New synthetic capsules may aid cross-membrane ion transport.

New synthetic capsules may aid cross-membrane ion transport.

Molybdates have proven to be a particularly versatile building material in the laboratory of Achim M?ller, professor of inorganic chemistry at the University of Bielefeld, Germany. M?ller has broken records for the largest artificial molecule and constructed a wide variety of different, pseudobiological shapes and forms with pre-designed and controllable molecular properties (see Chem. Brit., July 2002, p16). Now he presents a study of the uptake and release of small ions into a synthetic capsule, representing a simple model for the crucial ion transport processes at the cell membrane.

The cell is represented by a spherical capsule made of 132 molybdenum centres, with 20 channels leading into the interior with well-defined coordination sites. M?ller and his co-workers studied the movements of lithium ions into and out of this capsule, making use of lithium NMR which gives characteristic sharp peaks for Li+ in bulk solution, while the ions bound inside the capsule have a broad, low intensity signal with an upfield shift.

The researchers shifted the equilibrium between lithium ions inside and outside the capsule simply by varying the temperature. At 20oC, most of the ions are in free solution, while about 15 per cent are bound in the capsule, and then step by step are released with increasing temperature: at 70oC this release process is complete.

Additional investigation using exchange spectroscopy confirmed the movement of the lithium ions. The researchers were also able to seal the openings of the capsule with guanidinium ions, which stopped all exchange of lithium between inside and out.

Lithium alone has only limited biological significance - its use as an empirical treatment of manic depression being the most reported connection it has with the life sciences. But the system studied here can be adapted to other ions and exchange processes using several kinds of ions. M?ller and his team are already studying lithium/sodium exchange processes. Watch this small space.

Michael Gross