Supramolecular Synthons in Protein-Ligand Frameworks

Supramolecular synthons, defined as reproducible intermolecular structural units, have greatly aided small molecule crystal engineering. In this paper, we propose that supramolecular synthons guide ligand-mediated protein crystallization. The protein RSL and the macrocycle sulfonato-calix[8]arene cocrystallize in at least four ways. One of these cocrystals is a highly porous cube comprising protein nodes connected by calixarene dimers. We show that mutating an aspartic acid to an asparagine results in two new cubic assemblies that depend also on the crystallization method. One of the new cubic arrangements is mediated by calixarene trimers and has a ∼30% increased cell volume relative to the original crystal with calixarene dimers. Crystals of the sulfonato-calix[8]arene sodium salt were obtained from buffered conditions similar to those used to grow the protein-calix[8]arene cocrystals. X-ray analysis reveals a coordination polymer of the anionic calix[8]arene and sodium cation in which the macrocycle is arranged as staggered stacks of the pleated loop conformation. Remarkably, the calixarene packing arrangement is the same in the simple salt as in the protein cocrystal. With the pleated loop conformation, the calixarene presents an extended surface for binding other calixarenes (oligomerization) as well as binding to a protein patch (biomolecular complexation). Small-angle X-ray scattering data suggest pH-dependent calixarene assembly in solution. Therefore, the calix[8]arene-calix[8]arene structural unit may be regarded as a supramolecular synthon that directs at least two types of protein assembly, suggesting applications in protein crystal engineering.

Investigation of supramolecular interaction of quercetin with N,N-dimethylamine-functionalized p-sulfonated calix[4,8]arenes using molecular modeling and their in vitro cytotoxic response towards selected cancer cells

Although quercetin is an effective bioactive compound preventing the progress of several human cancers, its impact is reduced due to low bioavailability.

Supramolecular stacking in a high Z′ calix[8]arene–porphyrin assembly

A co-crystal structure of sulfonato-calix[8]arene (sclx8) and trimethylanilinium-porphyrin (tmap) at 1.0 Å resolution is reported.

Calixarene-mediated assembly of a small antifungal protein

Synthetic macrocycles such as calixarenes and cucurbiturils are increasingly applied as mediators of protein assembly and crystallization. The macrocycle can facilitate assembly by providing a surface on which two or more proteins bind simultaneously. This work explores the capacity of the sulfonato-calix[n]arene (sclx n ) series to effect crystallization of PAF, a small, cationic antifungal protein. Co-crystallization with sclx4, sclx6 or sclx8 led to high-resolution crystal structures. In the absence of sclx n , diffraction-quality crystals of PAF were not obtained. Interestingly, all three sclx n were bound to a similar patch on PAF. The largest and most flexible variant, sclx8, yielded a dimer of PAF. Complex formation was evident in solution via NMR and ITC experiments, showing more pronounced effects with increasing macrocycle size. In agreement with the crystal structure, the ITC data suggested that sclx8 acts as a bidentate ligand. The contributions of calixarene size/conformation to protein recognition and assembly are discussed. Finally, it is suggested that the conserved binding site for anionic calixarenes implicates this region of PAF in membrane binding, which is a prerequisite for antifungal activity.

Micro-nano structure nanofibrous p-sulfonatocalix[8]arene complex membranes for highly efficient and selective adsorption of lanthanum(iii) ions in aqueous solution

In this study, micro-nano structured p-sulfonatocalix[8]arene (calix8) complex membranes for high efficient selective adsorption of La(iii) ions were prepared via a facile route.

An in vitro antibacterial and antifungal effects of cadmium(II) complexes of hexamethyltetraazacyclotetradecadiene and isomers of its saturated analogue

OBJECTIVE

To evaluate the antibacterial and antifungal effects of cadmium(II) complexes with hexamethyltetraazacyclotetradecadiene ligands.

METHODS

Five coordinated square pyramidal cadmium(II) complexes and six coordinated square octahedral cadmium(II) complexes have been synthesized by interaction of 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene (denoted by L.2HClO4) and C-chiral isomers of its saturated analogue (denoted by 'teta' and 'tetb') with different salts of Cd(2+) ion [e.g. CdI2, Cd(NO3)2·6H2O, CdCl2·2H2O and Cd(ClO4)2·6H2O] in methanolic solution. Complexes of the ligands were investigated for antibacterial activity by disc diffusion method and antifungal effect by poisoned food technique.

RESULTS

The newly synthesized cadmium(II) complexes of the ligands were screened as potential antimicrobial agent against a number of medically important bacteria (Staphylococcus aureus, Bacillus cereus, Salmonella typhi, Shigella dysenteriae and Escherichia coli) and against two fungi (Candida albicans and Aspergillus aculeatus). The growth inhibiting activity of the ligands and complexes against bacteria and fungi were compared with the standard antibiotic ampicillin and commercially important antifungal agent, griseofulvin respectively. Among them some of the macrocyclic complexes were found to be more fungitoxic and antibacterial than the reference antifungal drug griseofulvin and antibacterial drug ampicillin respectively.

CONCLUSIONS

Hexamethyltetraazacyclotetradecadiene ligands and its complexes could be considered as very potential antibacterial and antifungal agent with further investigation.

Structural characterization of inclusion complexes of para-sulphonato-calix[8]arene with 1,2-bis(4-pyridyl)-ethane and 1,3-bis(4-pyridyl)-propane. New ‘double cone’ and ‘up–flat–down’ conformations of para-sulphonato-calix[8]arene

Two new conformations of para-sulphonato-calix[8]arene have been found in supramolecular complexes with BPE and BPP, the gas take-up has been demonstrated for S8C–BPE complex.

Investigation of the inclusion interaction of p-sulfonatocalix[6]arene with triamterene

The inclusion complexation behavior of p-sulfonatocalix[6]arene (SCX6) with triamterene (TA) was investigated by fluorescence spectroscopy, UV-Vis spectroscopy, (1)H and 2D NMR, FT-IR, SEM and DSC. The results indicate that TA is able to form an inclusion complex with SCX6. The inclusion complex has a stoichiometry of 1:1 at pH 6.50. In addition, the water solubility of TA was obviously increased in the inclusion complex with SCX6.

Sulfonatocalix[8]arene as a potential reaction cavity: photo- and electro-active dicationic guests arrest conformational equilibrium

Smaller members of water-soluble sulfonated calixarenes have been extensively explored in the context of host-guest complexation, supramolecular chemistry, and potential sensors. However, larger members especially eight-membered calixarene (CA[8]) has received much less attention because of its floppy nature and tendency to exist as a mixture of conformational isomers. Our continued interest in identifying molecules with an internal cavity as reaction vessels has led us to examine the host-guest complexation of CA[8] with photoactive bispyridyl ethylenes. We find that 4,4'-bispyridyl ethylene and 3,3'-bispyridyl ethylene upon complexation to CA[8] arrest the conformational equilibrium and force the latter to adopt a single conformation in solution. During complexation, bispyridyl ethylenes are protonated by the sulfonic acid groups of CA[8]. The host-guest complex is stabilized via an electrostatic interaction between the cationic bispyridyl ethylenes and anionic sulfonated calix[8]arene, and we propose the complex to have an inverted capsular structure. This model is also consistent with the electrochemical behavior of 4,4'-dimethylviologen included within CA[8]. Rigidification of bispyridyl ethylenes by the host has a consequence on the excited-state chemistry of the former. Generally, prevalent geometric isomerization of bispyridyl ethylenes are prevented by CA[8] upon complexation.