The Solid-State Complex of para-Sulphonato-Calix[8]Arene Anion with Dimethylammonium Cations

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.

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.

Olefin Metathesis Reaction in Water and in Air Improved by Supramolecular Additives

A range of water-immiscible commercially available Grubbs-type precatalysts can be used in ring-closing olefin metathesis reaction in high yields. The synthetic transformation is possible in pure water under ambient conditions. Sulfocalixarenes can help to boost the reactivity of the metathesis reaction by catalyst activation, improved mass transfer, and solubility of reactants in the aqueous reaction media. Additionally, the use of supramolecular additives allows lower catalyst loadings, but still high activity in pure water under aerobic conditions.

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.

Solid-state interactions of calixarenes with biorelevant molecules

Among all crystalline complexes of calix-type calix[n]arenes those with organic molecules which are biologically relevant are especially of interest due to their potential medical and pharmaceutical applications. The co-crystallization of drugs with water-soluble calixarenes offers the opportunity to modify chemical and physical properties of APIs (active pharmaceutical ingredients) and to control drug conformation. Such co-crystallization can improve solubility, bioavailability and stability of pharmaceutically active molecules and/or eliminate polymorphism. In this article the solid-state interactions between calixarene hosts and biologically relevant guest molecules are summarized for the currently available structures solved by single-crystal X-ray crystallography.

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.