopy. It was observed that all flavylium cations form 1:1 inclusion complexes with association constants that depend on the nature and position of the substituents. The results indicate that CB7 displays higher affinity for more hydrophobic flavylium compounds and for those bearing amino substituents. ¹ H NMR spectroscopy was used to elucidate the structure of the complexes. While for 7-hydroxyflavylium and 4-methyl-7-hydroxyflavylium the phenyl group (ring B) is included within the host's cavity leaving the benzopyrilium group (rings A and C) outside, in 4',7-dihydroxyflavylium and 3',4',7-trihydroxyflavylium the macrocycle shuttles between rings A and B. For compounds with amino substituents it was found that CB7 is attracted towards these groups regardless of their position in ring A or B. In addition, it was observed that the dimethylamino group tends to be positioned near the carbonyl-decorated portal while the diethylamino motif prefers the hydrophobic cavity of CB7.

Tuning the surface activity of gemini amphiphile by the host-guest interaction of cucurbit[7]uril

This research is aimed to develop an effective supramolecular route for tuning the surface activity of the surfactant. To this end, cationic gemini amphiphiles and cucurbit[7]uril (CB[7]) were complexed in water, and each hydrophobic chain of the gemini amphiphiles was bound with a CB[7]. The steric hindrance of CB[7] prevented the two hydrophobic chains from getting closed to each other, leading a significant change of surface activity. Before supramolecular complexation, the surface activity of the gemini amphiphile is relatively high, which can generate the foams easily. However, the foam generated by gemini amphiphile can be destructed by adding CB[7], suggesting that the suface activity is lowed after the supramolecular complexation. The surface activity can recover after adding 1-adamantanamine hydrochloride, which has a stronger ability to bind CB[7]. Therefore, a controllable foaming and defoaming process can be realized. It is highly anticipated that supramolecular chemistry for tuning amphiphilicity of surfactants may find application in the fields that fast foaming and defoaming are needed.

Exploring the charged nature of supramolecular micelles based on p-sulfonatocalix[6]arene and dodecyltrimethylammonium bromide

Kinetic probes were used together with capillary electrophoresis experiments to get insights into the interfacial and solubilizing properties of supramolecular micelles made of an anionic calix[6]arene and a cationic surfactant.

Stimuli-responsive supramolecular micellar assemblies of cetylpyridinium chloride with cucurbit[5/7]urils

This article demonstrates, for the first time, construction of novel cucurbituril (CB)-adorned supramolecular micellar assemblies of a cationic surfactant, cetylpyridinium chloride (CPC), through noncovalent host-guest interactions. The distinct cation receptor features and cavity dimensions of the CB5 and CB7 homologues assert that the macrocyclic hosts remain complexed with the CPC monomers and take part in the micelle formation, a unique observation in contrast to that of the classical host, β-cyclodextrin. The cooperative contributions of the CB macrocycles in the micelle formation have been documented by the photochemical, surface tension, conductivity, DOSY NMR, and SANS measurements. The contrasting downward and upward shifts in the cmc of the CPC surfactant, respectively, with CB5 and CB7 hosts provide a unique opportunity for the controlled tuning of the micellization region for CPC from 0.57 to 1.6 mM, by using a combination of the macrocyclic hosts. The article also establishes the reversible response of these soft supramolecular micellar structures to thermal-stimuli, which projects their utility for on-demand smart drug-delivery vehicles.

The formation of host-guest complexes between surfactants and cyclodextrins

Cyclodextrins are able to act as host molecules in supramolecular chemistry with applications ranging from pharmaceutics to detergency. Among guest molecules surfactants play an important role with both fundamental and practical applications. The formation of cyclodextrin/surfactant host-guest compounds leads to an increase in the critical micelle concentration and in the solubility of surfactants. The possibility of changing the balance between several intermolecular forces, and thus allowing the study of, e.g., dehydration and steric hindrance effects upon association, makes surfactants ideal guest molecules for fundamental studies. Therefore, these systems allow for obtaining a deep insight into the host-guest association mechanism. In this paper, we review the influence on the thermodynamic properties of CD-surfactant association by highlighting the effect of different surfactant architectures (single tail, double-tailed, gemini and bolaform), with special emphasis on cationic surfactants. This is complemented with an assessment of the most common analytical techniques used to follow the association process. The applied methods for computation of the association stoichiometry and stability constants are also reviewed and discussed; this is an important point since there are significant discrepancies and scattered data for similar systems in the literature. In general, the surfactant-cyclodextrin association is treated without reference to the kinetics of the process. However, there are several examples where the kinetics of the process can be investigated, in particular those where volumes of the CD cavity and surfactant (either the tail or in special cases the head group) are similar in magnitude. This will also be critically reviewed.