Interaction between cucurbit[8]uril and viologen derivatives

The interaction between cucurbit[8]uril (Q[8]) and a series of symmetric viologen derivatives having aliphatic substituents of variable length [N,N′-dialkyl-4,4′-bipyridinium dianions; alkyl = CH₃(CH₂) _n –, n = 0 (MV²⁺), 1 (EV²⁺), 2 (PV²⁺), 3 (BV²⁺), 4 (FV²⁺), 5 (HV²⁺) or 6 (SV²⁺); BPY²⁺ = diprotonated 4,4-bipyridine], determined by ¹H NMR and electronic absorption spectroscopy methods, is described. Some different binding models were observed in this work when compared to the interactions between cucurbit[7]uril (Q[7]) and these guests. The experimental results revealed that the binding site of the guests by Q[8] depended strongly on the length of the aliphatic substituents on the 4,4′-bipyridinium nucleus. While a 1:2 complex was observed for Q[8]-BPY²⁺ under acidic conditions, a 1:1 complex was formed for Q[8]-viologen derivatives with chains shorter than four carbon atoms. However, multiple Q[8] molecules could be threaded on the longer-chain FV²⁺, HV²⁺ or SV²⁺ molecules to form 2:1 and even possibly 3:1 complexes.     

Sequence recognition and self-sorting of a dipeptide by cucurbit[6]uril and cucurbit[7]uril

Cucurbit[7]uril forms very strong complex with zwitterionic dipeptide Phe-Gly with affinity exceeding 10(7) M(-1) and effectively recognizes peptide sequence of Phe-Gly over Gly-Phe as well as Tyr-Gly over Gly-Tyr and Trp-Gly over Gly-Trp with relative affinities of 23 000, 18 000 and 2000, respectively.     

Binding modes of cucurbit[6]uril and cucurbit[7]uril with a series of bis-pyridinium compounds

Binding behaviors of cucurbit[6]uril (CB[6]) and cucurbit[7]uril (CB[7]) with a series of bis-pyridinium compounds N, N’-hexamethylenebis(1-alkyl-4-carbamoyl pyridinium bromide) (HBPB-n) (alkyl chain length, n = 6, 8 and 10) guests were investigated using ¹H-NMR, ESI–MS and single crystal X-ray diffraction methods. The results show that CB[6] and CB[7] can form [2]pseudorotaxanes with HBPB-n easily. When increasing the length of tail alkyl chain, the binding site of CB[6] at guest molecules changed from the tail to the middle part, while CB[7] remained located over the tail chain. As CB[6] and CB[7] were added in HBPB-8 aqueous solution, a [3]pseudorotaxane was formed by the inclusion of the internal middle site in CB[6] and the tail chain in CB[7].     

A hemicyanine and cucurbit[n]uril inclusion complex: competitive guest binding of cucurbit[7]uril and cucurbit[8]uril

The interaction between the hemicyanine indole derivative H and the cucubit[n]urils Q[7] and Q[8] has been studied using ¹H NMR and UV spectroscopy as well as by fluorescence experiments. Competitive studies on the inclusion of H by Q[7] and Q[8] have also been conducted, and reveal that on changing the size of the Q[n] cavity, the binding behaviour can be very different.     

Binding modes of cucurbit[6]uril and cucurbit[7]uril with a tetracationic bis(viologen) guest

Binding behaviors of two cucurbit[n]urils (CB[n]) hosts with the [CH3bpy(CH2)6bpyCH3]4+ (bpy = 4,4'-bipyridinium) guest were investigated by 1H NMR and MALDI-TOF-MS experiments. While the CB[6] and CB[7] form [2]pseudorotaxanes with the host located over the hexamethylene chain of the guest, only the CB[7] forms a [3]pseudorotaxane with both host molecules residing over the bipyridinium groups. The initial CB[7] host vacates the inclusion of the hexamethylene chain as a result of the electrostatic and steric repulsions that would arise in simultaneous binding of adjacent aliphatic and aromatic portions of the guest.     

Recognition of glycine by cucurbit[5]uril and cucurbit[6]uril: A comparative study of exo- and endo-binding

Recognition features of glycine (Gly) with cucurbit[5]uril (Q[5]) and cucurbit[6]uril (Q[6]) both in aqueous solution and solid state were investigated by ¹H NMR spectroscopy and X-ray crystallography. ¹H NMR data indicate that the Gly is located outside of the portals of the Q[5], exhibiting exo binding with the Q[5]. In the case of the Q[6], the Gly shows endo binding or a dual binding mode (endo and exo binding) with the host, which depends on the amount of the host in the aqueous solution. X-ray crystallography clearly display that the Gly forms 2:1 exclusion complex with the Q[5], and 2:1 inclusion complex with the Q[6]. Interestingly, hydrogen bondings between the encapsulated Gly molecules in the Q[6] were observed.     

