Airy accumulators for biomolecules

The organometallic framework structures are also attracting increasing interest from biologists. This is because they offer plenty of space for proteins and chiral compounds.

Metalorganic framework compounds, so-called MOFs, are highly porous. Their interior is permeated by a regular system of cavities and channels. Gas molecules can be stored in the pores. The scaffolding compounds thus absorb them like a sponge. A vessel filled with MOFs can even absorb more gas than an empty vessel of the same size. For this reason, the scaffolding compounds, which were first described in 1999, are currently being tested as natural gas storage facilities for trucks. The storage substances for this purpose have been manufactured on an industrial scale for two years. However, the application potential of the scaffold connections is far from exhausted. An international research group has now produced organometallic compounds with exceptionally large pores. Large molecules such as proteins or other biomolecules can now also be stored in them.

MOFs have a modular and therefore regular structure that resembles a shelf system. They consist of metal-containing complexes that are linked to each other via organic assemblies - so-called linkers - to form a three-dimensional network. By selecting the components, the dimension of the shelf compartments - the pore size of the framework - can be specifically adjusted. A pore size of 4.7 nanometers was previously considered the record. Omar Yaghi from the University of Berkeley in California and his colleagues have now produced MOFs with cavities down to ten nanometers in diameter.

The scientists based their research on the well-known scaffolding compound MOF-74. This consists of rod-shaped units containing zinc or magnesium. The rods, which are arranged in parallel, are linked together and kept apart by a large number of organic bifunctional units of terephthalic acid. This results in a framework with regularly arranged hexagonal channels. Yaghi and his colleagues now extended the spacing by using linkers containing between two and eleven benzene groups instead of terephthalic acid. The longer the linkers were, the larger the channels became. With eleven benzene groups lined up side by side, the result was a scaffold with pores of almost ten nanometers ("Science", Vol. 336, p. 1018). For the first time, the pores of the new MOF family now also have room for biomolecules, such as the green fluorescent protein GFP, vitamin B12 or myoglobin.

Until now, all attempts to synthesize MOFs with such large pores have usually failed at two points: Either two lattices formed that penetrated each other, resulting in only small pores. Or the substances were unstable and collapsed rapidly. However, the new scaffold connections are robust and can even be heated to 300 degrees without the structure giving way.

Apart from storage purposes, the framework structures can be used for the separation or purification of substances. An iron-containing scaffolding substance could be used in oil refineries, where it could help to save a cost- and energy-intensive process step, as research by the group led by Jeffrey Long, also from the University of Berkeley, shows. Up to now, after cracking, which takes place at 500 degrees, the cracked gases have had to be strongly cooled in order to be able to separate the resulting gases: Unsaturated hydrocarbons such as ethylene and propylene then migrate to polymer production, while ethane and propane are used as fuels. According to Long and his colleagues, this separation can also be achieved with framework connections and without strong cooling.

For demonstration purposes, the researchers allowed a hydrocarbon mixture to flow at a temperature of 45 degrees through an iron-containing scaffold substance. This showed that their MOFs selectively held the unsaturated hydrocarbons inside, while allowing alkanes such as ethane or propane to pass through unhindered ("Science", Vol. 335, p. 1606). Ethane was obtained with a purity of 99 percent. The porous substance loaded with unsaturated hydrocarbons only released it again when it was heated.

Researchers from the Ruhr University of Bochum and the Karlsruhe Institute of Technology (KIT) recently reported on chiral scaffolding structures that are suitable for the rapid and inexpensive separation of enantiomers and could thus become interesting for the production of drugs. Many drugs are chiral, i.e. there are always two mirror-image forms of these molecules, so-called enantiomers. Often, the two forms have different pharmacological properties, which is why the enantiomers produced during the production process have to be separated from each other.

The scientists led by Christof Wöll and Roland Fischer constructed their MOFs layer by layer by immersing a solid body successively in solutions containing the starting substances. As usual, a framework substance with a modular structure was created. The chemists used camphoric acid, a chiral organic molecule, as the bracing material. The synthesized MOFs were therefore sensitive to molecules of a certain chirality. These were separated from the other enantiomers. Researchers have demonstrated this one-sided mode of action on the chiral alcohol hexanediol ("Angewandte Chemie", Vol. 124, p. 831). Now they are working on increasing the pore size of their MOFs in order to be able to use the separation process for larger molecules as well.