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Molecular Hedgehog
March 29, 2012   
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Researchers at the Polish Academy of Sciences’ Institute of Physical Chemistry in Warsaw have obtained, crystallized, and examined what they call a molecular container with an exceptional structure. Resembling a rolled-up hedgehog in appearance, the molecular aggregate has a spacious cavity in its central part that can be used to transport other molecules, for example drugs.

The aggregate is composed of 12 calixarene molecules. The researchers from the Institute of Physical Chemistry have managed to crystallize the new structure, which allowed them to get a deeper insight into its molecular structure.

Calixarenes are organic compounds with cyclic molecules composed of phenyl units. The simplest representative of the family is calix[4]arene, consisting of four phenyl units with a cavity in the center. The cavity is large enough to host another molecule, either in whole or in part, thus forming a molecular complex.

“In practice, we consider calixarenes to be molecular equivalents of chalices or baskets that can be used to transport other molecules, for instance drugs,” says Kinga Suwińska, a researcher with a postdoctoral degree at the Institute of Physical Chemistry. “Our research group, together with colleagues from the Institut de Biologie et Chimie des Protéines in Lyon, France, has an international patent for producing co-crystals of calixarenes with drugs.”

Using calixarenes in medicine is not easy. These compounds are usually soluble only in organic solvents. For this reason, the researchers at the Institute of Physical Chemistry study modified calixarenes, substituted with sulphonate (-SO3H) or phosphonate (-PO3H2) groups. Such calixarenes are converted into an acidic form and become water-soluble. Tests on mice, performed two years ago by a research team from the Institut de Biologie et Chimie des Protéines, collaborating with the Institute of Physical Chemistry, showed that sulphonated calixarenes are not toxic in low and medium concentrations. In addition, sulphonated calixarenes are biologically active, which means they have antiviral and antibacterial properties.

Modified calixarenes are potential drug carriers. This is especially important as drugs which are water-insoluble in pure form can become water-soluble in a complex with calixarene. Moreover, complex formation may enhance the drug’s bioavailability profile. This means that drugs—which may have a negative effect on patients’ tissues and organs—can in the future be administered in lower, safer doses.

By designing an appropriate complex with calixarene, the researchers are able to protect complexed molecules against external factors like light or moisture. This has practical significance. In pure form, a drug may decompose, for instance in the upper part of the digestive tract. With calixarenes, the drug can be protected like in a molecular capsule and in this form can be delivered precisely where its presence will be most effective.

The most recent discovery of the institute’s supramolecular chemistry research group are modified calixarene molecules self-assembling into aggregates with an exceptionally complex and visually appealing structure. It all began with obtaining a new calixarene derivative that crystallized as regular crystals.

“Crystalline compounds are excellent research objects,” says Suwińska. “Their structure can be directly analyzed using X-ray diffraction. That’s why we were able to determine so precisely how the new calixarene molecules self-assemble.”

It turns out that under certain conditions, 12 calixarene molecules assemble, forming a spherical structure with alcylocarbonyl groups pointing outward. The emerging supramolecular supermolecule resembles a rolled-up hedgehog.

The picturesque supermolecule discovered at the Institute of Physical Chemistry is about five nanometers (billionth parts of a meter) in size. Particularly important for its potential future applications is the presence of a large internal void with a volume of about 1,000 cubic angstroms.

“On the micro-world scale such a space is really something,” says Suwińska. “You can place more than 30 water molecules inside.” Due to its special structure, the molecular hedgehog seems to be an ideal candidate for transporting other molecules.

The research conducted at the Institute of Physical Chemistry has been exploratory in nature so far, Suwińska says. The development of methods for producing such calixarene supermolecules—and the examination of the properties of their complexes with other molecules including drugs, and testing such complexes for their possible toxicity and/or biological activity—will require long studies in collaboration with other research groups.
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