Synthesis and binding behaviors of monomethyl cucurbit[6]uril

By reacting 3a-methylglycoluril and glycoluril with paraformaldehyde, monomethyl cucurbit[6]uril was synthesized in 14% yield. The new host molecule forms stable inclusion complexes with diamine and hexylaminocarbazole derivatives.     

Supramolecular capsules of cucurbit[6]uril and controlled release

Supramolecular capsules of THF and acid molecules inside cucurbit[6]uril have been prepared via [C₂mim]Br route. The 1:1 ratio of host–guest complexes have been characterized by ¹H NMR, thermal gravimetric analysis and elemental analysis in solution and in solid state. Two types of release have been observed in NaCl aqueous solution, including partial release of THF due to stronger binding and complete release of acid molecules (C₃–C₆) due to weaker binding.     

Interaction between tetramethylcucurbit[6]uril and some pyridine derivates

Interaction between tetramethylcucurbit[6]uril (TMeQ[6], host) with hydrochloride salts of 2-phenylpridine (G1), 2-benzylpyridine (G2), and 4-benzylpyridine (G3) (guests) have been investigated by using 1H NMR spectroscopy and electronic absorption spectroscopy and theoretical calculations. The 1H NMR spectra analysis established an interaction model in which the host selectively included the phenyl moiety of the HCl salt of the above three guests, and formed inclusion complexes with a host-guest ratio of 1:1. Absorption spectrophotometric analysis allowed quantitative measurement of the stability of these host-guest inclusion complexes. Particularly, we have established a competitive interaction in which one host-guest inclusion complex pair is much more stable than another host-guest inclusion complex pair. The stability constants for the three host-guest inclusion complexes of TMeQ[6]-G1, TMeQ[6]-G2, and TMeQ[6]-G3 are approximately 2x10(6), 60.7, and 19.9 mol-1.L, respectively. To understand how subtle differences in the structure of the title guests lead to a significant difference in the stability of the corresponding host-guest inclusion complexes with the TMeQ[6], ab initio theoretical calculations have been performed, not only for the gas phase but also the solution phase (water as solvent) in all cases. The calculation results revealed that when the phenyl moiety of the three pyridine derivate guests was included, the host-guest complexation reached the minimum, and the corresponding energy differences for the formation of the title host-guest inclusion complexes are qualitatively consistent with the experimental results.     

Monofunctionalised cucurbit[6]uril synthesis using imidazolium host-guest complexation

Monohydroxylated cucurbit[6]uril was prepared for the first time through the controlled oxidation of CB[6] in the presence of a tailor-made bisimidazolium guest, as verified by (1)H NMR, ESI-MS and X-ray crystallography. Further chemical modification of monohydroxylated CB[6] was also readily achieved.     

Synthesis of disubstituted cucurbit[6]uril and its rotaxane derivative

Synthesis of diphenyl cucurbit[6]uril (CB[6]) has been achieved via co-oligomerization of diphenyl glycoluril and unsubstituted glycoluril. The unsymmetrically substituted CB[6], Ph(2)CB[6], was further converted to a rotaxane incorporating bis(dinitrophenyl)spermine. [reaction: see text]     

Complexation behavior of cucurbit[6]uril with short polypeptides

The binding properties of cucurbit[6]uril towards various peptides have been investigated in acidic aqueous solution. Stability constants and thermodynamic values of the complex formation between following peptides: glycyl-l-alanine, l-leucyl-l-valine, glycyl-l-asparagine, l-leucyl-l-phenylalanine, l-leucyl-l-tryptophan, glycyl-l-histidine, l-glutathione reduced (γ-l-glutamyl-l-cysteinyl-glycine, GSH), and dl-leucyl-glycyl-dl-phenylalanine) with cucurbit[6]uril in aqueous formic acid (50%, v/v) have been calculated from calorimetric titrations. From these results it can be seen that the peptides form exclusion complexes with cucurbit[6]uril. Due to the polar peptide bond they are not included within the hydrophobic cavity of cucurbit[6]uril. The complex formation is favoured by entropic contributions. The release of water molecules from the polar amino groups of the peptides and the carbonyl groups of cucurbituril are responsible.     

The cucurbit[n]uril family

In 1981, the macrocyclic methylene-bridged glycoluril hexamer (CB[6]) was dubbed "cucurbituril" by Mock and co-workers because of its resemblance to the most prominent member of the cucurbitaceae family of plants--the pumpkin. In the intervening years, the fundamental binding properties of CB[6]-high affinity, highly selective, and constrictive binding interactions--have been delineated by the pioneering work of the research groups of Mock, Kim, and Buschmann, and has led to their applications in waste-water remediation, as artificial enzymes, and as molecular switches. More recently, the cucurbit[n]uril family has grown to include homologues (CB[5]-CB[10]), derivatives, congeners, and analogues whose sizes span and exceed the range available with the alpha-, beta-, and gamma-cyclodextrins. Their shapes, solubility, and chemical functionality may now be tailored by synthetic chemistry to play a central role in molecular recognition, self-assembly, and nanotechnology. This Review focuses on the synthesis, recognition properties, and applications of these unique macrocycles.     

Ion binding to cucurbit[6]uril: structure and dynamics

Molecular dynamics simulations are used to study the microscopic structure and dynamics of cations bound to cucurbit[6]uril (CB[6]) in water and in aqueous solutions of sodium, potassium, and calcium chloride. The molarities are 0.183 M for the salts and 0.0184 M for CB[6]. The cations bind only to CB[6] carbonyl oxygens. They are never found inside the CB[6] cavity. Complexes with Na(+) and K(+) mostly involve one cation, whereas with Ca(2+) single- and double-cation complexes are formed in similar proportions. The binding dynamics strongly depends on the type of cation. A smaller size or higher charge increases the residence time of a cation at a given carbonyl oxygen. When bound to CB[6], sodium and potassium cations jump mainly between nearest or second-nearest neighbors. Calcium shows no hopping dynamics. It is coordinated predominantly by one CB[6] oxygen. A few water molecules (zero to four) can occupy the CB[6] cavity, which is limited by the CB[6] oxygen faces. Their residence time is hardly influenced by sodium and potassium ions. In the case of calcium the residence time of the inner water increases notably. A simple structural model for the cation activity as "lids" over the CB[6] portal cannot, however, be identified. The slowing of the water exchange by the ions is a consequence of the generally slower dynamics in their presence and of their stable solvation shells.     

Direct force measurement between cucurbit[6]uril and spermine using atomic force microscopy

The rupture forces of individual host-guest complexes between surface-confined cucurbit[6]uril (CB[6]) and spermine derivatives were measured directly by atomic force microscopy (AFM). While 1,2-dithiolane-attached spermine was immobilized on a gold-coated AFM tip, perallyloxyCB[6] was attached to an allyl-terminated self-assembled monolayer on a gold substrate by olefin metathesis reaction. A histogram and autocorrelation function analysis yielded a rupture force of approximately 120 pN, which is the highest value ever reported for a synthetic host-guest system.     

Supramolecular shuttle based on inclusion complex between cucurbit[6]uril and bispyridinium ethylene

Cucurbit[6]uril (CB6) and bispyridinium ethylene form a stable inclusion complex. A rotaxane derived from this complex was prepared in which a CB6 wheel shuttles along an axle in an NMR time-resolved regime.     

Molecular-recognition properties of a water-soluble cucurbit[6]uril analogue

The molecular-recognition properties of the cucurbit[6]uril analogue (1) in aqueous buffer (sodium acetate, 50 mM, pH 4.74, 25 degrees C) toward a variety of guests including alkanediamines (6-12), aromatics (14-32), amino acids (33-36), and nucleobases (37-42) were studied by fluorescence spectroscopy. For the alkanediamines studied (H2N(CH)nNH2, n = 6, 7, 8, 9, 10, 11, 12), the association constants increase as the length of the alkane (n) is increased. Host 1 is capable of forming strong complexes with guests containing aromatic rings with association constants (Ka) ranging from 10(2) to 10(6) M(-1) as a result of the favorable pi-pi interactions that occur between host 1 and the aromatic rings of the guest when bound in the cavity of 1. Biologically relevant guests such as amino acids and nucleobases are also bound in the cavity of 1 with Ka values ranging from 10(3) to 10(6) M(-1). Consequently, cucurbit[6]uril analogue 1 functions as a versatile fluorescent sensor for the presence of a wide range of chemically and biologically important substances in aqueous solution including nitroaromatics, neurotransmitters, amino acids, and nucleobases.     

Theoretical study on the protonation of cucurbit[6]uril

Abstract  

The most probable structures of the cucurbit[6]uril·H₃O⁺ and cucurbit[6]uril·(H₃O⁺)₂ cationic complex species have been derived by quantum mechanical DFT calculations. In these two complexes, each of the H₃O⁺ ions is bound by three strong linear hydrogen bonds to three carbonyl oxygen atoms of the parent macrocycle.